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Plant morphology

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McGRAW-HILL PUBLICATIONS IX THE BOTANICAL SCIENCES

Edmund W. Sinnott. Consulting Editor

PLANT MORPHOLOGY

McGRAW-HILL PUBLICATIONS IN THE BOTANICAL SCIENCES

Edmund W. Sinnott, Consulting Editor

Arnold — An Introduction to Paleobotany

Avery et al. — Hormones and Horticulture

Babcock and Clausen — Genetics

Curtis and Clark — An Introduction to Plant Physiology

Eames — Morphology of Vascular Plants

Eames and MacDaniels — An Introduction to Plant Anatomy

FiTz PATRICK — The Lower Fungi

Gates — Field Manual of Plant Ecology

Gaumann and Dodge — Comparative Morphology of Fungi

Haupt — An Introduction to Botany

Haupt — Laboratory Manual of Elementary Botany

Haupt — Plant Morphology

Hill — Economic Botany •

Hill, Overholts, and Popp — Botany

Johansen — Plant Microtechnique

Kramer — Plant and Soil Water Relationships

Lilly and Barnett — Physiology of the Fungi

Maheshwari — An Introduction to the Embryology of Angiosperms

Miller — Plant Physiology

Pool — Flowers and Flowering Plants

Sharp — Fundamentals of Cytology

Sharp — Introduction to Cytology

Sinnott — Botany: Principles and Problems

Sinnott — Laboratory Manual for Elementary Botany

Sinnott, Dunn, and Dobzhansky — Principles of Genetics

Smith — Cryptogamic Botany

Vol. I. Algae and Fungi

Vol. II. Bryophytes and Pteridophytes Smith — The Fresh-water Algae of the ITnited States Swingle — Textbook of Systematic Botany Weaver — Root Development of Field Crops Weaver and Clements — Plant Ecology

There are also the related series of McGraw-Hill Publications in the Zoo- logical Sciences, of which E. J. Boell is Consulting Editor, and in the Agri- cultural Sciences, of which R. A. Brink is Consulting Editor.

H ^'1

PLANT MORPHOLOGY

ARTHUR W. HAUPT

Professor of Botany University of California, Los Angeles

"Morphology .. .is one of the most interesting departments of natural history, and may almost be said to be its very soul." — Charles Darwin, Origin of Species.

New York Toronto London McGRAW-HILL BOOK COMPANY, INC.

1953

PLANT MORPHOLOGY

Copyright, 1953, by the McGraw-Hill Book Company, Inc. Printed in the United States of America. All rights reserved. This book, or parts thereof, may not be reproduced in any form without permission of the publishers.

Library of Congress Catalog Card Number: 52-13810

THE MAPLE PRESS COMPANY, YORK, PA.

OGlC/17

,3: LIBRakY ^

MASS. Qv

PREFACE

This book deals A\'ith the principal groups of plants from the standpoint of their structure, reproduction, and development. It presents a survey of the plant kingdom Avith emphasis upon relationships as revealed by basic similarity in bodil}' organization and life histories. It gives an account of the general course of evolution that existing groups appear to have followed. It endeavors to interpret, as far as possible, the struc- tural and developmental complexities of the higher plants in terms of the simpler conditions prevailing among the lower plants.

The principal groups of plants are taken up in an ascending sequence based on ever-increasing structural complexity. This order of pre- sentation does not impl}^ direct phylogenetic relationship between successive groups, even though in some cases such relationship may exist. It merely denotes different degrees of progress from what is assumed to have been a more primitive condition. Conclusions as to the derivation of one group from another are based on substantial morphological evidence, but are always tentative and subject to confirmation by paleontological evidence. A true understanding of phylogeny can rest onl\^ on the fossil record and, with a few notable exceptions, this is very incomplete.

The system of classification used as a basis for the presentation of the principal plant groups is at once simple and conservative. The older classification, which continues to be the one most widely used, has certain limitations, but these arise mainly from uncertainties regarding the affinities of many groups, particularly the lower ones. As long as these uncertainties remain, there is little justification for abandoning an established system of classification for a newer one. A somewhat simpli- fied classification is adopted because of its greater convenience and because more detailed schemes may be found in advanced works dealing ^vith special plant groups. It is comprehensive enough to embrace, under almost every class, the most important orders; but it generally does not include families.

Usually the outstanding features of each order are developed through a detailed discussion of one or, more frequently, of several of its repre- sentative genera. The distinguishing characters of the order are then given in the form of a summar3^ Likewise the characters of each class are summarized after all its members have been considered. These are usually presented with the characters of related classes, so that a com-

vi PREFACE

parison can be made. General conclusions are given at the end of the account of each of the major divisions of the plant kingdom, viz., algae, fungi, bryophytes, pteridophytes, and spermatophytes. Here are emphasized the evolutionary tendencies within the group, its contribu- tions to the evolution of the plant kingdom, and the interrelationships of its classes.

This book is designed for use in a two-semester course with adequate laboratory work. It is intended to follow a course in general botany, where the student has gained a knowledge of such material as the many available elementary textbooks present. In particular, the student should understand the cytological relations involved in alternation of generations, including the behavior of the chromosomes in vegetative mitosis, fertilization, and meiosis. Much material properly belonging to the special fields of plant anatomy, cytology, and taxonomy has been omitted from the present work, especially in the treatment of the angio- sperms. Emphasis is placed throughout on the evolution of the plant kingdom as revealed by a comparative study of the morphology of the main groups. At the end of the book a list of supplementary readings has been added. These will serve to introduce the student to the current literature dealing with special groups and topics.

More than two-thirds of the illustrations are original, and most of these have not hitherto been pubhshed elsewhere. Some have been taken from the author's earlier writings. Of the figures borrowed from the works of others, for which credit is given in every case, almost all have been redrawn and are designated in the legends by the word "after."

The author is indebted for many valuable suggestions to his colleague, Prof. Orda A. Plunkett, and to H. R. Bennett of Chicago, who read Chap. IV; to Prof. P. Maheshwari, University of Delhi, India, who read Chaps. VIII and IX, and to Prof. Paul D. Voth, The University of Chicago, who read the entire manuscript. The author is also grateful to his wife for making some of the slides from which illustrations have been made and for much assistance in proofreading.

Arthur W. Haupt Los Angeles, Calif. April, 1953

CONTENTS

Preface v

I. INTRODUCTION 1

Classification of Plants 1

Plant Life of the Past 4

II. thallophyta: algae 7

1. Cyanophyceae 8

2. Euglenophyceae 14

3. Chrysophyceae 16

4. Dinophyceae 18

5. Xanthophyceae 18

6. Bacillariophyceae 21

7. Chlorophyceae 25

1. Volvocales 26

2. Chlorococcales 32

3. Ulotrichales 38

4. Oedogoniales 45

. 5. Conjugales 48

6. Siphonocladiales 54

7. Siphonales 57

Summary of Chlorophyceae 61

III. thallophyta: algae (continued) 63

8. Charophyceae 63

9. Phaeophyceae 66

1. Ectocarpales 67

2. Sphacelariales 69

3. Cutleriales 70

4. Dictyotales 72

5. Laminariales 76

6. Fucales 80

Summary of Phaeophyceae 85

10. Rhodophyceae 85

Comparison of the Classes of Algae 93

General Conclusions 94

IV. thallophyta: fungi 100

1. Schizomycetes 100

2. Myxomycetes 104

3. Phycomycetes 103

vii

67807

viii CONTENTS

1. Chytridiales 108

2. Monohlepharidales Ill

3. Plasmo(iiophoralps 112

4. Saprolegnialos 113

5. Peronosporales 114

6. Mucorales 117

7. Entomophthorales 120

4. Ascomycetes 121

1. Protoascales 122

2. Protodiscales 123

3. Plectascales 124

4. Perisporiales 127

5. Pezizales 129

6. Helvellales 133

7. Tuberales 134

8. Pyrenomycetales 134

9. Laboulbeniales 140

5. Basidiomycetes 140

1. Ustilaginales 140

2. Uredinales 142

3. Auriculariales 146

4. Tremellales 146

5. Exobasidiales 146

6. Hymenomycetales 147

7. Gasteromycetales 151

Fungi Imperfecti 153

Lichenes 153

Comparison of the Classes of Fungi 156

General Conclusions 157

V. BRYOPHYTA 160

1. Hepaticae 161

1. Marchantiales 161

2. Sphaerocarpales 173

3. Jungermanniales 177

4. Anthocerotales 186

2. Musci 192

1. Sphagnales 192

2. Andreaeales 197

3. Bryales 198

Comparison of Liverworts and Mosses 205

General Conclusions 205

VI. PTERIDOPHTTA 209

The Vascular System 210

1. Psilophytinae 212

1. Psilophytales . 212

2. Psilotales 214

2. Lycopodiinae 218

1. Lycopodiales 218

CONTENTS ix

2. Selaginellales 230

3. Lepidodcndrales 238

4. Isoetales 241

3. Equisetinae 247

1. Hyeniales 247

2. Sphenophyllales 248

3. Equisotales 250

4. Calamitales 258

VII. PTERIDOPHYTA (CONTINUED) 260

4. Filicinae 260

1. Coenopteridales 260

2. Ophioglossales 261

3. Marattiales 270

4. Filicales 276

5. Hydropteridales 292

Comparison of the Classes of Pteridophytes 303

General Conclusions 304

VIII. SPERMATOPHYTA 309

1. Gymnospermae 310

1. Cycadofilicales 311

2. Bennettitales 315

3. Cycadales 319

4. Cordaitales 330

5. Ginkgoales 333

6. Coniferales 340

7. Gnetales 354

IX. SPERMATOPHYTA (CONTINUED) 362

2. Angiospermae 362

Vegetative Organs 363

The Flower 372

Chief Orders of Angiosperms 399

Comparison of Gymnosperms and Angiosperms 408

General Conclusions 409

X. EVOLUTION OF THE PLANT KINGDOM 412

Prominent Evolutionary Tendencies 415

Evolution of Sex 417

Alternation of Generations 422

Selected References 427

Glossary 431

Index 445

CHAPTER I INTRODUCTION

Morphology deals with the form and structure of plants. It is con- cerned with both gross, external features and minute, internal details. It includes a study of the development of plants throughout all their growth stages, called ontogeny, as well as their evolutionary development, or phyJogeny, by means of which all existing plants have been derived from those of past ages. Morphology considers the interrelationships of the groups forming the larger units of classification, but does not deal with species, the study of which belongs to taxonomy. One of the main objec- tives of morphology is the determination, so far as possible, of lines of descent.

A sound knowledge of the structure and development of plants is a necessary foundation for successful specialization in any phase of botany, whether it be taxonomy, physiology, ecology, pathology, or genetics. A study of the lower plants is often neglected, since the higher ones are more famihar and, in general, more important. Many of the problems encountered in the higher plants, however, are more easily studied in the lower plants, whose structure and functions are much simpler. The logical procedure is to study simple plants before attempting to under- stand complex plants.

CLASSIFICATION OF PLANTS

For many years the system of classification most widely adopted by botanists has been one in which the plant kingdom is separated into four major divisions: Thallophyta, Bryophyta, Pteridophyta, and Spermato- phyta. Each division comprises a number of classes. A class is made up of orders, an order of Jamilies, a family of genera, and a genus of species. Categories of intermediate rank are designated by the prefix suh.

At one time the two subkingdoms Cryptogamia and Phanerogamia were recognized, the former including the three lower divisions and the latter the fourth division. These names have fallen into disuse because they are inappropriate. Cryptogam means "fertilization concealed" and phanerogam means "fertiUzation evident." The names were given because stamens and pistils, the organs once thought to produce directly the cells which unite in fertilization, are present in seed plants but not in plants without seeds. After the true nature of fertiUzation was dis-

1

2 PLANT MORPHOLOGY

covered, it was found to be actually more evident in the so-called crypto- gams than in the phanerogams.

Often all plants above the level of the Thallophyta are grouped together as the Embryophyta, plants in which the zygote gives rise to an embryo that undergoes its early development within either an archego- nium or an embryo sac. A less suitable name for these plants, but one sometimes used, is Cormophyta, meaning "plants with a stem." Many bryophytes have a stem, but it is not homologous with the stem of pteridophytes and spermatophytes. Sometimes the bryophytes and pteridophytes are combined into a single group, the Archegoniatae, a name that is not distinctive because archegonia are present in nearly all gymnosperms, which form the lower class of spermatophytes. A recent tendency is to place the pteridophytes and spermatophytes together under the name of Tracheophyta, which signifies that they are vascular plants.

Classification of Thallophyta. Some botanists disapprove of the term Thallophyta on the ground that it includes a heterogeneous assemblage of plants which are not closely related. This objection is more valid v/hen the term is applied to one of the four divisions of the plant kingdom rather than to one of two subkingdoms; for the same objection could be raised against the term Embryophyta. A partial solution of the difficulty is to consider the Thallophyta as a subkingdom and to raise the algae and fungi to the rank of divisions, as follows:

A. Thallophyta

I. Phycophyta (Algae)

II. Mycophyta (Fungi)

B. Embryophyta (Cormophyta)

I. Bryophyta

II. Pteridophyta

III. Spermatophyta

The thallophytes comprise a number of subordinate groups. These may either be considered as classes and assigned to the algae or the fungi, or may be distributed among a larger number of divisions. The first arrangement is a convenient one, but some of the groups classified as algae or fungi have little in common with the others. Furthermore, it makes the presence or absence of chlorophyll the basis for establishing the two divisions Phycophyta and Mycophyta, a distinction which cannot be maintained among the flagellates, where both green and colorless forms occur. The flagellates were formerly regarded as constituting a distinct class of thallophytes, but are now broken up into a number of separate groups.

Some of the groups commonly included among the algae and fungi are so distinctive that their separation seems justified. The.se include the

INTRODUCTION 3

Cyanophj^ceae, Schizomyoetes, M^^\omycetes, Bacillariophyceae, and possibly some of the flagellate groups. The remaining classes of algae might then be retained in one division and the remaining classes of fungi in another, or some or all of these classes might be raised to the rank of divisions. Much difference of opinion exists as to which classes should be placed together.

Two different arrangements for classifying the thallophytes are as follows :

A.

Thallophyta

I. Phycophyta'

I.

Schizophyta

1. Cyanophyceae

1. Cyanophyceae

2. Xanthophyceae

2. Schizomycetes

3. Bacillariophyceae

II.

Myxomycophyta (Myxomycetes)

4. Chlorophyceae

III.

Bacillariophyta (Diatomeae)

5. Charophyceae

IV.

Euphycophyta (Euphyceae)'

6. Phaeophyceae

1. Xanthophyceae

7. Rhodophyceae

2. Chlorophyceae

II. Mycophyta

3. Charophyceae

1. Schizomycetes

4. Phaeophyceae

2. Myxomycetes

5. Rhodophyceae

3. Phycomycetes

V.

Eumycophyta (Eumycetes)

4. Ascomycetes

1. Phycomycetes

5. Basidiomycetes

2. Ascomycetes

3. Basidiomycetes

' Several other classes, consisting almost entirely of flagellates, are generally recog- nized. These are the Euglenophyceae, Chrysophyceae, Cryptophyceae, and Dinophyceae.

The elevation of a great number of classes to the rank of divisions, thus making each coordinate with the bryophytes, pteridophytes, and sperma- tophytes (or even with the tracheophytes, if the last two are combined), tends to conceal relationships and gives a prominent place to small, obscure groups. Moreover, when the names of all the divisions are given the termination phyta, in order to make them consistent through- out the plant kingdom, many lose their distinctive meanings, and we find the various groups of algae called Chlorophyta (green plants), Phaeophyta (brown plants), Rhodophyta (red plants), etc.

Classification of Embryophyta. The position of the Bryophyta as a division of the plant kingdom seems secure. Those botanists who classify the higher plants on the basis of vascular anatomy discard the names Pteridophyta and Spermatophyta and designate all vascular plants as Tracheophyta. They point out that a marked tendency toward seed formation was present in several extinct groups of pterido- phytes, and that the most primitive group of seed plants, the extinct Cycadofilicales, were very fern-like. The Tracheophyta, constituting a division, are separated into four classes, the Psilopsida, Lycopsida,

PLANT MORPHOLOGY

Sphenopsida, and Pteropsida. The first three correspond to established classes of pteridophytes under the older classification, while the Pterop- sida include the ferns (Filicinae), gymnosperms, and angiosperms.

The presence of leaf gaps in the vascular cylinder is thought to indicate a closer relationship between the ferns and seed plants than exists between the ferns and other pteridophytes. However, the basis used in distinguishing the ferns from the gymnosperms and angiosperms, when the three are grouped together as Pteropsida, is the same as when the ferns are placed in the Pteridophyta and the other two groups in the Spermatophyta. Furthermore, if the existing classes of pteridophytes represent collateral lines of descent from the psilophytes of the Devonian, a view widely accepted, their relationship to one another can better be expressed by including them in a division of their own. Certainly no greater degree of relationship is expressed by placing them in a division that also includes the gymnosperms and angiosperms.

The t"svo different schemes of classifying the embryophytes are as follows:

B. Embryophyta

I.

I. Bryophyta

1. Hepaticae

2. Musci

II. Pteridophyta

1. Psilophytinae

2. Lycopodiinae

3. Equisetinae

4. Filicinae III. Spermatophyta

1. Gymnospermae

2. Angiospermae

Bryophyta

1. Hepaticae

2. Musci

II. Tracheophyta

1 . Psilopsida

2. Lycopsida

3. Sphenopsida

4. Pteropsida

a. Filicinae

b. Gymnospermae

c. Angiospermae

PLANT LIFE OF THE PAST'

The plants of today are the modified descendants of other plants that have lived on the earth throughout the course of geologic history. They are the products of a process of evolution that has been in operation since life first began. Our knowledge of the plants of the past has come from a study of fossil remains found embedded in the layers of rock that form the earth's crust. These remains constitute a direct record of the changes undergone by plants down through the ages. This record, incomplete as it is, helps us to follow the course of evolution and to understand the relationships that occur among the various existing plant groups.

It is not known how or when life arose on the earth. It is not even

known in what form it arose, although much evidence indicates that the

first living things were extremely simple and from them forms more and

' This subject is presented in much greater detail in Arthur W. Haupt, An Introduc- tion to Botany, 2d ed., Chap. XX, New York, 1946.

INTRODUCTION 5

more complex have been evolved. Some groups have made more progress than others. That is why existing groups are at different levels of development. Along with the tendency toward ever-increasing complexity, much retrogression has occurred and, as a result, some modern groups are more or less degenerate.

Paleobotany, the study of fossil plants, has made great progress because methods have been developed making it possible to study thin sections of petrified material under the microscope. Many of these sections show such an amazing wealth of structural detail that almost as much can be learned from them as from sections of living plants. Unlike petri- factions, fossils in the form of casts or impressions, made when some part of a plant falls into soft earth that later hardens into stone, show no internal structvu-e but preserve many external features. Most fossils are of this kind.

Geologic time, whose total duration is about 2 billion years, is divided into five great eras. The Archeozoic era came first. Then followed, in order, the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras. Each era is divided into periods. The Archeozoic and Proterozoic, with an estimated duration of 800 million and 650 million years, respectively, comprise nearly three-fourths of all geologic time. Most of the evidence for the existence of Hfe during these two great eras is indirect, consisting of extensive deposits of graphite, limestone, and iron ores, substances that are formed, at least to some extent, by organisms. The earliest plants may have been similar to certain existing bacteria and blue-green algae.

The fossil record of nonwoody plants is very fragmentary. Because of their soft and perishable nature, few have left any direct evidence of their existence. Remains are more numerous of such algae as diatoms, which have siliceous shells, and of lime-secreting seaweeds. Bryophytes have been poorly preserved and their remains are scanty. Vascular plants, on the other hand, are represented by an abundance of well- preserved fossil material, and much is known of the geologic history of many groups.

The fossil record really begins with the Paleozoic era, since so little is known of the life of the Archeozoic and Proterozoic. The periods into which the Paleozoic, Mesozoic, and Cenozoic eras are divided are given in the table on page 6. The figures in the time scale denote millions of years.

Fossil algae furnish the only record of plant life during the Cambrian and Ordovician, and the diversity of types which have been found indi- cates that all four of the great algal groups were represented in both periods. Silurian deposits have yielded remains of the oldest known land plants, the psilophytes, but these are scanty. During the Devonian so

6

PLANT MORPHOLOGY

much progress was made, that not only were many kinds of primitive land plants in existence, but even such highly developed forms as large lycopods, ferns, and primitive gymnosperms were abundant.

The Carboniferous was characterized by a wonderful display of plant life. Tree lycopods and horsetails, as well as fern-like and other primi- tive gymnosperms, formed a most luxuriant growth surpassing even the

Time scale

Periods

Eras

0

Recent

1

Quaternary

Cenozoic

60

Tertiary-

100

Upper Cretaceous \

125 160

Lower Cretaceous

■

Jurassic

Mesozoic

195

Triassic

220

Permian \

255

Upper Carboniferous 1

305

Lower Carboniferous/

355

Devonian \

Paleozoic

395

Silurian I

480

Ordovician j

550

Cambrian /

densest tropical jungles of today. The accumulated remains of the plants that lived in the vast Carboniferous swamp forests have formed our most extensive coal deposits.

The plant life of the Mesozoic, except during the Upper Cretaceous, was dominated by the gymnosperms, these being of much more advanced types than had lived during the Paleozoic. Nearly all the large pterido- phytes of the late Paleozoic, as well as the primitive gymnosperms, became extinct early in the Mesozoic. A striking feature of the Creta- ceous was the rise of the angiosperms, as a result of which they came to dominate the vegetation of the entire earth, a position they have main- tained ever since. With the rise of the angiosperms, the gymnosperms have become a subordinate group.

CHAPTER II THALLOPHYTA: ALGAE

The thallophytes comprise a large and diverse assemblage of simple plants forming the lowest division of the plant kingdom. They number about 88,000 species. The plant body may be unicellular but, where multicellular, as is generally the case, it is a thallus — a body without differentiation into true vegetative organs, such as characterize the higher plants. This distinction is not absolute, however, as some of the marine algae have parts that superficially resemble true vegetative organs, while some of the bryophytes have thallus bodies. A more tenable distinction is based on the structure of the reproductive organs. The sporangia of thallophytes, with only a few exceptions, are unicellular; those of the higher plants are always multicellular. The gametangia of thallophytes are prevailingly unicellular but, where multicellular, have no outer layer of sterile cells (except in the Charophyceae). In the thallophytes the zygote does not produce an embryo within the female sex organ, as it does in all the higher groups.

The Thallophyta include two main series, the algae (Phycophyta) and the fungi (Mycophyta), the former with 18,000 species and the Latter with 70,000. The algae, having chlorophyll, are able to make food by photo- synthesis and so are independent {autotrophic) plants. The fungi, lack- ing chlorophyll, must obtain their food from an external source and so are dependent (heterotrophic) plants. This distinction, being physiologi- cal, is a convenient one but does not necessarily express relationship; thus it may be without phylogenetic significance. For this reason the various classes of algae and fungi are often regarded as separate and more or less coordinate groups of thallophytes rather than as members of two different series.

Algae live in both fresh and salt water, while a few grow on moist soil, wet rocks, tree trunks, or in other terrestrial habitats. They include the pond scums, kelps and other seaweeds, and a host of less famihar forms. Many are microscopic, but some kelps reach a large size. Because of their perishable nature, algae have left few reliable records of their exist- ence during geologic times. Most of those preserved as fossils are lime- secreting seaweeds and forms with sihceous shells (diatoms). As here presented, the algae are distributed among 10 main classes, the Cyano- phyceae, Euglenophyceae, Chrysophyceae, Dinophyceae, Xanthophy-

7

8 PLANT MORPHOLOGY

ceae, Bacillariophyceae, Chlorophyceae, Charophyceae, Phaeophyceae,

and Rhodophyceae.

1. CYANOPHYCEAE

The Cyanophyceae,^ or blue-green algae, are the simplest and lowest group of green plants. They are characterized by having, in addition to chlorophyll and carotinoids, a blue pigment, phycocyanin, the combina- tion resulting in a blue-green color. Some of the Cyanophyceae, how- ever, also possess a red pigment, phycoerythrin, the presence of which, in varying amounts, produces shades of red, brown, or purple. The Red Sea is said to have received its name from a floating species, Trichodes- mium erythraeum, which is red and sometimes occurs in such abundance as to color the water. The Cyanophyceae are unicellular plants, the cells being nearly always grouped to form colonies of various kinds. About 1,500 species are known.

The Cyanophyceae comprise two orders: (1) the Coccogonales, whose cells are either solitary or arranged in nonfilamentous colonies; and (2) the Hormogonales, whose cells are in filamentous colonies. Some of the genera belonging to the Coccogonales are Chroococcus, Gloeocapsa, Meris- mopedia, Coelosphaerium, and Chamae siphon. The main genera of the Hormogonales include Oscillatoria, Lynghya, Nostoc, Anahaena, Rivula- ria, Gloeotrichia, Tolypothrix, Scytonema, and Stigonema.

Distribution and Habitat. Blue-green algae are found in all parts of the world where plants can grow. Most of them live in fresh water, some occur on moist earth, rocks, and trees, while others live in the ocean. They commonly form scums, slimy mats, or gelatinous lumps. They are especially prevalent in stagnant water, where large quantities of organic matter accumulate. Some live in hot springs at temperatures as high as 75°C. Many forms extract calcium and magnesium from the water and cause minerals, which are often brightly colored, to be depos- ited on rocks in the vicinity. Some species of Nostoc and Anahaena live as endophytes in the intercellular cavities of other plants, as in the thallus of Anthoceros, the leaves of Azolla, and the roots of cycads. Some blue-green algae enter into the formation of lichens.

The Cyanophyceae living in hot springs thrive under conditions that

1 Sometimes called Schizophyceae or Myxophyceae. Schizophyceae means "split- ting algae"; Myxophyceae means "slime algae." These names are used by some botanists in preference to Cyanophyceae, which means "blue algae," because not all the members are blue-green. But it is also true that some of them lack the slippery feel. As long as we retain the names Chlorophyceae, Phaeophyceae, and Rhodo- phyceae for other algal groups, we might as well retain the name Cyanophyceae for the sake of uniformity, especially since some of the Chlorophyceae are not green, some of the Phaeophyceae are not brown, and some of the Rhodophyceae are not red. The things most desired in a name are that it shall express the most prominent feature of the group and that it shall be consistent with the names of coordinate groups.

THALLOPHYTA: ALGAE 9

would be fatal to almost all other forms of life. For this reason, because of their simplicity in cellular organization, and because they are auto- trophic, members of this group may have lived on the earth before con- ditions were favorable for the existence of other organisms, with the possible exception of bacteria. Their great antiquity is indicated by the presence, in rocks of Proterozoic age, of what seem to be fossil Cyano- phyceae, as well as numerous calcareous deposits resembling those made by blue-green algae now living in hot springs. There is more certain

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'• 'V.

D

Fig. 1. Some simple colonial blue-green algae. A, Gloeocapsa, X750; B, Merismopedia, X750; C, Nostoc, X 1,000; D, Oscillatoria, X600. Except in Oscillatoria, the cells are embedded in a mucilaginous matrix.

evidence of their existence in the Paleozoic era, particularly in the Cambrian, Silurian, and Devonian periods.

Plant Body. All the Cyanophyceae are unicellular and in nearly all of them the cells are organized to form colonies (Fig. 1). None has a truly multicellular body, although this condition is approached by the higher members of the group. In some species of Chroococcus the cells are solitary, while in Gloeocapsa they form small irregular colonies loosely held together in a gelatinous matrix. In Merismopedia the colonies are plate-hke, the cells being arranged in regular rows. In Coelosphaerium the colonies are globular and hollow, in Nostoc they resemble a string of beads, while in Oscillatoria they form a compact filament. The fila- mentous type of colony is most common.

10

PLANT MORPHOLOGY

Although ciUa are never present, many of the filamentous blue-green algae have the power of movement. If a mass of Oscillatoria growing on mud is placed in a flat dish, the filaments soon creep out in all direc- tions. Under the microscope the filaments are seen to shift frequently their position laterally in the water. In performing these movements, the cause of which is unknown, the cells of the colony function as a unit, thus approaching a condition characteristic of multicellular plants — a cooperation of cells in the performance of their functions.

Fig. 2. Longitudinal sections through cells of Anabaena circinalis, some of which are dividing, X 2,750. The nuclear material is in the form of irregular masses. The spherical bodies are cyanophycin granules and represent reserve food. {After Haupt.)

Cell Structure. The Cyanophyceae are characterized by a very primi- tive cell structure. A thin cell wall, composed of cellulose and pectic compounds, seems always to be present. Generally it becomes mucilag- inous and forms a matrix around the cell. The protoplast lacks the degree of organization seen in the higher plants. It consists of an outer colored portion, containing the blue and green pigments, and a central colorless portion. The latter, representing an incipient nucleus, con- tains a mass of scattered chromatin granules not surrounded by a mem- brane and without a nucleolus (Fig. 2). Plastids are not organized, the pigments being merely diffused throughout the peripheral region of the cell. Carbohydrate food is stored as glycogen, starch being absent. Reserve food often occurs also as minute oil droplets and as spherical bodies (cyanophycin granules) that are probably protein in nature.

THALLOPHYTA: ALGAE 11

These granules usually lie in the outer part of the cell, in many fila- mentous forms being commonly grouped along the cross walls.

Cell division is accomplished by a ring-like wall that develops from the outside toward the center, finally cutting the cell in half (Fig. 2). At the same time the chromatin separates into two approximately equal masses without the formation of chromosomes or other features of mitosis.

Reproduction. Because the Cyanophyceae are unicellular plants, cell division results in reproduction, a method called fission. The division of a cell to form two new individuals directly is the simplest method of reproduction in the plant kingdom. In most of the Cyanophyceae the cell walls break down to form abundant mucilage. Generally, as in Gloeocapsa, this holds together a group of cells derived from a single cell by repeated division, thus forming a colony (Fig. L4). Here the mucilage surrounding the cells is in concentric layers; but in many other genera it is in a continuous mass made up of the confluent sheaths of the individual cells.

In some of the filamentous types, such as Lynghya, a firm mucilaginous sheath is present around the whole colony, but in Oscillatoria, a related genus, the cell walls are more resistant and no sheath is formed (Fig. \D). In both genera the cells are compactly arranged in the colony, each cell, except the terminal one, being shortly cylindrical. That their shape results from mutual pressure is shown by the fact that the free surface of the end cell is convex. This is also true of cells adjacent to a dead cell in the filament.

The type of colony produced depends on the way in which the cells divide. In a filament all the divisions occur in one plane. Where the cells divide in two planes, a plate or a hollow sphere one layer thick is produced. Divisions in three planes usually result in a somewhat massive type of colony.

In most of the filamentous forms, with the exception of Oscillatoria and its relatives, differentiated cells, called heterocysts, appear in the colony. They may be seen in such common genera as Nostoc and Anabaena (Fig. IC). A heterocyst is an enlarged vegetative cell that becomes thick- walled and transparent. Heterocysts usually occur singly but at rather frequent intervals, thus dividing the filament into segments called hormogonia. These become detached and move away from one another to form new colonies. A hormogonium is merely an isolated portion of the original filament. In Oscillatoria and related genera hormogonia are formed by the death of unmodified cells here and there in the colony (Fig. ID).

Although none of the Cyanophyceae produces zoospores or gametes, most of the filamentous members form nonmotile resting spores. One

12

PLANT MORPHOLOGY

kind, called an akinete, arises from a v^egetative cell that enlarges by the accumulation of food and develops a thick cell wall (Fig. 3A, B). These cells are very resistant to unfavorable conditions. Akinetes may be separated in the filament or several may occur together. In some forms they always appear next to a heterocyst, either at the end or in the middle of the filament. Another kind of resting cell, called an endospore, is developed in some genera, as in Chamae siphon and in the marine genus, Dermocarpa (Fig. 3C). Endospores are small thick- walled spores that

B C

Fig. 3. Formation of resting spores in the blue-green algae. A, portion of filament of Anabaena with a heterocyst and an akinete containing many food granules, X750; B, Gloeotrichia, showing a young filament and two stages in the development of an akinete, X500; C, Dermocarpa, an epiphytic form, with two empty cells and others containing endospores. (C, aftei- Bornet and Thuret.)

arise from a protoplast by divisions within the cell cavity and from which they later are liberated.

Rivularia is a filamentous form in which the basal cell of a filament is always a heterocyst, while the other cells become gradually smaller toward the very slender apex. A thick mucilaginous sheath, confined to the base of the filament, begins next to the heterocyst. Gloeotrichia is similar to Rivularia except that the first basal vegetative cell becomes transformed into an elongated akinete (Fig. 35).

Branching. In some of the filamentous members branching occurs (Fig. 4). In Tolypothrix the filaments exhibit "false branching." Here the cells on one side of a heterocyst grow out beyond it to form a branch. In Scytonema the false branches arise laterally in pairs but

THALLOPHYTA: ALGAE

13

usually not in connection with a heterocyst. In Stigonema a true branch arises as a lateral outgrowth from a single cell of the filament. This type of branching is rare in the Cyanophyceae, occurring in only a few genera. Summary. The Cyanophyceae are an ancient group of plants showing an extremely primitive condition of structural organization. In addition to chlorophyll and carotinoids, a blue pigment (phycocyanin) is present and often a red pigment (phycoerythrin) as well. The plant body is unicellular, the cells nearly always forming colonies. The cell wall is

A B C

Fig. 4. False branching in Tolypothrix (A) and Scytonema (B), and true branching in Stigonema (C) ; A, X750; B, X200; C, X300.

more or less unstable, usually producing abundant mucilage. The protoplast shows little organization. The pigments forming the charac- teristic blue-green color are diffused throughout the peripheral part of the protoplast, no plastids being present. Reserve carbohydrate food is stored as glycogen. A nucleus is represented only by scattered chromatin granules, there being no nuclear membrane or nucleolus. Reproduction occurs by fission and by nonmotile spores. It is entirely asexual. Ciliated cells are never produced. The resting ceUs (akinetes) are merely enlarged protoplasts with a thick wall. There is a tendency toward cellular differentiation, in some forms resulting in the establish- ment of a distinct apex and base. The relationships of the Cyanophyceae to the other algae are obscure. They appear to be closely related to the bacteria. In fact, the blue-green algae and bacteria are sometimes placed in the same group, the Schizophyta, and made an independent class of thallophytes.

14

PLANT MORPHOLOGY

2. EUGLENOPHYCEAE

Flagellates are unicellular organisms combining characters of both plants and animals. Zoologists regard them as one-celled animals, while botanists consider at least those with chlorophyll as plants, as well as certain colorless ones evidently derived from them. Nearly all flagellates are solitary and free-swimming, but some form loose gelatinous

colonies and some are attached. Most of them live in fresh or salt water, some occur on damp earth, and some are parasitic. Formerly they were placed in a separate group, the Flagellatae, but they show so many differences among themselves that they are now distributed, so far as possible, into other groups. One of these, the Euglenophyceae, includes about 350 species of green or colorless, mainly fresh-water flagellates. The best-known genus, Euglena, is widely distributed and common in stagnant pools and ditches, often occurring in such abundance as to color the water a deep green.

Cell Structure. Euglena is somewhat pear-shaped, being blunt at its anterior end and gradually tapering behind (Fig. 5). As in other flagellates, there is no cell wall, each cell consisting of a naked protoplast. In Euglena the outer part of the protoplast is differ- entiated into a thin pellicle that is somewhat firm but flexible enough to permit the cefl to undergo frequent changes in shape. In some flagellates the pellicle is more rigid, giving the cell a constant form, while in others it is wanting. Some flagellates that lack a pellicle are amoeboid, putting out slender pseudopodia. Flagellates are characterized by having, in the vege- tative condition, one or two (rarely more) cilia — slender protoplasmic threads that lash back and forth in the water. Long whip- like ciHa are called flageUa,! the possession of which gives the flagellates their name. The flagella are generally borne at the anterior end of the cell and, where two are present, they are either equal or unequal in length.

Euglena has a single flagellum attached anteriorly. Near its base is a conspicuous red eyespot, which is thought to be sensitive to fight. Although avoiding direct sunlight, the organism tends to swim toward the best-illuminated part of the water. At the anterior end of the cell

1 If a distinction is to be made between cilia and flagella, the latter are not only longer than the cell that bears them but coarser and fewer in number. Cilium means "eyelash"; flagellum means "whip."

Fig. 5. Euglena viridis, X750. The cell contains a large nucleus and a num- ber of chloroplasts. At the anterior end is a long flagellum, a narrow gullet lead- ing to the reservoir, a contractile vacu- ole, and an eyespot.

THALLOPHYTA: ALGAE 15

is a short narrow tube, called the gullet, that leads to a spherical cavity, the reservoir. The flagelliim is inserted inside the reservoir and projects through the gullet. Near the reservoir is a contractile vacuole (more than one in some species), which alternately contracts and expands. The contractile vacuole discharges its contents into the reservoir. Con- tractile vacuoles, usually regarded as organs of excretion, are found in many one-celled animals, as well as in the motile cells of many algae. Flagellates show an advance over the blue-green algae in having a definite nucleus. Moreover, their photosynthetic pigments, where present, are always confined to definite plastids. Euglena has many small green plastids (chloroplasts) , but some flagellates, not belonging to the Euglenophyceae, have yellow or brown plastids. All these colored forms carry on photosynthesis. Other flagellates, some belonging to the Euglenophyceae, are colorless and live either as saprophytes, absorbing organic matter in solution through the plasma membrane, or as animals, ingesting solid particles of food either through the gullet or by means of pseudopodia. A few flagellates are parasitic on animals, one of these, Trypanosoma, causing a disease of man known as African sleeping sickness. Some forms with chlorophyll ingest solid food particles through the gullet. Some species of Euglena carry on photosynthesis in the light, but, if kept in darkness and supplied with organic matter in solution, become colorless and saprophytic. In all the Euglenophyceae food is stored as paramylon, a starch-like carbohydrate, and often as oil. The presence of paramylon granules is very characteristic, even in colorless cells. True starch is not formed. Flagellates belonging to other groups of algae differ with respect to the type of food stored.

Reproduction. As in all flagellates, reproduction in Euglena occurs by fission, the cell dividing longitudinally. In the presence of unfavor- able conditions, encystment often occurs. The protoplasm retracts the flagellum, rounds up, secretes a thick gelatinous covering about itself, and goes into a resting stage. Although later the cyst usually produces a single motile protoplast, sometimes it divides internally into a number of smaller protoplasts that escape, develop flagella, and grow to mature size. Sexual reproduction in the Euglenophyceae is of doubtful occurrence.

Colacium, a member of the Euglenophyceae, is interesting in being an attached form lacking flagella in the vegetative condition. Its cells are surrounded by a gelatinous wall and are united into small irregular colonies. When reproduction occurs, the cell contents escape as a naked euglenoid protoplast with a flagellum.

Relationships. Flagellates are related on the one hand to various groups of algae and on the other hand to the Protozoa. The fact that they are intermediate between plants and animals strongly suggests

16 PLANT MORPHOLOGY

that the earliest forms of life may have been similarly undifferentiated and that from such a common ancestry both plants and animals may have arisen. Although the relationship of the Cyanophyceae to the flagellates, if any, is very obscure, most of the higher algal groups are thought to have been derived from flagellate ancestors. Where intermediate forms occur, such a derivation seems almost certain.

Summary. The Eugienophyceae are a group of flagellates with bright green chloroplasts containing only chlorophyll and its associated carotinoids, the chlorophyll predominating, as in the green algae. Some members are colorless, these being either saprophytic or animal-like in their nutrition. All are unicellular and uninucleate, the cells being solitary or rarely in colonies. The cells have one or two cilia (flagella) that may be equal or unequal but are always attached anteriorly. Except in colonial forms, the cells are motile and lack a cell wall. Reserve food is stored as paramylon and often also as oil. Reproduction occurs by longitudinal fission. Resting cells (cysts) are commonly formed. Sexual reproduction is doubtful.

3. CHRYSOPHYCEAE

The Chrysophyceae, or golden-brown algae, are a small group number- ing only about 200 species and occurring mainly in fresh water. Their plastids contain chlorophyll and an excess of yellow and brown carotinoid pigments, giving them a golden-brown color. Most members are flagel- lates, being unicellular motile forms without a cell wall. The cells are solitary or in colonies and may be either free-swimming or attached. Motile cells have one or two, rarely three, cilia (flagella) attached anteri- orly. The two cilia may be equal or unequal in length. A few forms have a cell wall and are either filamentous or palmelloid, the latter with cells loosely held together in a gelatinous matrix. All members are uninucleate. Food is stored as oil or as leucosin, which is a protein-like substance of unknown composition. Some forms are colorless, while a few, with chlorophyll, may ingest soUd food. Reproduction occurs by fission, mainly longitudinal. A characteristic feature is the occurrence of cysts with a silicified cell wall having a small plug at one end. Zoospores may be produced in members that are not flagellates. Sexual repro- duction is of doubtful occurrence.

Chromulina is a motile unicellular form (Fig. 6). Chrysamoeba is amoeboid. Symira and Uroglena are globular free-swimming colonies. Dinobryon has species in which the cells form a dendroid colony. Hydru- rus and Phaeocystis are palmelloid forms. Phaeothamnion is a branched filament and represents the highest type of organization attained by the group.

THALLOFHYTA: ALGAE

17

Fig. 6. Group of golden-brown algae. A, Chromulina ovalis, X 1,450; B, Chrysamoeba radians, X960; C, Synura uvella, X600; D, Dinohryon sertularia, X900; E, Hydrurus foetidus, X480; F, Phaeothamnion confervicolum, X-440. (A and B, after Klebs; C, after Stein; D, after Senn; E, after Berthold; F, after G. M. Smith.)

Another group composed almost entirely of flagellates are the Crypto- phyceae, yellow-green and brown forms that store food as starch or a related substance. Motile cells have two unecjual cilia (flagella). Reproduction occurs by fission. Sexual reproduction has been reported in only one species. The Cryptophyceae comprise only 30 species, mostly occurring in fresh water, and are rarely seen. In some respects they resemble the next class.

18

PLANT MORPHOLOGY

4. DINOPHYCEAE

The Dinophyceae comprise a group of nearly 1,000 species of organisms, most of which are known as dinofiagellates. Although some occur in fresh water, most of them are free-swimming marine forms. A few have naked protoplasts, but nearly all have sculptured walls of cellulose usually

composed of a definite number of jointed plates (Fig. 7). All the dinofiagellates are unicellular and most of them are solitary ; some occur in chain-like colonies. The cells are small and generally have a pair of laterally attached cilia (flagella). A characteristic feature is the occurrence of two grooves, one encircling the cell trans- versely and the other extending longi- tudinally along one side. The cilia arise at the point of intersection of the grooves. One lies in the transverse groove and the other is directed backward.

The dinofiagellates have a definite nu- cleus and usually a number of brownish yellow plastids, in which there is a predom- inance of carotinoids over the chlorophyll. Some are colorless. The colorless forms live either as saprophytes or as animals, the latter ingesting solid food particles. Some are parasitic. Reserve food occurs either as starch or as oil. Many of the dinofiagellates are phosphorescent. The prevailing method of reproduction is by fission, but some members produce zoo- spores. As in the other flagellates, cysts are often formed. Sexual repro- duction has been reported in only one member of the class.

In addition to the dinofiagellates, the Dinophyceae include a few forms with a higher type of cellular organization, such as Gloeodinium, a palmel- loid form, and Dinothrix and Dinodadium, both of which are filamentous.

Fig. 7. Ceratium hirudinella, a fresh-water dinoflagellate, X400.

5. XANTHOPHYCEAE

The Xanthophyceae,! or yellow-green algae, are a small but distinct group of only about 200 species characterized by having an excess of

1 Also called Heterocontae.

THALLOPHYTA: ALGAE 19

yellow pigments, especially carotin, in their plastids. Xanthophijll and carotin are the two carotinoid pigments associated with chlorophyll in other green plants, but here their proportions are different. Although a few are marine, most yellow-green algae are found in fresh water. They are either unicellular or multicellular. Many are flagellates. The group was formerly classified with the Chlorophyceae, but seems to have had an independent origin from a flagellate ancestry and to have fol- lowed a line of evolution parallel to that of the green algae. Three rep- resentative genera are Chlorochromonas, Tribonema, and Botrydium.

Chlorochromonas. This is a naked unicellular flagellate with two yel- low-green plastids (Fig. 8). It has two cilia (flagella) of unequal length attached anteriorly, a contractile vacuole, and a cen- .

tral nucleus. It stores food as leucosin and probably also as oil. A leucosin granule, contained in a vacuole, lies at the posterior end of the cell. Reproduction takes place by fission. From such a form as Chloro- ckromonas, the other Xanthophyceae appear to have evolved.

Tribonema. This is a filamentous alga, widely distributed in fresh-water pools (Fig. 9). The fila- ments are unbranched and composed of elongated cylindrical cells. The walls are made up of two over- lapping pieces that appear H-shaped in a longitudinal section. The cells contain a nucleus and a number of chromonas minuta, yellow-green plastids. Asexual reproduction occurs x 2,000. {After by the formation of aplanospores, akinetes, or zoo- spores. Aplanospores are nonmotile spores with a wall distinct from the wall of the parent cell. Akinetes are also nonmotile but are derived from an entire vegetative cell whose wall becomes the wall of the spore. Zoo- spores are ciliated and naked. In Tribonema one or more aplanospores may be produced within a cell, while the zoospores are usually formed singly. Sexual reproduction, which is rare, takes place by the fusion of isogametes formed in ordinary cells. Usually one gamete settles down before the other unites with it. The motile cells have two cilia of unequal length, attached anteriorly, and the reserve food is stored as oil or leu- cosin, never as starch.

Botrydium. Botrydium is a terrestrial alga often found on wet muddy flats. The vegetative body is unicellular and multinucleate, consisting of a balloon-shaped bladder about 1 to 2 mm. in diameter (Fig. 10). It is fastened to the soil by means of branched colorless rhizoids. The cytoplasm, containing many nuclei and, in the aerial portion, numerous yellow-green plastids, forms a thin layer lining the cell wall and enclosing

20

PLANT MORPHOLOGY

a large central vacuole. Such a multinucleate body, without any cross walls, is called a coenocyte. Food is stored as oil or leucosin.

Asexual reproduction may occur either by zoospores or aplanospores. When covered with water, the entire aerial portion may release numerous uninucleate zoospores through a terminal pore. The zoospores have two

D

Fig. 9. Tribonema homhycinum, a yellow-green alga. A, portion of vegetative filament; B, aplan- ospores; C, zoospores; D, structure of cell wall, as revealed by special treatment. {A, B, C, after Gay; D, after Bohlin.)

Fig. 10. Botrydium granulatum, showing balloon-shaped aerial portion and branched subterranean portion, X20.

cilia of unequal length, attached anteriorly. They may either germinate immediately or form a wall and go into a resting stage. In the absence of sufficient moisture, the aerial portion may give rise to aplanospores or all the cytoplasm may move into the rhizoidal portion and there produce aplanospores. The aplanospores of Botrydium may be either uninucleate or multinucleate and, after a dormant period, may give rise either to zoo- spores or to new plants directly. Sexual reproduction is accomplished by small biciliate isogametes, each with a single nucleus, that fuse to

THAU/)PHVTA: ALGAE

21

form thick-walled zygotes. These germinate immediately, giving rise directly to a new vegetative body. Sometimes the gametes conjugate before being liberated.

6. BACILLARIOPHYCEAE

The Bacillariophyceae,* or diatoms, constitute an isolated group whose relationships to the other algae are very uncertain. They include over 5,000 species of unicellular plants occurring almost universally in fresh and salt water, as well as on damp soil. Some of the more common genera are Melosira, Coscinodiscus, Bid- dulphia, Pinnularia, Surirella, Cocconeis, Navicula, and Pleurosigma. Diatoms may be either free-floating or attached. Frequently they form slimy brown coat- ings on mud at the bottom of shallow bodies of water, as well as on sticks, stones, shells, other aquatic plants, etc. That they were more numerous in geologic times is shown by the great accumula- tions of diatomaceous earth found in var- ious parts of the world. This consists of the shells (cell walls) of dead diatoms. Deposits of diatomaceous earth were formed mainly during the Tertiary, but the fossil record of diatoms extends as far back as the Jurassic.

Although most diatoms are solitary, some form colonies of diverse types, the individuals being held together by a sheath of mucilage. Their color, usually a golden brown, is due to the presence of chlorophyll in association with an excess of carotinoids, particularly carotin and several brown xanthophyll pigments. Diatoms are distinguished from other algae by their silicified cell wall. This consists of two valves, one overlapping the other like the lid and bottom of a pillbox (Fig. 11). The place where the valves overlap is called the girdle. The cell wall is composed mainly of pectin impregnated with a large amount of silica. It is variously marked with numerous fine transverse lines that form regular and elaborate patterns. These make diatoms among the most striking and beautiful objects to be seen under the

1 Also called Diatomeae.

A B

Fig. 11. Two views of the shell of Pinnularia viridis. A, girdle view; B, valve view; g, girdle; pn, polar nodule, en, central nodule; r, raphe. {After Pfitzer.)

22

PLANT MORPHOLOGY

Fig. 12. Group of common diatoms. A, Triceratium; B, Aitlacodiscits; C, Isthmia; D, Sitrirella, E, Navicula; F, Amphipleura; G, Pleurosigma. {Adapted from a Turtox classroom chart.)

THALLOPHYTA: ALGAE

23

microscope. A good microscope will show that the striations on the silici- fied cell wall generally consist of rows of very minute pores. They appear as lines because the pores are very close together.^

Two views of a diatom are possible — girdle (side) view and valve (top) view (Fig. 11). Many diatoms possess a raphe, which is a longitu- dinal slit extending down the center of the valve. Such forms have the power of locomotion, movement apparently being accomplished by a streaming of protoplasm along the raphe.

The Bacillariophyceae comprise two orders: (1) the Centrales, which are radially symmetrical in valve view, often circular, and have no raphe

Fig. 13. Triceratium, a large marine diatom, as seen in optical section, X400. The nucleus lies in the center of the cell, while numerous small plastids lie just inside the cell wall.

(Fig. 12A-C) and (2) the Pennales, which are usually bilaterally sym- metrical, not circular, and generally have a raphe (Fig. \2D-G). The difference in symmetry is clearly shown by the pattern of markings on the valves, being radial in the Centrales and bilateral in the Pennales.

In most diatoms the nucleus is suspended in the center of the cell by slender strands or by a broad transverse band of cytoplasm connected with a thin laj^er lying next to the cell wall (Figs. 13 and 14/1). Embed- ded in the peripheral layer are one or more plastids that are usually brown, frequently yellow, or rarely green. In the Centrales the plastids are small and numerous. In the Pennales they are large and few in number; commonly there are two. The plastids of diatoms vary greatly in shape, being often irregular and sometimes elaborately lobed. The cell contains no starch, food being stored mainly as oil.

1 Some diatoms have striae so fine that they are used as test objects in determining the efficiency of microscope lenses. A good oil immersion objective will resolve mark- ings that are as fine as five striae to the micron.

24

PLANT MORPHOLOGY

Reproduction occurs chiefly by fission, the cell always dividing in the plane of the valves (Fig. 14). The two valves separate and each daugh- ter protoplast forms a new wall on its naked side, the new wall fitting inside the old one. One of the cells is always as large as the parent cell,

B

Fig. 14. Cell division in Surirella calcarata, X275. Laiderborn.)

A to D, successive stages. {After

but the other is smaller. Thus, as cell divisions continue, some of the individuals become constantly smaller. After a minimum size for the species has been reached, the original size is regained through the forma- tion of auxospores.

In the Centrales an auxospore is formed by the escape from its cell wall of a protoplast that soon grows to the original size and develops a new cell wall. An auxospore may directly become a new individual or may form two new individuals by dividing in half. In most of the

THALLOPHYTA: AWAE

25

Pennales auxospore formation is due to a fusion of cells. In some forms two vegetative protoplasts escape and conjugate to produce a single auxospore (Fig. 15). In other forms two diatoms unite to produce two auxospores. Here the two fusing cells lie within a gelatinous matrix and each produces two gametes. Then each of the gametes derived from one cell conjugates with one of those derived from the other cell. It is apparent that an "auxospore" formed by sexual fusion is really a zygote.

Fig. 15. Conjugation in Cocconeis placentula, X 1,500. A and B, meiosis in conjugating cells; C, fusion of protoplasts; D, zygote with sexual nuclei not yet fused. (After Geitler.)

Just previous to conjugation the nucleus of each of the pairing proto- plasts undergoes a reduction of chromosomes, giving rise to four haploid nuclei. Some of these degenerate.

In some of the Centrales many small biciliate protoplasts arise within a vegetative cell and later escape into the water. These have been called "microspores." Some observers think that they function as zoo- spores, while others regard them as gametes, claiming that they fuse in pairs. The occurrence of these ciliated cells in the Bacillariophyceae suggests that the group may have been derived from flagellates with brown plastids. The connection, however, is a remote one.

7. CHLOROPHYCEAE

The Chlorophyceae, or green algae, are predominantly fresh-water forms whose plastids contain a preponderance of chlorophyll over its associated carotinoids, the green and yellow pigments occurring in approximately the same proportions as in the groups above the thal- lophyte level. ^ Onlj^ a comparatively few members are marine, but

' In a few members accessory pigments in the cell sap may mask the green color of the chloroplasts.

26 PLANT MORPHOLOGY

some of these are widely distributed and often abundant. Some green algae grow as scums on the surface of quiet water, while others are attached to various objects beneath the surface. A few forms grow on moist soil, rocks, or tree trunks. Most of the green algae are multicellular but some are unicellular, these occurring either as isolated cells or as colonies. The Chlorophyceae are generally regarded as the group of algae from which the bryophytes and other higher groups of green plants have been derived. Lime-secreting forms are known as fossils as far back as the Ordovician. The Chlorophyceae number over 5,000 species, nearly all of which are included in seven principal orders: Volvocales, Chlorococcales, Ulotrichales, Oedogoniales, Conjugales, Siphonocla- diales, and Siphonales.

1. Volvocales

The Volvocales are a distinct group of primitive green algae that are widely distributed in fresh water. Only a few members are marine. They appear to have been derived from green flagellates, which they resemble in many ways, and to have given rise, in turn, to the other groups of Chlorophyceae. The Volvocales include about 50 genera and 300 species. The main genera are Chlamydomonas, Sphaerella, Gonium, Pandorina, Eudorina, and Volvox.

Chlamydomonas. This is a unicellular alga that does not form per- manent colonies. It is widely distributed in pools and ditches and on damp ground. The vegetative cell, which is free-swimming, is generally spherical or egg-shaped (Fig. 16A). A cell wall is always present. At the anterior end are a pair of cilia, equal in length, a red eyespot, and two (rarely more) small contractile vacuoles. Surrounding the nucleus is a small m.ass of colorless cytoplasm lying in the depression of a large cup-shaped chloroplast. Embedded in the chloroplast is a conspicuous spherical pyrenoid. Pyrenoids are protein bodies that function as centers of starch formation. Although occurring in some members of certain other algal groups, they are especially characteristic of the Chlorophyceae.

Chlamydomonas reproduces asexually by means of zoospores. The vegetative cell becomes quiescent by retraction of the ciha and then its protoplast divides internally to form two, four, or eight daughter proto- plasts, each of which, after enlarging slightly, forms a new cell wall and a pair of cilia while within the parent cell (Fig. 165, C). By the breaking down of the original cell wall, the small cells are set free as zoospores, each soon undergoing further enlargement to become an adult vegeta- tive cell (Fig. 16-D). Under conditions unfavorable for vegetative activ- ity, Chlamydomonas may pass into a "palmella" stage. The daughter cells, produced by the internal division of a vegetative cell, increase in number but, instead of escaping, become surrounded by abundant muci-

THALLOPHYTA: ALGAE

27

lage derived from the cell walls (Fig. XQE). Later, when favorable con- ditions return, the cells develop cilia and swim out of the mucilaginous matrix.

Sexual reproduction in Chlamydomonas occurs by the union of similar gametes. These arise from a (luiescent vegetative cell by division of its protoplast into 16 or 32 daughter protoplasts (Fig. 16F). The gametes are smaller than the zoospores and are usually without a cell wall, but

Fig. 16. Chlamydomonas, a free-swimming, unicellular green alga, X 1,000. A, vegetative cell, showing large cup-like chloroplast with embedded pyrenoid, nucleus, eyespot, two contractile vacuoles, and two cilia; B and C, formation of zoospores within parent cell wall; D, two escaped zoospores; E, "palmella" stage; F, formation of gametes; G, two escaped gametes; H, gametes fusing; /, zygote; /, four zoospores escaping from zygote.

otherwise have the same structural features. They escape and swim about in the water. Finally, they come together in pairs and fuse, each pair forming a zygote (Fig. 16G, H). The zygote soon loses its cilia, secretes a heavy wall about itself, and goes into a resting stage (Fig. 16/). While the wall is forming, the two nuclei inside the zygote unite. Upon germination, the protoplast of the zygote divides internally to form four zoospores that escape and enlarge to become new vegetative cells (Fig. 16J). The reduction in chromosome number from the diploid to the haploid state occurs in connection with the formation of the four zoo- spores from the zygote. Because, in most species, the pairing gametes are alike in size, Chlamydomonas is said to be isogamous. The fusing of similar gametes (isogametes) is known as conjugation.

In Chlamydomonas eugametos there are two sexually differentiated

28

I'LANT MORPHOLOGY

strains, designated as plus and minus. A zygote may be formed only by the union of a plus gamete with a minus gamete. Of the four zoo- spores arising from the zygote, two belong to the plus strain and two to the minus. These strains may soon undergo another sexual fusion or may be perpetuated asexually for an indefinite period. In Chlamy- domonas hraunii and a few other species the gametes of the plus strain are slightly larger than those of the minus strain, and so here a visible as well as a physiological sexual differentiation exists. Such species show that Chlamijdomonas displays a slight tendency toward heterogamy.

A ' B

Pig. 17. Two species of Gonium, X900. A, side and top views of four-celled colony of Gonium sociale; B, top view of sixteen-celled colony of Gonium pectorale.

In Sphaerella, a close relative of Chlamijdomonas and common in rain- w^ater pools, the inner portion of the cell wall is gelatinous and thick and is traversed by many delicate cytoplasmic strands. Generally it con- tains a bright red pigment, haematochrome , that masks the chlorophyll. This is present in the cell sap.

Gonium. This is a colonial form, each colony consisting of a flat plate of cells numbering either four or sixteen, according to the species (Fig. 17). The cells are regularly arranged and held together by a mucilaginous matrix derived from their cell walls. Each cell is biciliate and otherwise similar to an adult Chlamijdomonas. By division of its protoplast, any cell may form a new colony that escapes from the parent cell. Sexual reproduction occurs by the fusion of similar gametes (isogametes), the two coming from separate colonies. The number of gametes formed in a cell corresponds to the number of cells in the colony. They escape separately. The zygote becomes thick-walled and dormant. Later it produces four biciliate zoospores. In the four-celled species these usu- ally remain together as a colony; in the sixteen-celled species they separate and each forms a new colony.

Pandorina. This form is similar to Gonium except that the colony is spherical or nearly so and consists usually of 16 biciliate cells crowded

THALLOPHYTA: ALGAE

29

together within a mucilaginous matrix and surrounding a small central cavity (Fig. 18A). Sometimes the colony consists of only 8 cells or, less frequently, of 32 cells. Each cell resembles that of Chlamydomonas. In asexual reproduction each cell divides simultaneously to produce a group of as many daughter cells as were in the parent colony (Fig. ISB). Each group then escapes as a new colony. In sexual reproduction each vegeta- tive cell similarly produces a group of daughter cells as numerous as the cells in the colony, the groups separate, and the daughter cells escape individually as biciliate gametes. Although Pandorina is isogamous, one

Fig. 18. Pandorina morum, X750. A, free-swimming vegetative colony of 16 cells, those lying below not shown; B, colony undergoing ase.xual reproduction; C, a large and a small gamete; D, gametic union; E, a zygote.

of the fusing gametes is slightly larger and less active than the other, thus showing a tendency toward heterogamy (Fig. 18C, D). The zygote remains motile for a while, finally settling down and secreting a cell wall (Fig. 18E). Upon germination the zygote divides internally into four protoplasts, but generally only one becomes a zoospore. The zoospore produces a new colony.

Eudorina. Eudorina is a spherical colony usually consisting of 16, 32, or 64 biciliate cells, each like a cell of Chlamydo?nonas. The cells are loosely arranged in a single layer near the surface of a mucilaginous matrix. As in the preceding genera, any cell may give rise to a new colony by internal division of its protoplast, but an advance is seen in sexual reproduction (Fig. 19). Some of the cells divide to form groups of sperms, as many as 64 usually arising from a single vegetative cell. The other cells enlarge slightly by the accumulation of food and become eggs. Although both male and female gametes are biciliate, only the sperms escape from the colony and become free-swimming. At first the

30

PLANT MORPHOLOGY

sperms hang together as a plate, but finally separate and fuse with the eggs. The union of a sperm and egg, called fertilization, results in the formation of a thick-walled resting zygote. Upon germination, the zygote produces four biciliate zoospores but only one functions, the other three degenerating inside the zygote. Because the pairing gametes are differentiated into sperms and eggs and are therefore unlike, Eudorina

Fig. 19. Eudorina elcgans. A, colonj' of 32 cells, many of which are dividing to form daughter colonies, X 500; B, a female colony surrounded by numerous sperms, two groups of which are still intact, while others, having separated, are uniting with the eggs. (After Gocbel.)

is heterogamous. Some species show a further degree of sexual differentia- tion in being dioecious. Here all the cells in the male colony give rise to sperms, while all those in the female colony become eggs.

Volvox. This is the most highly developed member of the Volvocales. It lives in quiet bodies of fresh water, especially pools, ponds, and lakes. It consists of a hollow globular colony composed of hundreds or some- times thousands of biciliate cells embedded in mucilage and arranged in a single layer (Fig. 20A). Often the colony reaches a diameter of nearly 2 mm. It is free-swimming, as in other members of the order. Each cell is like an adult Chlamijdomonas, with two cilia, an eyespot, contractile vacuoles, a nucleus, and a single chloroplast with a pyrenoid. In most species the cells are connected by very fine protoplasmic strands, and thus the colony approaches the multicellular condition of organiza- tion. This is also shown by the fact that most of the cells function only vegetatively during the entire life of the colony, while others become

THALLOPHYTA: ALGAE

31

reproductive cells. Such a "division of labor" is not seen in lower mem- bers of the order.

Volvox reproduces asexually by the formation of new colonies inside the old one. A few of the vegetative cells, seldom over 10 or 12, retract

B

Fig. 20. Volvox. A, mature colony with young colonies inside; B, j'oung colony in rim of mature colony; at the right, a vegetative cell has lost its cilia and is starting to form a new colony; C, a group of sperms derived from a single vegetative cell, one of which, to the left, has lost its cilia and is enlarging; D, an egg shortly before fertilization and, to the left, an egg beginning to develop from a vegetative cell; E, a mature zygote; A, X170; B to E, X780. {After Chamberlain.)

their cilia and increase slightly in size. Each divides to form a small group of cells that enter the colony and give rise to a new colony, remain- ing inside until the old colony dies (Fig. 20B).

In sexual reproduction, Volvox is heterogamous. Any cell may retract its cilia, enlarge by the accumulation of food, and become an egg (Fig. 20D). Another cell may enlarge and, at the same time, divide to form many small biciliate sperms (Fig. 20C). These arise as a hollow sphere or plate of cells that later separate. The sperms and egg? escape into

32 PLANT MORPHOLOGY

the colony and there fertilization occurs. The zygote becomes heavy- walled and remains dormant for several months (Fig. 2()E). In some species it then gives rise to a single biciliate zoospore, while in others it forms a new colony directly. In connection with the germination of the zygote, the number of chromosomes is reduced one-half. As is Eudorina, some species of Volvox are monoecious, others dioecious.

Summary. The Volvocales are distinguished from the other Chlo- rophyceae by the fact that their vegetative cells are cihated and motile. They exhibit a range of development from single isolated cells to com- plex globular colonies. Each cell has one nucleus and generally one chloroplast. Asexual reproduction occurs by zoospores and by the formation of a new colony from a single parent cell. The number of cells in the colony is definite and is determined during early development. It is not subsequently increased by vegetative cell divisions. Sexual reproduction shows an advance from isogamy to heterogamy, while dioecism is attained by some species of Eudorina and Volvox.

2. Chlorococcales

The Chlorococcales constitute a large order of diverse forms that are probably not closely related. They are chiefly fresh- water algae, only a few occurring in the ocean. Some live in moist places on land. Some are endophytic in the intercellular spaces of certain seed plants, while others live symbiotically in the lower animals. Others are lichen formers. The order contains 90 genera and approximately 700 species. Some characteristic genera are Chlorococcum, ChloreUa, Scenedesmus, Pedias- trum, Hydrodictyon, and Protosiphon.

Chlorococcum. This simple alga grows on damp soil or rocks. It is unicellular, spherical, and nonmotile. At first it has a single nucleus and a large cup-hke chloroplast with one or more pyrenoids (Fig. 21). Later the cell becomes multinucleate and the protoplast divides to form a variable number of bicihate zoospores that escape. After coming to rest, a zoospore loses its ciha, secretes a wall, and becomes a vegetative cell. Asexual reproduction may also occur by aplanospores. These arise in the same way as zoospores but have no ciha and develop a cell wah before being freed. As in Volvox, a "palmella" stage may develop by gelatinization of the cell walls in a group of cells. Sexual reproduction is accomplished by the production of a large number of bicihate isoga- metes by a vegetative cell. These escape and fuse in pairs. In general, Chlorococcum is like Chlamydomonas except that the vegetative cells have lost their motility.

ChloreUa. ChloreUa lives on the bark of trees, damp walls, and soil; also in various infusoria, the fresh-water sponge, and the green hydra. It can be grown easily in water cultures and is much used in experiments

THALLOPHYTA: ALGAE

33

on photosynthesis. The cells are spherical and solitary. They have a single nucleus and a cup-shaped peripheral chloroplast usually without a pyrenoid. Chlorella resembles Chlorococcum except that it produces only aplanospores, no motile cells of any kind. A protoplast divides to form as many as 16 daughter protoplasts, each of which, before escaping, secretes a cell wall. Gametes are unknown.

Fig. 21. Chlorococcum infusionum. A, section of vegetative cell with single nucleus and pyrenoid; B, multinucleate stage; C, appearance of cleavage furrows, isolating uninucleate protoplasts with a pyrenoid fragment; D, section of nearly mature sporangium; E, escape of zoospores in a gelatinous vesicle; F, two zoospores; A, X 2,000; others, X 2,700. {After Bold.)

Scenedesmus. This alga is common and widely distributed in fresh water. It is a colonial form with generally four or eight cells arranged in a short row (Fig. 22). The end cells often bear conspicuous spine- like projections. Each cell contains a single nucleus, a large peripheral chloroplast, and a pyrenoid. In reproduction, a protoplast divides within its own cell wall to form a new colony that escapes as a whole. Neither zoospores nor gametes are produced.

Pediastrum. Pediastrum is a free-floating form widely distributed in fresh water. It consists of a colony of cells symmetrically arranged in a flat plate (Fig. 23). The number of cells may be 2, 4, 8, etc., up to 128, but is most commonly 16 or 32. The cells are nearly all alike, except that the peripheral ones often bear short spine-like projections.

34

PLANT MORPHOLOGY

In young colonies the cells are uninucleate but later become multi- nucleate (coenocytic), as many as eight nuclei being present. Young cells have a single peripheral chloroplast with one pyrenoid, while older cells have several pyrenoids, the chloroplast becoming diffuse.

In asexual reproduction a protoplast divides generally into as many daughter protoplasts as there are cells in the colony, but often into twice as many. These become biciliate zoospores that escape as a group enclosed in a common membrane (Fig. 24A). The zoospores then come together and form a new colony within the membrane (Fig. 24i?, C).

Fig. 22. Four-celled colony of Scenedesmus, X 750. Each cell contains a small nucleus and a large peripheral chloro- plast with a pyrenoid.

Fig. 23. Young colony of Pediastrum horyanum, its cells forming a plate, X 750. Some of the cells have become binucleate. Each has a peripheral chloroplast and a pyrenoid.

Sexual reproduction also takes place, Pediastrum being isogamous. Division of a vegetative protoplast results in the formation of many biciliate gametes. These escape separately and fuse in pairs to form zygotes. After increasing in size, the zygote gives rise to a group of zoospores. These escape into the water, swim freely and, after coming to rest, develop into thick- walled polyhedral cells (Fig. 24Z)). The polyhedrons enlarge and divide internally to form a group of zoospores that escape in a common membrane, within which they construct a new colony by coming together without further division (Fig. 2^E-G). Hydrodictyon. This remarkable alga, common in fresh water, is a free-floating colony having the form of a large hollow net, the polyg- onal meshes of which are made up of elongated cylindrical cells arranged end to end (Fig. 2b A). Each mesh consists usually, but by no means always, of six cells. A single colony may reach a length of 20 to 30 cm. At first each cell contains a single nucleus and a chloroplast with one pyrenoid, but later there are many nuclei and a large number of pyre-

THALLOPHYTA: ALGAE

35

noids, the chloroplast becoming reticulate and diffuse (Fig. 255). Mature cells have a peripheral layer of cytoplasm surrounding a large central vacuole. In asexual reproduction as many as 7,000 to 20,000 biciliate zoospores may arise from a single vegetative cell by progressive cleavage of its protoplast. These do not escape but swim around within the parent cell, finally coming together to form a new net (Fig. 25C). Later the cell walls of the old net dissolve and the young colonies are set free. These grow to the adult size without any cell division.

Fig. 24. Pediastrum horyanum. A, formation of zoospores and escape of one group in a common vesicle; B and C, zoospores forming a new colony; D, a thick-walled resting cell (polyhedron); E, F, G, zoospores within a polyhedron forming a new colony. {A, B, C, after A. Braun; D to G, after Askenasy.)

In sexual reproduction a single protoplast may give rise to as many as 30,000 to 100,000 biciliate isogametes. These escape from the parent cell through a small pore and fuse in pairs to form thin-walled zygotes (Fig. 25D--F). After undergoing a short resting period, the zygote turns green and increases in size. It then produces four large zoospores and, in connection with their formation, the number of chromosomes is reduced one-half (Fig. 25G, H). As in Pediastrum, the zoospores escape into the water, settle down, and become large heavy-walled polyhedrons (Fig. 25/). These remain dormant until the following spring and represent the real resting stage. Upon germination, a poly- hedron produces 200 to 300 small zoospores that escape enclosed in a membrane, where they arrange themselves to form a new net (Fig. 25 J). These nets are much smaller than the ones developed later by the zoo- spores arising within the vegetative cells of the colony.

36

PLANT MORPHOLOGY

Protosiphon. Protosiphon is a unicellular coenocytic alga occurring on damp earth. It shows a striking resemblance to Botrydium, one of the Xanthophyceae, and often grows with it in the same habitat. The plant has a green aerial portion that is tubular or bladder-like and a

Fig. 25. Hydrodictyon reUcnlaium. A, portion of colony, X 150; B, single cell with many nuclei and pyrenoids, X350; C, young net formed within a parent cell; D, a gamete; E, gametes fusing; F, zygote; G, four zoospores escaping from zygote; H, a zoospore escaped from zygote; /, polyhedron formed by a zoospore; /, young net escaping from polyhedron. (C to F, after Klebs; G to J, after Pringsheim.)

colorless underground portion that resembles a rhizoid (Fig. 26A). It is entirely without cross walls. The cytoplasm, in a thin layer sur- rounding a large central vacuole, contains numerous scattered nuclei (Fig. 26B). When young, the cell has a large reticulate chloroplast

THALLOPHYTA: ALGAE

37

with many pyrenoids; later there may be several chloroplasts. Reserve food occurs chiefly as starch. The aerial portion may bud off new plants that later become detached. When covered with water, the protoplast may give rise to a number of biciliate zoospores or isogametes that escape

E ^-^ F ^<=^ G H

Fig. 26. Protosiphon hotryoides. A, longitudinal section of vegetative plant; B, upper portion, showing scattered nuclei; C, an older stage, the cytoplasm undergoing progressive cleavage; D, formation of zoospores; E and F, gametic union; G, a zygote; H, germinating zygote with four nuclei. (After Bold.)

through an apical pore (Fig. 26C-H). Gametes from the same plant may pair and fuse. The zj^gote, which becomes thick-walled and dormant, produces a new plant directly. If the soil becomes dry, the vegetative protoplast may form aplanospores by progressive cleavage of the cyto- plasm. These may be either small and uninucleate or larger and multi- nucleate. The latter, upon germination, usually give rise to zoospores or isogametes, but may develop into a new vegetative plant directly.

38

PLANT MORPHOLOGY

Summary. The Chlorococcales range from simple isolated cells to complex colonies. In this and succeeding orders nonmotility is the permanent condition of the vegetative cells. Although usually uninu- cleate, frequently these are multinucleate and often contain more than one chloroplast. Colonies are formed by the coming together of free cells (usually zoospores) derived from a single parent cell and there is no subsequent division of vegetative cells. Cell division occurs only in connection with the formation of reproductive cells. Reproduc- tion is accomplished by zoospores, aplanospores, or akinetes, and usually also by isogametes.

3. Ulotrichales

The Ulotrichales have been called the representative group of the Chlorophyceae. Most of them live in fresh water but some are marine.

.Q.--

-<3'

•.--G>.0;:

K®

®-.

'01

feiO-

<^

iSti

ABC

Fig. 27. Ulothrix zonata, vegetative and reproductive stages, X700. A, basal portion of filament, showing holdfast cell and three vegetative cells, each with a single nucleus and a peripheral band-like chloroplast with many pyrenoids; B, formation and escape of zoospores; C, formation and escape of gametes, some of which are pairing.

A few live in damp places on land. Trichophilus grows inside the hair of the South American sloth. To this order belong 85 genera and approxi- mately 500 species, the principal genera being Ulothrix, Chaetophora, Draparnaldia, Stigeoclonium, Protococcus, Ulva, and Coleochaete.

Ulothrix. This alga is of widespread occurrence in streams, lakes, and ponds, where it grows attached to objects in the water. A few of its species are marine. The plant body is multicellular, consisting of a simple unbranched filament (Fig. 27 A). The basal cell is elongated

THALLOPHYTA: ALGAE 39

and modified to serve as a holdfast, but all the other cells are alike, being shortly cylindrical. Each contains a central nucleus and a periph- eral band-like chloroplast usually with many pyrenoids. The chloro- plast may form either a complete or a partial band. Any cell in the filament, except the basal one, may divide by the formation of a cross wall between two daughter protoplasts, thus resulting in growth of the filament.

In asexual reproduction, the contents of any vegetative cell, except the holdfast, may divide to form mostly 2, 4, 8, or 16 zoospores (Fig. 21 B). These escape through a pore in the cell wall and swim by means of four equal cilia attached apically. When discharged, the zoospores are enclosed in a common membrane that soon disappears. As in the vegetative cells of the Volvocales, each zoospore has a red eyespot and a contractile vacuole. After a period of free swimming, a zoospore comes to rest, withdraws its cilia, and secretes a cell wall. It then gives rise to a new filament by repeated cell divisions. Sometimes aplanospores are formed instead of zoospores. They frequently germinate within the parent cell.

Sexual reproduction takes place in Ulothrix by the conjugation of isogametes (Fig. 27C). These originate from the vegetative cells in the same way as the zoospores do, but are smaller, more numerous (usually 32 or 64 in a cell), and have only two cilia. They escape through a pore in the cell wall, enclosed in a common membrane that soon breaks down. Following pairing and fusing of the gametes, the resulting zygotes secrete a heavy wall and generally do not germinate until the following spring. Then each produces 4 to 16 zoospores (or sometimes aplanospores) that, in turn, give rise to new vegetative filaments. The zygote is the only diploid cell in the life history. When its nucleus divides, the chromosome number is reduced one-half. Although isogamous, Ulothrix shows some degree of sexual differentiation in that the gametes of one filament fuse only with those of another.

Chaetophora. Some of the I lotrichales are branching filaments, often with cells showing a differentiation in size. One such member is Chaetophora, frequently found in standing water attached to submerged objects. The cells of the branches become progressively smaller and end in hair-like appendages that taper to a point. In a closely related form, Draparnaldia, common in clear, cool streams, the cells of the main filament are much larger than those of the branches (Fig. 28). Cell structure and reproduction in both genera are much the same as in Ulothrix. Stigeoclonium, another relative of Chaetophora, is differentiated into a cushion-like basal portion from which arise a number of sparingly branched upright filaments. When exposed to dry conditions, the cells round off and separate, giving rise to a "palmella" stage. These cells

40

PLANT MORPHOLOGY

are thiek-walled and divide in any plane. They may remain in groups or become separate. With the return of favorable conditions, they produce a new filamentous body.

Protococcus. One of the commonest and most widely distributed of the green algae is Protococcus,^ a terrestrial form growing on the shaded side of damp tree trunks, moist rocks, walls, etc. It is a unicellular alga,

Fig. 28. Drapamaldia, portion of plant, a branching filament with a marked differentiation in size of vegetative cells, X 200. Each cell has a central nucleus obscured by the peripheral band-like chloroplast with many pyrenoids.

consisting of a spherical protoplast enclosed by a cell wall (Fig. 29). It has a small nucleus and a large, peripheral, irregularly lobed chloroplast usually without pyrenoids. Reproduction occurs entirely by cell division, spores and gametes being unknown. Permanent colonies are not formed but, instead of separating immediately, the cells usually hang together temporarily in small groups. In the presence of excessive mois- ture, the number of cells in a group is greatly increased and sometimes some of them grow into short filaments.

In most unicellular algae the division of a cell involves the formation of a new cell wall completely around each daughter protoplast and the disintegration of the wall of the parent cell. In Protococcus, however, a

' Often called Pleurococcus.

THALLOPHYTA: ALGAE

41

cross wall is developed across the parent cell, a method characteristic of Ulothrix and multicellular algae in general. If a second wall appears in one or both of the daughter cells before they separate, it comes in at right angles to the first one. Later divisions may be in the third plane. Thus there is a slight tendency in Protococcus toward the development of a multicellular body.

Protococcus is now generally regarded, not as a primitive form, but as one that has become reduced from more highly developed ancestors, probably as a result of its terrestrial mode of life. This is indicated by

Fig. 29. Protococcus viridis, a unicellular green alga, X 1,000. Some of the cells have divided to form small temporarj' groups. Each cell has a central nucleus and a peripheral lobed chloroplast.

its advanced method of cell division combined with a failure to develop an extensive multicellular plant body like that of other Ulotrichales and by the absence of zoospores and gametes, which even such truly primitive forms as Chlamydomonas possess.

Ulva. This is a widely distributed marine alga commonly known as "sea lettuce." It grows along seacoasts between the high- and low-tide lines. The vegetative body consists of a plate-like thallus two layers of cells in thickness (Fig. 30). It is attached to rocks and other objects in the water by means of a basal holdfast consisting of long colorless rhizoids. The thallus may reach a length of 30 cm. or more. Each cell is uni- nucleate and has a single chloroplast with a pyrenoid.

Reproduction in Ulva closely resembles that of Ulothrix. Zoospores arise from ordinary vegetative cells situated along the thallus margin, four or eight zoospores being produced in each cell. They are liberated into the water through an opening in the cell wall and swim by means of four cilia. Upon germination, a zoospore gives rise to a plant that produces only gametes. These are smaller than the zoospores, more numerous (16 or 32 in a cell), and biciliate. Two similar gametes^ coming from

' Although some species of Ulva are strictly isogamous, others produce two kinds of gametes that differ slightly in si?e.

42

PLANT MORPHOLOGY

separate plants unite to form a zygote. Instead of becoming a thick- walled resting cell, the zygote germinates immediately and produces a plant that bears only zoospores.

Thus Ulva illustrates the phenomenon of alternation of generations. Two separate plants, one producing gametes and the other spores, are involved in each life cycle and, although they look alike, the gamete- producing plants are haploid and the spore-producing plants are diploid.

Fig. 30. Ulva lactuca, the sea lettuce, about one-half natural size. The bright green thallus is only two layers of cells thick. (After Thuret.)

The doubling of chromosomes, resulting from the conjugation of two gametes, is carried over by the zygote to the cells of the spore-producing plant. The reduction of chromosomes takes place when the zoospores are produced. These haploid spores give rise to the gamete-producing plants. The haploid plants are called gametophytes and the diploid plants sporophytes. Because the two kinds of plants are alike vegeta- tively, Ulva displays an isomorphic alternation of generations.

Coleochaete. Coleochaete is a small fresh-water alga that usually grows attached to leaves and stems of aquatic seed plants, such as water

THALLOPHYTA: AWAK

43

lilies and cattails. Depending on the species, the vegetative body is either a branching filament, a cushion with free branches, or a circular disk with radiating rows of cells (Fig. 31 A). When disk-like, it rarely exceeds 5 mm. in diameter. Some of the cells bear hair-like outgrowths,

Fig. 31. Coleochaete scutata, a discoid species. A, a small vegetative plant with numerous zygotes overgrown by the surrounding cells, X150; B, a small group of vegetative cells, one of which is giving rise to a zoospore, and an escaped zoospore, X500; C, vegetative cells giving rise to antheridia, and an escaped sperm, X500; D, cross section of portion of thallus, showing a zygote, X350.

each with a sheath at its base. Each cell has a single nucleus and a chloroplast with one or sometimes two pyrenoids. Growth is always apical, in the discoid species occurring by means of a marginal meristem. Biciliate zoospores, formed singly, may arise in any vegetative cell (Fig. 31JB). They escape through a pore in the cell wall.

In being heterogamous, Coleochaete makes an advance over the other Ulotrichales that have been considered. In the discoid species antheridia are formed by the division of a vegetative cell into smaller cells, the

44

PLANT MORPHOLOGY

protoplasts of which escape into the water as biciHate sperms (Fig. 31C). An oogonium is formed near the margin of the thallus by the enlargement of a vegetative cell, its protoplast becoming a nonmotile egg. In the branched species the antheridia and oogonia are borne at the ends of separate branches. Here the oogonium has a long, slender extension (trichogyne) with a terminal opening. A few species are dioecious.

A sperm enters the oogonium and fertilizes the egg, the zygote enlarg- ing and becoming thick-walled. At the same time adjacent vegetative cells grow up around the oogonium and form a case (Figs. 3 ID and 32.4).

Fig. 32. Coleochaete pulvinata. A, section of oogonium containing a zygote and sur- rounded by jacket produced by adjacent vegetative cells; B, section of oogonium containing a group of cells derived from the zygote, each of which gives rise to a zoospore. (After Oltmanns.)

After undergoing a period of rest, the zygote germinates inside the oogonium and produces a spherical body consisting of 16 or 32 cells, each cell in turn producing a biciliate zoospore (Fig. 32J5). This escapes and gives rise to a new vegetative plant. In the discoid species the zygote produces an eight-celled body. The reduction of chromosomes takes place when the zygote germinates. Consequently, the body of spore-producing cells that develops from it is haploid and so cannot be regarded as a sporophyte. Thus Coleochaete is without a true alternation of generations.

Summary. The plant body of the Ulotrichales is multicellular (except in Protococcus) , being either a simple filament, a branched filament, or a flat plate-like thallus. The cells contain one nucleus and a single chloro- plast. Growth occurs by division of the vegetative cells. Nearly all members produce zoospores, these being either biciliate or quadriciliate. Asexual reproduction may also occur by aplanospores or by akinetes. Sexual reproduction ranges from isogamy to heterogamy.

THALLOPHYTA: ALGAE 45

4, Oedogoniales

The Oedogoniales are related to the Ulotrichales and are often classified with them. They are a fresh-water group including onl}^ 3 genera and approximately 400 species. The two chief genera are Oedogonium and Bulbochaete, both occurring throughout the world.

Oedogonium. This widely distributed alga, comprising nearly 300 species, generally lives in ponds, lakes, and quiet streams, often attached to sticks, stones, and other aquatic plants. It consists of a simple un- branched filament that, when young, has a basal holdfast cell but later is usually free-floating. The cells are elongated and uninucleate. Each contains a peripheral chloroplast with many pyrenoids. The chloroplast is band-like and reticulate. Any vegetative cell except the basal one may divide.

Oedogonium has a peculiar method of cell division seen only in the other members of its order (Fig. 33). It results in the formation of distinctive "apical caps." The nucleus divides near the upper end of the cell, where simultaneously a ring-like thickening of cellulose is developed on the inside of the lateral wall above the dividing nucleus. A groove appears in this ring and the cell wall splits transversely opposite the groove. A thin cross wall now appears between the daughter nuclei and the protoplast is divided in half. The ring stretches into a cylinder as each daughter protoplast elongates, the new cross wall moving upward to the top of the parent cell, where it unites with the lateral wall very close to where the transverse split occurred. The upper cell, which has a new cell wall, con- tinues to elongate until it reaches the size of the lower cell, which possesses the old cell wall.

Asexual reproduction occurs by the formation of large zoospores, each of which arises from the entire contents of an ordinary vegetative cell (Fig. 34: A, B). This escapes as a naked protoplast that bears a crown of cilia. The liberation of the zoospore is accompanied by a transverse splitting of the cell wall at the apical end. After a period of free swim- ming, the zoospore comes to rest with its ciliated end downward, retracts its cilia, forms a cell wall, and gives rise by repeated divisions to a new filament. Oedogonium may also produce akinetes, although these are relatively uncommon. The akinetes may occur either singly or in a linear series. They germinate directly into new filaments.

Oedogonium is heterogamous. An antheridium arises as a short cell that is cut off at the apex of an ordinary vegetative cell. It may remain the only one, but generally more (from 2 to 40) are produced by continued division of the lower cell or by division of antheridia already formed (Fig. 34C). Each antheridium gives rise to one or, more commonly, to two sperms, either by a vertical or a transverse division of the protoplast,

46

PLANT MORPHOLOGY

depending on the species. The sperms escape into the water and, like the zoospores, swim by means of a crown of cilia. An oogonium also commonly arises from the smaller upper cell produced by the division of an ordinary vegetative cell, but this cell subsequently enlarges by the accumulation of food (Fig. 34D). The oogonia may occur separately or

wmm^

mmm\

Fig. 33. Nuclear and cell division in Oerfogo/iiMm gra?ic?e, X320. A, elongation of nucleus and appearance of young ring; B, metaphase; C, anaphase; D, formation of cross wall and separation of nuclei; E, broken outer layer of cell wall and stretching of ring; F, straighten- ing of ring and migration of cross wall upward to unite with inner layer of cell wall. {After Ohashi.)

several may be cut off in a series. The entire protoplast of the oogonium becomes a large nonmotile egg.

A sperm enters an oogonium through a pore in its wall and unites with the egg. The zygote becomes a heavy-walled resting cell that later produces four zoospores (Fig. ME, F). When liberated, these are enclosed by a common membrane that soon disappears. From each of the zoospores a new filament is developed. The reduction of chromo- somes occurs in connection with the germination of the zygote, and so the four zoospores are haploid.

THALLOPHYTA: ALGAE

47

Some species of Oedogonium are monoecious, the antheridia and oogonia occurring in the same filament. Other species are dioecious, the two kinds of sex organs being borne on separate filaments. In some dioecious species the male and female filaments are approximately equal in size.

i^^

B

CD E

Fig. 34. Reproduction in Ocdoj7ont?/m, X500. ^ and B, the entire fontents of a vegetative cell escaping as a zoospore; C, portion of filament with two groups of antheridia; also a single escaped sperm; D, portion of filament with an oogonium containing a mature egg in which are many pyrenoids and starch grains; E, heavy-walled zygote still within the oogonium; F, group of four zoospores produced by the zygote. {A and B, after Him; F, after Juranyi.)

In others the male filaments are very small, consisting of only a few cells. These dwarf filaments are produced by special small zoospores, called androspores, that originate singly in rows of small cells resembling antheridia. The androspores germinate on the female filaments near or on an oogonium (Fig. 35). The dwarf filament usually consists of a single vegetative cell that cuts off one or several terminal antheridia, each producing two sperms. Figure 35 shows three dwarf filaments of different ages. In the one on the right the single vegetativ-e cell has cut off a small, undivided, antheridial cell. In the middle filament a second antheridium

48

PLANT MORPHOLOGY

has been formed by the vegetative cell, while the first antheridium has produced two sperms. In the male filament on the left two sperms have escaped from the upper antheridium, but two more have been formed in the lower one.

Bulbochaete is a genus closely resembling Oedogonium, differing chiefly in having branches, most of the cells of which bear long one-celled hairs

that are swollen at the base.

Summary. The Oedogoniales are a small order differing from the Ulotrichales mainly in having a peculiar method of cell division and motile re- productive cells with a crown of cilia. The vege- tative body is multicellular and filamentous, the cells having one nucleus and a single chloroplast. Asexual reproduction occurs by zoospores, some- times by akinetes. All the members are heter- ogamous.

5. Conjugales

The Conjugales constitute a distinct and highly specialized order of green algae that occupy an isolated position. In fact, they are sometimes removed from the Chlorophyceae and made an independent class. All of them occur in fresh water. They include 38 genera and over 2,400 species. Some representative genera are Closterium, Cosmarium, Mougeotia, Spirogyra, and Zygnema.

Desmids. These algae are widely distributed in bogs, ponds, and small lakes, usually becoming abundant late in the season. They number about 2,250 species. Closterium is a genus of nearly 200 species, while Cosmarium has over 800. The des- mids are unicellular and the cells display a great variety of form. Like the diatoms, they have won the favor of microscopists by their great beauty. Desmids are typically solitary, but some develop into filamentous colonies. Many desmids have the power of movement, which appears to be caused by exudation of mucilage through pores in the cell wall.

In most desmids the cell is organized into two symmetrical halves that are generally separated by a median constriction called the isthmus (Figs. 36 and 37). In each half there is usually one large chloroplast (some- times two) with one or more pyrenoids. The chloroplast is often elabo- rately lobed. The nucleus lies in the isthmus. In Closterium, at each

Fig. 35. A species of Oedogonium having dwarf male filaments, three of which have developed on the cell below the oogonium, X300.

THALLOPHYTA: ALGAE

49

end of the cell, is a small group of calcium sulphate crystals that show Brownian movement. In some desmids, the outer surface of the cell wall displays warts, spines, ridges, or other markings, most of which show a regular arrangement.

Asexual reproduction occurs mainly by fission, rarely by apian ospores. Zoospores have never been observed. In cell division the nucleus divides

m

J I

ffii

'•■^m*

<*

Fig. 36. Closterium, a common desmid. A, vegetative cell, showing nucleus at isthmus, a large lobed c-hloroplast with a row of pyrenoids in each half of the cell, and at each end a vacuole containing a few crjstals, X300; B and C, another species, showing two stages in conjugation, X200.

first and then a cell wall is formed across the isthmus. Each of the two chloroplasts splits transversely. The daughter cells then separate and each forms a new half similar to itself. In sexual reproduction two cells come together and secrete a common mucilaginous sheath (Fig. 365, C). Their walls generally break at the isthmus. Then the protoplasts escape and fuse to form a zygote. In a few desmids each cell sends out a short tube. These meet, become continuous, and the two protoplasts fuse in the tube. The desmids are isogamous but their gametes, each represent-

50

PLANT MORPHOLOGY

B

D

Fig. 37. Several desmids, showing variety of form. A, end view and B, front view of Staurastrum; C, Docidium; D. Cosmarium, E, Micrasterias; C, X250; others, X400.

G^:

mm

Fig. 38. Single cells of Moiigeotia, showing the plate-like chloroplast as seen in side (A) and face (B) views, X500; C, conjugating filaments, with three zygotes formed in the conjugat- ing tubes. (C, after Wittrock.)

ing an entire vegetative protoplast, are nonciliated. The zygote becomes thick-walled and, after a period of rest, its protoplast escapes and divides generally into two daughter protoplasts, each of which becomes a new individual. As a result of two successive divisions of the zygote nucleus, during which the reduction of chromosomes occurs, each daughter proto-

THALLOPHYTA: ALGAE

51

plast has two haploid nuclei. Then one nucleus in each protoplast degenerates.

Mougeotia. This alga consists of a deli(;ate unbranched filament. Each cell displays a nucleus and a peculiar, axial, plate-like chloroplast containing two or more pyrenoids (Fig. 38/1, B). The chloroplast can change its position in the cell, presenting its flat surface to dull light and

BCD

Fig. 39. Spirogyra. A, a vegetative cell, showing the central nucleus and the peripheral, band-like, spiral chloroplast with many pyrenoids, X500; B, C, D, stages in conjugation, X250.

its edge to bright light. Reproduction occurs by fragmentation, aplano- spores, and by the conjugation of isogametes. The cells of two filaments lying parallel to each other put out short bud-like outgrowths that come into contact and form tubes. The protoplasts of two conjugating cells pass into one of these tubes and there fuse, producing a heavy-walled zygote (Fig. 38C). Upon germination, four cells are formed. Three of these die, the fourth producing a new filament. It is probable that the chromosome reduction takes place when the zygote germinates.

Spirogyra. Spirogyra is a well-known green alga very common in ponds, lakes, and streams, where it forms slimy bright green masses on or

52

PLANT MORPIIOLOG Y

beneath the surface of the water. It is a large genus of over 100 species. The vegetative bod}^ is an unl)ran('hed filament with cylindrical cells that are usually elongated. Each cell has a single nucleus suspended in the center by strands of cytoplasm (Fig. 39.4). It also has one or more peripheral, ribbon-like chloroplasts with many pyrenoids. The chloro- plasts have the form of spiral bands, the number in each cell depending

G H

Fig. 40. Nuclear changes in the zygote of Spirogyra longata {A to G) and germination of the zygote of Spirogyra neglecta (H). A, B, C, first meiotic division of fusion nucleus in the zygote; D and E, second division; F and G, degeneration of three of the haploid nuclei. (After Trondle.)

on the species. Any cell may divide by the formation of a cross wall, thus resulting in growth of the filament. In some species the cross walls possess characteristic infoldings.

In sexual reproduction the cells of the two filaments lying side by side put out lateral projections that come in contact (Fig. 395-Z)). The contiguous portions of the cell walls at the ends of these projections then break down and form tubes leading from one filament to the other. Through these conjugating tubes the protoplasts of one filament pass to fuse with those of the other filament, forming zygotes. An entire vege- tative protoplast thus becomes a large gamete. The zygote develops a heavy wall and goes into a resting stage. Upon germination, which usually occurs in the following spring, it directly produces a new filament

THALLOPHYTA: ALGAE

53

(Fig. 40//). The zygote becomes diploid by the fusion of the two nuclei derived from the conjugating protoplasts. When germination takes place, the fusion nucleus undergoes two successive divisions that result in a reduction of chromosomes (Fig. 40A-G). Of the four haploid nuclei thus formed, three degenerate, leaving one to function. In this way the zygote gives rise to a haploid filament.

Zoospores are never produced in Spirogyra. If conjugation fails to occur, a protoplast may round up and become a heavy-walled cell that, after a period of rest, gives rise to a new filament. Such a cell is often

Fig. 41. A species of Spirogyra with lateral conjugation, gametic union occurring between adjacent cells of the same filament. A, B, C, development of conjugating tubes and forma- tion of zygote, X300.

called an aplanospore but would be more appropriately designated as a gamete that develops without undergoing conjugation.

Spirogyra, like the other Conjugales, is peculiar because an entire vege- tative protoplast becomes a single large gamete that is not ciliated and does not escape into the water. Although the gametes show no differ- entiation in size, the active ones are regarded as male and the passive ones as female. The ordinary type of conjugation is known as scalariform (ladder-like) conjugation. In a few species lateral conjugation occurs (Fig. 41). In this type conjugating tubes are developed between adjacent cells of the same filament. At its completion a zygote is formed in one of the conjugating cells.

Zygnema. This is a genus closely related to Spirogyra and resembling it in many ways. Both forms grow in the same sort of places and look much alike to the naked eye. The filaments of Zygnema are unbranched and consist of cyUndrical, more or less elongated cells (Fig. 42). Each has two spherical chloroplasts between which, at the center of the cell, lies the nucleus. Each chloroplast has a single pyrenoid surrounded by

54

PLANT MORPHOLOGY

radiating starch grains. As in Spirogyra, sexual reproduction takes place by the passage of isogametes, each representing an entire vegetative pro- toplast, through conjugating tubes and their fusion in the cells of one of the filaments. There is also the same degeneration of three of the haploid nuclei derived from the nucleus of the zygote.

A B

Fig. 42. Single cells of Zygiiema, X750. A, vegetative cell, showing central nucleus and two spherical chloroplasts, each with a pyrenoid surrounded by radiating starch grains; B. young zygote with four chloroplasts and the two gametic nuclei not yet fused.

Summary. The Conjugales are an aberrant order of green algae show- ing no close relationship to any of the other orders. The plant body may be either unicellular or multicellular, in the latter case consisting of a simple unbranched filament. The cells are uninucleate and have one or more peculiar chloroplasts. The distinguishing feature of the order is the absence of all ciliated cells in the life history. No zoospores are produced, but aplanospores may occur. Sexual reproduction is accomplished by the conjugation of two noncihated isogametes, each derived from the entire protoplast of a vegetative cell. These either escape and fuse, unite in a conjugating tube, or pass through a conjugating tube and fuse in one of the cells.

6. Siphonocladiales

This is a group whose members are often distributed among other orders, although its characters are rather well defined. They are repre- sented in both fresh and salt water, but most of them are marine, being found principally in tropical and subtropical seas. Many of the marine forms are incrusted with lime. Representatives of the group have been found as fossils as far back as the Ordovician. The Siphonocladiales include about 37 genera and 450 species, the best-known genera being Cladophora, Sphaeroplea, and Acetabularia.

Cladophora. Cladophora is a genus of about 150 species, world-wide in distribution. It is found in great abundance in streams, ponds, and lakes, usually attached to stones and piers. Some of its species are marine. The vegetative body is filamentous and much branched, its cells being elongated and cylindrical (Fig. 43A). A branch originates as an out- growth from the upper end of a cell lying near the end of a filament. Each cell is a coenocyte, containing many nuclei. The cytoplasm usually sur-

THALLOFHYTA: ALGAE

55

rounds a large central vacuole. When young, a cell has a large, periph- eral, reticulate chloroplast with many pyrenoids. Later the chloroplast often appears to break up into numerous small chloroplasts, some of which have pyrenoids.

Many quadriciliate zoospores are formed, usually in cells at or near the ends of branches (Fig. 43i^). The zoospores, which are uninucleate.

Fig. 43. Cladophora. A, portion of plant, a branching filament, X65; B, a vegetative cell, a sporangium, and two escaped zoospores. X300. Each vegetative cell has many nuclei and a large, peripheral, reticulate chloroplast with a large number of pyrenoids.

escape singly through a small pore in the cell wall. They develop into new filaments, but these, in turn, produce only isogametes. The gametes may arise in any vegetative cell. They escape into the water and swim by means of two ciha. The gametes pair and fuse, but fusion occurs, as a rule, only between gametes coming from different plants. The zygote, without undergoing a period of rest, gives rise to a new filament directly. This plant produces only zoospores. Although alike vegetatively, the gamete-producing plants are haploid and the spore-producing plants are diploid. The reduction of chromosomes occurs in connection with the

56

PLANT MORPHOLOGY

formation of zoospores. Thus, as in Ulva, the life cycle of Cladophora involves a distinct alternation of generations of the isomorphic type.

Sphaeroplea. This is a fresh-water alga that grows in wet meadows and occasionally in pools. Although widely distributed, it is not com- mon. The vegetative body consists of an unbranched filament with very long cylindrical cells, each containing numerous nuclei and chloroplasts (Fig. 44). The chloroplasts, some of which have pyrenoids, are parietally placed and grouped into wide annular bands of cytoplasm separated by

W m m

ii.!.iJU4^j5Sjp*.,uiu...r,i

r

-^ — ■ I n'*

Fig. 44. Sphaeroplea annulina, X400. A, portion of a vegetative cell with ring-like bands of cytoplasm containing many small nuclei, chloroplasts, and pyrenoids; B, anther- idia producing sperms; C, portion of an oogonium with nianj- eggs ready for fertilization.

wide vacuoles. The vegetative cells of Sphaeroplea do not produce any zoospores. Sexual reproduction is heterogamous, the two kinds of sex organs usually being borne in different filaments. Any vegetative cell, without undergoing a change in shape, may become an antheridium or an oogonium. The antheridium produces a large number of small biciliate sperms, while the oogonium gives rise to many large nonmotile eggs. The eggs are at first multinucleate, but later all the nuclei degenerate except one. The sperms escape through small pores in the cell wall, enter the oogonium through similar pores, and there fertilization takes place. Each zygote becomes thick-walled and, after undergoing a long resting period, gives rise usually to four biciliate zoospores. Each of these forms a new filament. The reduction of chromosomes occurs when the zygote germinates, and so the spores and vegetative filaments are haploid.

THALLOPHYTA: AWAE

57

Acetabularia. This is a marine genus occurring in tropical and sub- tropical regions. It is called the mermaid's-wineglass. Acetabularia crenulata is a common species off the coast of Florida and throughout the West Indies. Its vegetative body, reaching a height of 6 to 9 cm., con- sists of a stalk bearing rhizoid-like holdfasts below and expanded above into a cup-like disk about 1 cm. in diameter (Fig. 45). The disk is com- posed of a whorl of elongated branches that are laterally coherent, each branch being a coenocyte. The plants are more or less incrusted with lime. At first the plant has a single nucleus that soon gives rise to many small nuclei. These pass up the stalk and enter the disk, which has now become divided into cells.

Reproduction begins by the formation of a large number of aplanospores (cysts) within the fertile branches composing the disk. The aplanospores are at first uninucleate but later become multinucleate. They are liberated into the water and, after a resting period, each gives rise to a large number of biciliate isogametes that escape and fuse in pairs. The zygote germinates immediately to form a new plant. The vegetative plant is diploid, the reduction of chromosomes occurring when the nucleus of the aplanospore divides.

Acetabularia has been widely used by students of genetics and develop- ment, especially in experiments on regeneration and polarity.

Summary. The Siphonocladiales are multicellular algae with large multinucleate cells, these usually containing many small chloroplasts. The plant body is thus partially coenocytic. Vegetative growth takes place by cell division. Asexual reproduction usually occurs by zoospores, aplanospores, or akinetes. Sexual reproduction may be either isogamous or heterogamous. This order is related both to the Chlorococcales and to the Siphonales.

Fig. 45. Acetabularia crenu- lata, natural size.

7. Siphonales

The Siphonales are a distinct group of mostly marine algae, only a few being found in fresh water. They are especially abundant in tropical seas. As in the Siphonocladiales, many marine forms secrete lime. Fossil members are known as far back as the Ordovician. The order includes 50 genera and about 350 species. Representative genera are Vaucheria, Codium, Bryopsis, and Caulerpa.

Vaucheria. This well-known alga grows in felt-like masses in fresh water and on damp soil. Some of its species live in brackish water and

58

PLANT MORPHOLOGY

some live in the ocean. The plant body consists of a sparsely branched coenocytic filament without any cross walls in the vegetative portion. It is attached by means of colorless rhizoid-like holdfasts. Numerous small nuclei and chloroplasts are scattered throughout the cytoplasm, which surrounds a large central vacuole. There are no pyrenoids or starch grains, but oil droplets are usually present in abundance. In this respect Vaucheria differs from the other Siphonales.

Vaucheria displays three methods of vegetative reproduction, as follows: (1) A branch may be constricted at the base, thus producing a

>^

T^i

s^V*^

i^p.

A C

Fig. 46. Fawc/ieria, a coenocytic green alga, X250. A, an escaping zoospore covered with many cilia; B, a zoospore giving rise to a new vegetative filament; C, two oogonia of Vaucheria sessilis, each with a zygote ; also an antheridium that has discharged its sperms ; D, two sperms, more highly magnified.

new plant body directly. (2) The tip of a branch may swell slightly and become cut off by a cross wall to form a club-shaped sporangium (Fig. 46A). The multinucleate protoplast in the branch rounds up and becomes a large zoospore entirely covered by cilia. The cilia are in pairs and beneath each pair is a nucleus. For this reason the zoospore is regarded as compound. It escapes into the water through a terminal pore and, upon germination, gives rise to a new filament (Fig. 465). (3) The contents of an entire filament may break up into aplanospores, each developing a thick wall.

Vaucheria is heterogamous. The antheridia and oogonia are not transformed vegetative cells but are developed on special branches of the filament (Fig. 46C). In most species a short branch, sooner or later cut off by a wall, becomes a globular oogonium. Its protoplast is organized as an egg, which becomes uninucleate. It is uncertain whether this is

TH A LLOPH Y TA : A WA E

59

accomplished by the degeneration of all its nuclei except one, as some observers have claimed, or, as others contend, by the passage back into the filament of all but one nucleus before the wall is formed at the base of the oogonium. Arising close to the oogonium is a longer and more slender branch, its curved tip being cut off by a wall to form an anther- idium. In some species both kinds of sex organs are borne on the same branch, the antheridium being terminal and surrounded by two or more oogonia (Fig. 47) . The antheridium produces many small biciliate sperms that are liberated into the water through a terminal pore. A sperm

*#5^^^

.$,

'^

^

. *'■■

Fig. 47. Sex organs of Vaucheria geminata, two oogonia and an anther- idium borne on the same branch, X250.

Fig. 48. Thallus of Codium fragile, one- half natural size.

enters an oogonium through a terminal pore in its wall and fuses with the egg to produce a heavy-walled zygote. After remaining dormant, it gives rise to a new filament directly. The reduction of chromosomes probably occurs during germination of the zygote.

Codium. Codium is a widely distributed marine alga that grows on rocks between tide lines. The thallus is dark green and spongy, consist- ing of thick cylindrical branches composed of a dense mass of interwoven filaments (Fig. 48) . It is anchored by means of a basal disk-Uke holdfast. Like other members of the order, the vegetative body is without cross walls. The cytoplasm is peripheral and has numerous small nuclei and chloroplasts. There is no asexual reproduction by means of spores. Two kinds of gametangia are produced, generally on different plants. They arise on the sides of large club-shaped branches that form a sort of cortex, and are cut off by a basal wall. The male gametangium, which is smaller than the female one, hberates many thousands of biciliate male gametes. In the female gametangium some of the nuclei degenerate, while others enlarge. Several hundred biciliate female gametes are

60

PLANT MORPHOLOGY

organized. These escape through a terminal pore and are fertilized in the water. The zygote gives rise at once to a vegetative plant. The vegetative plants of Codhim are diploid and the reduction of chromosomes takes place in connection with the formation of the gametes.

Bryopsis. Some of the marine Siphonales are highly branched, one of these being Bryopsis. The thallus is composed of a prostrate rhizome- like portion, anchored by rhizoids, and an upright feathery portion, the

Fig. 49. A small portion of the vegetative body of Bryopsis, showing branches of limited growth, X75.

latter consisting of an axis with branches of limited growth (Fig. 49). In the formation of a gametangium, a branch is cut off by a cross wall and gives rise to numerous bicilate gametes. A gametangium produces either male or female gametes and these are usually borne on different plants. The female gametes are about three times as large as the male ones. Both escape into the water, where they pair and fuse. The zygote secretes a cell wall and germinates immediately to form a new vegetative plant. There are no spores of any kind in the life cycle. The reduction of chromosomes occurs when the gametes are formed, and thus the vegetative plant is diploid.

Caulerpa. This is a marine form of interest because of the high degree of differentiation of its coenocytic plant body (Fig. 50) . It consists of a

THALLOPHYTA: ALGAE

61

creeping axis with root-like holdfasts and erect leaf-like shoots of various form. In some species the shoots reach a height of 30 cm. Cross walls are absent in the vegetative part of the plant, but the central cavity is traversed by numerous slender strands. Asexual reproduction occurs only by fragmentation, sexual reproduction by biciliate isogametes.

Summary. The Siphonales are characterized by a completely coeno- cytic plant body that is usually much branched and often differentiated in form. Cross walls appear only in connection with the formation of

ABC

Fig. 50. Three species of Caiilerpa, a coenocyte with a high degree of structural differentia- tion, one-half natural size. A, Canlerpa prolifera; B, Caulerpa crassifolia; C, Caiilerpa macrodisca.

r?productive organs. The vegetative body contains innumerable nuclei and small chloroplasts. It is really a single multinucleate cell. Asexual reproduction may be accomplished by fragmentation of the thallus, by zoospores, aplanospores, or akinetes. Sexual reproduction ranges from isogamy to heterogamy. This is a highly specialized order related both to the Chlorococcales and the Siphonocladiales.

Summary of Chlorophyceae

The Chlorophyceae are algae with only chlorophyll and its associated carotinoids in their plastids, these being present in the same proportions as in the higher plants. In vegetative organization they are highly diversified. Some are unicellular but most of them are multicellular, the thallus being most commonly filamentous, sometimes plate-like, and rarely massive. Some are partially or completely coenocytic. There is relatively little cellular differentiation. A definite cell wall composed of cellulose is nearly always present, this seldom becoming mucilaginous. The cells contain a well-organized nucleus (often more than one) and one or more distinct plastids. Pyrenoids are usually present. Reserve food is stored generally as starch, sometimes as oil. Asexual reproduction

62 PLANT MORPHOLOGY

occurs by fission (in some unicellular forms), fragmentation, or by zoo- spores, aplanospores, and akinetes. Sexual reproduction is either isoga- mous or heterogamous. In the heterogamous forms the sperms are ciliated, the eggs nearly always noncihated. Motile reproductive cells generally have two or four cilia, equal in length and apically attached. The zygote nearly always becomes a resting cell.

Within the Chlorophyceae, three main evolutionary trends can be recognized. The occurrence of ciliated reproductive cells in practically all members, except the Conjugales, indicates that the common ancestor of the group must have been a form like Chlamydomonas. The Volvo- cales, retaining motility in vegetative cells, represent one hne of evolution. It emphasizes the ciliated colonial type of organization that culminates in

Volvox.

A second line of development, represented by the Chlorococcales, also emphasizes the colony but shows a loss of motility by the vegetative cells. A tendency toward the formation of multinucleate cells appears in this order. This leads to the development of coenocytic bodies, which reaches a climax in the Siphonales. Protosiphon is a connecting link between the Chlorococcales and Siphonales. Some regard the Siphono- cladiales as a transitional stage leading to the evolution of the Siphonales; others consider them an offshoot of that order, the incomplete formation of walls being a recent development. Still others think that at least some of the Siphonocladiales have arisen from the Ulotrichales.

A third line of development within the Chlorophyceae is represented by the Ulotrichales, an order in which several different types of multi- cellular bodies have appeared. All of these grow by division of uni- nucleate vegetative cells. The Oedogoniales may represent an offshoot from this order, but a connection between the Conjugales and the Ulotrichales seems rather remote, the lack of ciliated cells and pecuHar type of sexual reproduction in the Conjugales being the chief obstacles. The Ulotrichales are of great interest in being the order of green algae most closely resembling the probable ancestors of the higher green plants. The occurrence of plate-hke forms is particularly significant, inasmuch as the vegetative body of the simpler bryophytes is a plate-like thallus.

CHAPTER III

THALLOPHYTA : ALGAE (CONTINUED)

8. CHAROPHYCEAE

The Charophyceae, or stoneworts, constitute a very isolated group of highly organized green thallophytes with uncertain affinities. Although often included in the Chlorophyceae, they are so distinct that they belong in a separate and coordinate class. The Charophyceae are multicellular plants in which the only pigments present are chlorophyll and its asso- ciated carotinoids, these occurring in essentially the same proportions as in the green algae. They include 6 genera and about 200 species, nearly all of which belong to Chara and Nitella. The stoneworts grow in streams, ponds, and lakes attached to the bottom. They also live in brackish water but not in the ocean. Most species of Chara extract calcium carbonate from the water and deposit it in their walls, thereby becoming rough and brittle. Fossils belonging to the Charophyceae have been identified in deposits of the Cretaceous and later geologic periods. There is some evidence of their existence even as far back as the Devonian.

Vegetative Body. The vegetative body of the stoneworts consists of a slender cylindrical stem bearing many short branches in whorls (Fig. 51 A). It grows erect and often reaches a height of 20 to 30 cm. The stem is attached to the substratum by means of colorless branched rhizoids. It is made up of short nodes and long unicellular internodes, the branches arising from the nodes. There are two kinds of branches: branches of unlimited growth, comprising the main axes, and branches of limited growth, the so-called leaves, in whose axils the main axes arise. All the cells contain numerous small spherical chloroplasts without pyrenoids. Reserve food is stored as starch.

Both kinds of branches grow by means of an apical cell, hemispherical in shape, that cuts off a longitudinal series of segments by successive transverse walls (Fig. olB). Each segment again divides transversely into two cells, the lower one becoming the long internodal cell and the upper one the nodal cell. The latter, by vertical divisions, gives rise to a plate of cells that produce the branches. The internodal cell, often attaining a length of 10 cm., may become coenocytic by fragmentation of its nucleus. Its cytoplasm gives a striking demonstration of protoplasmic streaming. In Chara the internodal cells become ensheathed by cells that

63

64

PLANT MORPHOLOGY

arise from the nodes and form a one-layered cortex. Half of the cortical cells are derived from the node below and half from the node above, the two halves meeting in a zigzag line midway between the nodes. In Niiella the Internodes remain uncovered.

Reproduction. No spores are produced in the Charophyceae. The nodes of the branches of limited growth bear unicellular branches and the sex organs, which are the most complex of all the algae. Most species are

Fig. 51. Chara. A, upper portion of plant, showing branches of Hmited and unlimited growth, natural size; B, median longitudinal section through the stem tip, showing promi- nent apical cell and alternating nodes and internodes derived from it, X200. The large internodal cell below is being ensheathed by a layer of cortical cells arising from adjacent nodes.

monoecious, an antheridium lying below an oogonium at the same node (Fig. 52). The antheridium is a stalked globular body that is brilliant red or yellow. It develops from a single initial cell that at first divides in three planes to produce octants. Each octant then undergoes two peri- clinal divisions to form an outer, a middle, and an inner cell. A jacket of eight triangular plate-like cells, called shields, is derived from the outer cells. The rapid enlargement of the shields results in the formation of a cavity within the antheridium. Projecting inward from the center of each shield is an elongated cell, the manubrium, that bears a rounded terminal cell, called a primary capitulum, which often divides in two. The manubria and primary capitula are derived from the middle and

THALLOPHYTA: ALGAE

65

inner cells, respectively, of the young antheridium (Fig. 53.4). The primary capituliim forms about six secondary capitnla. Each of these gives rise to a pair of long filaments consisting of 100 to 200 small cells, from every one of which a sperm is liberated (Fig. 53B). At maturity, the entire antheridium falls apart. A single antheridium of Chara produces 20,000 to 50,000 sperms. These are coiled and bicihate, resembling the sperms of bryophytes.

Fig. 52. Branch of Chara bearing an oogonium, with sterile jacket and crown, and an antheridium, with interlocking, shield-like wall cells, X50.

An oogonium is an enlarged apical cell. It produces a single large egg. A unique feature of the oogonium is the presence of five elongated, spirally wound cells that arise below and completely surround it (Figs. 52 and 53 A). At the top of the oogonium each jacket cell cuts off a small cell, these five cells forming a crown. In Nitella each spiral cell cuts off two crown cells, making ten in all. When the egg is ready for fertiliza- tion, the spirally twisted cells separate shghtly just below the crown, forming five slits through which the sperms enter the oogonium. After a sperm nucleus has united with the egg nucleus, the walls of the sur- rounding cells harden, the whole structure becoming nut-like. In this condition the zygote rests. Before germination, the fusion nucleus gives rise to four nuclei. Each probably has the haploid number of chromo- somes, although this has not been definitely established. Three of these

06

PLANT MORPHOLOGY

nuclei degenerate. Upon germination, the zygote sends out a simple green filament and a colorless rhizoid. The adult shoot arises from this filament as a lateral branch.

Summary. The Charophyceae are an aberrant group, standing apart from the other algae. They resemble the Chlorophyceae in containing an excess of chlorophyll over the carotinoids and in storing starch as reserve food. The vegetative body is distinctive, being an erect thallus differentiated into nodes and internodes and with two kinds of branches

A B

Fig. 53. Chara. A, longitudinal section of a young oogonium, invested by a sterile jacket, and a young antheridium, the latter consisting of a stalk cell, an outer layer of shield cells, four middle cells (manubria), and four inner cells (primary capitula), X200; B, a shield cell from a mature antheridium with manubrium projecting from it. At the tip of the manubrium is a primary capitulum to which are attached smaller secondary capitula, each bearing a pair of spermatogenous filaments.

arising at the nodes. There is no reproduction by spores. The sex organs are multicellular and complex, both being enclosed by a jacket of sterile cells. In this respect the Charophyceae resemble the bryophytes, although the development of the sex organs in the two groups is very different. The sperms are also like those of bryophytes.

9. PHAEOPHYCEAE

The Phaeophyceae, or brown algae, are nearly all marine in distribu- tion, occurring along most seacoasts but reaching their greatest display in cool waters. They range in color from olive green to dark brown as a result of the presence in their cells of chlorophyll and an excess of carotin and a unique xanthophyll, fucoxanthin, which is brown. There are no unicellular brown algae. Their multicellular bodies may be filamentous, plate-like, or may reach massive proportions and be highly differentiated in form. They are always attached. The Phaeophyceae are a special- ized group, probably derived independently from flagellate ancestors and apparently not related to any of the higher plants. There is no satis-

THALLOPHYTA: ALGAE

67

factory fossil evidence of their existence before the Jurassic. The Phaeophyceae number almost 1,000 species, nearly all of which are con- tained in six main orders: Ectocarpales, Sphacelariales, Cutleriales, Dictyotales, Laminariales, and Fucales.

1. Ectocarpales

The Ectocarpales include the simplest of the brown algae. They occur along all rocky seacoasts, growing attached to rocks, piers, and other plants. They include over 60 genera and 300 species, forming a diverse assemblage that is often broken up into several smaller orders. Of the many genera, perhaps the two that are best known are Ecto- carpus and Pylaiella.

Ectocarpus. Ectocarpus is a simple brown alga, widely distributed along seacoasts, where it grows attached to rocks or to other algae. It is filamentous and usually much branched, the older portions sometimes being surrounded by rhizoid- like branches. Otherwise the body is strictly mono sipho nous, each branch consisting of a single filament. An alga composed of parallel bundles of filaments is said to be polysiphonous. Growth of the filaments occurs mainly by intercalary cell divisions. Each cell contains a single nucleus and a number of small brown plastids.

Zoospores and isogametes are borne in spo- rangia and gametangia, respectively. These de- velop from the terminal cell of a short lateral branch, but may be either stalked or sessile. The sporangium is globular or somewhat elongated (Fig. 54A). It is unicellular and contains many (32 or 64) zoospores. It is at first uninucleate, becoming multinucleate and forming zoospores by cleavage of the cytoplasm. The gametangium

is longer than the sporangium and often ovate or cylindrical (Fig. 545). It is divided by cell walls into many small cubical cells, in each of which an isogamete is formed. Both the zoospores and gametes are laterally biciliate, the cilia being of unequal length. The pairing gametes are generally of the same size but, in some species, one is slightly larger than the other and swims less vigorously. Where this slight tendency toward heterogamy exists, all the gametes in a gametangium are either smaller (male) or larger (female). As in all the brown algae, the zygote germi- nates without going into a resting stage.

Fig. 54. A sporangium (A) and a gametangium (5) of Ectocarpus, X400; also a single escaped zoo- spore, more highly magni- fied.

68

PLANT MORPHOLOGY

■1^

M.

Ectocarpus displays a primitive type of alternation of generations and one that is not well established. Although all the plants of a species are alike vegetatively, some are gametophytes and some are sporophytes. The gametophytes, producing gametangia, are haploid. The zygotes give rise to sporophytes, which are diploid. These produce two kinds of sporangia. One is mvilticellular and looks like a gametangium but gives

rise to diploid zoospores that develop into other sporophytes. The other is the uni- cellular sporangium already described. The division of the nucleus in the young unicellular sporangium is reductional, and so the zoospores that it produces are haploid. These haploid zoospores always give rise to gametophytes. Some- times gametes, without pairing and fusing, develop directly into other hap- loid plants. It is apparent that much variation occurs in the behavior of the spores and gametes.

Pylaiella. This alga resembles Ecto- carpus in its habitat and general struc- ture. It differs chiefly in that the fila- ments are usually only slightly branched and any cell may become a sporangium or gametangium (Fig. 55). Conse- quently the reproductive organs are in- tercalary in position and usually appear in a linear series. They have the same structure as those of Ectocarpiis, the sporangia being unicellular and the gam- etangia multicellular. Sometimes mul- ticellular sporangia are produced on the plants bearing unicellular ones. Although Pylaiella is essentially isogamous, one of the pairing gametes is slightly larger than the other. An alternation of generations is seen also in this genus, the gamete- producing plants being haploid and the spore-producing plants diploid. The reduction of chromosomes occurs in the young unicellular sporan- gium.

Summary. The thallus of the Ectocarpales is usually composed either of freely branching filaments or wholly or in part of a plate-like or solid body composed of interlacing filaments. In some forms the thallus is parenchymatous. Vegetative growth is mainly intercalary, often being confined to basal portions of the branches. Reproduction occurs typi-

■C

■%

1»

A B

Fig. 55. A row of sporangia (A) and gametangia (B) oi Pylaiella, X500.

THALLOPHYTA: ALGAE 69

cally by zoospores borne in unicellular (and multicellular) sporangia and by motile isogametes borne in multicellular gametangia. There is an alternation of generations, the haploid plants being either similar to the diploid plants in size and vegetative structure, or much smaller and simpler.

A few heterogamous forms with unicellular sex organs borne on minute gametophytes, but otherwise resembling the Ectocarpales, are now segre- gated into two small orders: the Sporochnales and Desmarestiales. Some authors also segregate into the Chordariales, Punctariales, and Dictyo- siphonales isogamous forms with multicellular gametangia but with dis- similar haploid and diploid plants.

2. Sphacelariales

The Sphacelariales are a small but distinct order related to the Ecto- carpales. They are all littoral algae numbering 10 genera and 60 species, chiefly tropical but also occurring in temperate regions. The two chief genera are Sphacelaria and Stypocaulon.

Sphacelaria. This alga grows in small tufts attached to rocks and other algae. It occurs along both coasts of North America but is rather uncommon. The vegetative body is differentiated into a flat, plate-like, prostrate portion and a filamentous erect portion that is freely branched, the branches increasing in length by means of a large apical cell (Fig. 56). This cuts off a series of transverse segments that then divide both longi- tudinally and transversely to form a polysiphonous thallus. In most algae, growth is intercalary, which means that it occurs by division of all or many of its cells. Where there is an apical cell, all the cells of the body are descendants of it, even though some may later divide independently.

The sporangia and gametangia of Sphacelaria are similar to those of Edocarpus, the sporangia being unicellular and the gametangia multi- cellular. Both are short-stalked and borne on the axes. The zoospores and gametes are laterally biciliate and, in some species, one of the pairing gametes is slightly larger than the other. As in Edocarpus, there is an alternation of vegetatively similar generations and the number of chromo- somes is reduced one-half in the young unicellular sporangium. A form of vegetative reproduction common in Sphacelaria involves the production of propagules. These are short, flattened, modified branches that become detached and give rise to new plants.

Summary. The thallus of the Sphacelariales is filamentous, being monosiphonous near the tips and polysiphonous below. Growth takes place by means of an apical cell. Reproduction occurs by zoospores borne in unicellular sporangia and motile isogametes borne in multi- cellular gametangia. The order displays an isomorphic alternation of generations.

70

PLANT MORPHOLOGY

gmm

' ° " lyj ^.Ly. '

ti.ii4-j\'-i-

m

me

®

tm

B

Fig. 56. Sphacelaria, X200. A, tip of filament, showing large apical cell and segments derived from it; B, slightlj' older portion of thallus, showing development of branches.

3. Cutleriales

The Cutleriales are a very small order including only Cutleria, with 3 species, and Zanardinia, with 1. Both genera occur in the Mediterranean Sea, while Cutleria has been reported also from Florida, the West Indies, and the Gulf of California. The Cutleriales are more advanced than the two preceding orders, although apparently related to them.

Cutleria. The best-known species of Cutleria is found in the warmer parts of Europe. The plants grow just below the low-tide mark. Cut- leria displays a heteromorphic alternation of generations, the gametophyte and sporophyte being unlike \'egetatively. In fact, they are so different in general appearance that, before they were known to belong to the same Ufe history, they were placed in separate genera. The gametophyte was called Cutleria and the sporophyte Aglaozonia. The sporophyte is a small, flat, lobed disk several layers of cells in thickness and about 2 to 5 cm. in diameter (Fig. o7E). The lower side bears numerous rhizoids. On the upper side are enormous numbers of elongated unicellular spo-

THALLOFHYTA: ALGAE

71

rangia in crowded clusters. Each sporangium has a one-celled stalk and produces 8, 16, or 32 laterally biciliate zoospores (Fig. 57F). At first the sporangium has a single nucleus, the first two divisions of which result in a reduction of chromosomes. After three, four, or five simul- taneous free-nuclear divisions have occurred, uninucleate protoplasts are

Fig. 57. Cidleria muUifida. A, gametophyte, one-third natural size; B, male gametan- gium, X600; C, two female gametangia, X600; D, an egg and a sperm in the living condi- tion; E, voung sporophyte 30 days after fertilization; F, two nearly ripe sporangia, XGOO; G, a zoospore in the living condition. {A, after Thuret; D, E, G, after Yamanouchi.)

formed by cleavage of the cytoplasm. Thus the zoospores are haploid (Fig. blG).

The gametophytes, which are produced by the zoospores, are either male or female but are ahke vegetatively (Fig. 57.4). They are erect, ribbon-like, and dichotomously branched, reaching a length of about 20 cm. They are several layers of cells in thickness. The male and female gametangia, which are somewhat similar in appearance, are borne in

72 PLANT MORPHOLOGY

clusters on both sides of the thallus, intermixed with sterile hairs called paraphijses. Each has a short stalk and a number of gamete-producing cells. The male gametangium (antheridium) is a club-shaped organ consisting of over 200 small cells arranged in many tiers (Fig. 57 B). Each cell produces a single sperm. The female gametangium (oogonium) has fewer cells, about 20 to 60, each giving rise to an egg (Fig. 57C). The eggs are considerably larger than the sperms but both are laterally biciliate and free-swimming (Fig. 57 D). The eggs are less active than the sperms, however, and usually come to rest first. The zygote germi- nates at once, giving rise to a sporophyte.

Zanardinia. This genus differs from Cutleria in several ways. The gametophyte and sporophyte are ahke vegetatively, both being disk- like, several layers of cells thick, and about 5 cm. or more in diameter. Each sporangium produces four large bicihate zoospores, the reduction of chromosomes occurring when they are formed. The gametophytes are monoecious, the two kinds of gametangia being intermixed. The male gametangium produces about 250 sperms, the female gametangium about 12 to 36 eggs. The gametes resemble those of Cutleria, both being laterally biciliate.

Summary. The Cutleriales have a flat plate-like thallus that may be either erect or prostrate. Its growth is entirely or partially intercalary. The zoospores are borne in unicellular sporangia, the gametes in multi- cellular gametangia. The group has well-marked heterogamy, but both the sperms and eggs are ciliated. A distinct alternation of generations is present, the gametophyte and sporophyte being either vegetatively similar (Zanardinia) or dissimilar {Cutleria).

4. Dictyotales

The Dictyotales are a distinct group of brown algae occupying a some- what intermediate position with respect to the other groups. They are found in both tropical and temperate seas but always grow in warm waters. There are 18 genera and about 100 species. Dictijota, Padina, and Zonaria are well-known members.

Dictyota. Although found along both the Atlantic and Pacific coasts of North America, this genus does not occur north of about 35° latitude. The plants grow attached to rocks in tidepools and are always submersed. The vegetative body consists of a thin, flat, dichotomously branched thallus with a basal holdfast (Fig. 58). It is composed of three layers of cells: an upper and a lower layer of small photosynthetic cells with a layer of large colorless cells between them. The thallus grows by means of a large apical cell, one of which lies at the tip of each branch (Fig. 59) . The sporophyte and gametophyte are alike vegetatively, and so alternation of generations is isomorphic.

THALLOPHYTA: ALGAE

73

Numerous unicellular sporangia are scattered over both surfaces of the sporophyte (Fig. 60). Each sporangium, borne on a one-celled stalk, produces four nonmotile spores (apian ospores). In connection with the formation of four free nuclei from the single nucleus of the young spo- rangium, the number of chromosomes is reduced one-half. Two of the

Fig. 58. Dictyota binghamiae. Portion of plant showing dichotomous branching, three- fourths natural size.

Fig. 59. Longitudinal section of a bifurcating thallus of Dictyota dichotoma, cut parallel to its flat surface. The branch tip on the left shows a large undivided apical cell, while the one on the right has just undergone a second dichotomy.

spores from each sporangium give rise to male plants and two to female plants.

Like other members of the order, Dictyota displays well-developed heterogamy. The antheridia are borne in clusters of about 100 to 300 on both surfaces of the male plants (Fig. 61 A). The clusters are surrounded by several rings of sterile cells. Each antheridium is composed of a stalk

74

PLANT MORPHOLOGY

Fig. CO. Dictyota dichotoma. Cros.s section of thallus with a sporangium, showing three of the four spores. (After Mottier, Textbook of Botany, The Blakiston Company.)

B

Fig. 61. Sex organs of Dictyota dichotoma. A, cross section of thallus with group of antheridia; B, cross section of thallus with group of oogonia. {After Mottier, Textbook of Botany, The Blakistori Company.)

THALLOPHYTA: ALGAE

75

cell and about 1 ,500 small cells, each of which produces a sperm. Although the sperms are laterally biciliate, one cilium is very short. The oogonia are borne in groups of about 25 to 50 on both sides of the female plants (Fig. 615). The groups are not surrounded by sterile cells. Each

Fig. 62. Zonaria farlowii. A, portion of thallus with numerous groups of sporangia, Xl}i; B, young sporangium with eight free nuclei; C, mature sporangium, the eight aplanospores cut off by walls; D, young antheridia; E, young oogonia; F, mature antheridia; G, two mature oogonia; B to (?, X300. (After Haupt.)

oogonium consists of a small stalk cell and a single large nonmotile egg. The eggs are discharged into the water and there fertilized. The zygote gives rise to the sporophyte without undergoing any resting period.

Zonaria. Zonaria has about the same distribution along both coasts of North America as Dictijota. The thallus consists of an erect fan-like cluster of thin flat branches arising from a stalk-like portion that is

76

PLANT MORPHOLOGY

%

m

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attached by a disk-shaped mass of rhizoids (Fig. Q2A). It grows by- means of a row of apical cells extending around the distal margin of each branch. The mature thallus is about eight layers of cells in thickness.

The diploid sporophytes bear groups of sporangia intermixed with paraphyses. Each sporangivim, lack- ing a stalk cell, gives rise to eight large haploid aplan- ospores (Fig. 625, C). These produce the gameto- phytes, which are either male or female and resemble the sporophytes vegetatively. The antheridia and oogonia are, in general, similar to those of Dictyota (Fig. 62Z)-G). The zygote produces a sporophyte.

Summary. The thallus of the Dictyotales is flat, plate-like, and erect. It grows by means of a single apical cell or a marginal row of apical cells. Repro- duction occurs by aplanospores, four or sometimes eight being developed in a unicellular sporangium, and by heterogametes. Small biciliate sperms are borne in multicellular antheridia and large nonmotile eggs are borne singly in unicellular oogonia. A distinct alternation of generations is present, the gam- etophyte and sporophyte being similar vegetatively.

5. Laminariales

The Laminariales comprise the kelps, the largest of the brown algae. They are widely distributed throughout temperate and arctic regions, occurring mainly in cool waters and making their greatest dis- play along shores bordering the North Pacific Ocean. Most of the Laminariales grow below the low-tide line. They include about 30 genera and 100 species. Some of the best-known members are Laminaria, Macrocijstis, Nereocystis, Postelsia, and Egregia. Laminaria, with 30 species, is the largest genus.

Laminaria. Common along both coasts of North America, in cooler waters, are various species of Laminaria. Some are not more than 30 cm. long, while others reach a length of 9 to 12 m. They live attached to rocks just below the low-tide line. Alter- nation of generations is heteromorphic. The large vegetative plant is a sporophyte (Fig. 63). It consists of a long blade and a thick leathery stipe anchored by means of a branching basal holdfast. According to the species, the blade may be entire or divided lengthwise into segments. The cells of the stipe show a differentia-

i €

Fig. 63. Laminaria, a small kelp with a blade, stipe, and holdfast, about one- half natural size.

THALLOPHYTA: ALGAE

77

tion into an outer cortical region of photosynthetic tissue and a central pith that usually contains storage cells. Many of the central cells are elongated and have pores in their end walls, thus resembling the sieve tubes of vascular plants. Vegetative growth is not apical but results

WMMMM^'^M

Fig. 64. Sporangia of Lamiiiaria, intermixed with paraphyses, X400.

Fig. 65. Gametophjtes oi Laminaria yendoana. A and B, male gametophytes, X 1,200; C, a sperm, X 1,200; D, E, F, female gametophytes, X800; a, antheridia, some empty; e, egg; o, oogonia; s, young sporophytes arising from the fertilized egg. (After Kanda.)

from the activity of a meristem situated at the junction of the blade and stipe. The meristem forms a new blade each year, replacing the old one, which dies off.

Numerous unicellular, club-shaped sporangia, intermingled with long

IP

78

PLANT MORPHOLOGY

sterile cells (paraphyses) , arise in large pat ches on both sides of the thallus (Fig. 64). They produce 32 or 64 small, laterally biciliate zoospores. The reduction of chromosomes results from the division of the nucleus of the young sporangium. After four or five simultaneous free-nuclear divisions have taken place, the contents of the sporangium undergoes cleavage into uninucleate protoplasts, the zoospores. These are liberated

Fig. 66. Apical portion of a plant of Macrocystis pyrifera, one-fifth natural size.

and develop into minute male and female gametophytes (Fig. 65). The sperms are laterally biciliate and are borne singly in antheridia that arise at the ends of short, branched filaments. The female plant usually con- sists of only a few cells, one of which becomes an oogonium. This pro- duces a single nonmotile egg that is extruded through a terminal pore, to which it remains attached. The zygote germinates at once, giving rise to the large sporophyte.

THALLOPHYTA: ALGAE

79

Other Kelps. As in Laminaria, the bodies of nearly all the other kelps are differentiated into holdfast organs, stout stalks, and flat blades often much divided into narrow segments. Air bladders are freciuently present. Reproduction is similar in all members of the order. The greatest variety and largest of the kelps occur along the Pacific coast of North America, where they live in water 10 to 30 m. deep, their stalks attached to rocky reefs and their blades often floating on the surface. Macrocijstis may reach a length of 30 to 50 m. A sin- gle plant consists of a stalk with many blades, each blade having a float (Fig. 66). Another large kelp is Nereocijstis, with a large hollow bulb at the end of a thick stalk and a number of blades arising from the bulb (Fig. 67). It reaches a length of 25 to 30 m. Postelsia, known as the "sea palm," has a stout stalk up to 60 cm. long bearing at its tip nu- merous branches terminating in nar- row blades (Fig. 68). Egregia, the "feather-boa kelp," has a long stalk that bears two rows of lateral blades and floats, the blades producing sporangia being much narrower than the sterile ones.

Summary. The vegetative body of the Laminariales is highly differ- entiated both externally and inter- nally. It consists of a massive thallus usually with a holdfast, stipe, and one or more blades. Growth is due to an intercalary meristem. The large plant body is a sporo- phyte bearing unicellular sporangia that contain many zoospores. The gametophytes are microscopic, dioecious, and heterogamous. The sperms are biciliate and produced singly in unicellular antheridia. The eggs are nonmotile and borne in unicellular oogonia. The Laminariales have a heteromorphic alternation of generations.

Fig. 67. Young plant of Nereocystis luetkeana, one-quarter natural size.

80

PLANT MORPHOLOGY

Fig. 68. A sea palm {Postelsia palmaefonnis) growing on a rock exposed at low tide, about one-quarter natural size.

6. Fucales

The Fucales, commonly known as rockweeds, are a highly specialized order of brown algae standing apart from the others. They are widely distributed throughout tropical and temperate regions, most of them growing along rocky seacoasts in the intertidal zone. They comprise 32 genera and 325 species, representative forms being Fucus, Pelvetia, Ascophyllum, and Sargassum.

Fucus. Fucus is widely distributed in cool waters, being represented along both the eastern and western coasts of North America. The thallus, rarely exceeding a meter in length, is coarsely ribbon-like and

THALLOPHYTA: ALGAE

81

repeatedly forked, with a basal stalk arising from a disk-like holdfast (Fig. 69). It is rather tough and leathery. In some species air bladders, giving buoyancy to the plant, are conspicuous. Growth occurs by means of an apical cell that occupies a notch at the end of each branch. The

Fig. 69. size.

Fucus furcatus. Portion of thallus, showing conceptacles, two-thirds natural

apical cell is complex, having the form of a truncated quadrangular pyramid and cutting off cells in three planes. When the thallus branches, the apical cell divides vertically into two nearly equal parts, each of which becomes the apical cell of a new branch. Internally the thallus is differ- entiated into a firm outer cortex of photosynthetic tissue and a central colorless pith that is rather spongy. The only method of asexual repro- duction is by fragmentation of the thallus. There are no spores of any kind.

82

PLANT MORPHOLOGY

Within the swollen tips of some of the branches are numerous flask- shaped pits or chambers, called conceptades, each with a pore-like opening (Fig. 70). Sperms and eggs are produced inside the conceptades, the sperms in antheridia and the eggs in odgonia. The antheridia are oval

^^

Fig. 70. Longitudinal section of a conceptacle of Fucus furcatus, showing oogonia in various stages of development, small branching filaments bearing antheridia, and numerous paraphyses, X 100.

and sac-like; they appear on special branching filaments that arise from the Avail of the conceptacle (Fig. 7L4). Each antheridium produces 64 small, laterally biciliate sperms. The antheridium is unicellular and, when young, is uninucleate. The number of chromosomes is reduced one-half when its nucleus divides. Free-nuclear divisions continue until there are 32 nuclei. Then the cytoplasm undergoes cleavage to form an equal number of uninucleate protoplasts, each of which divides again to

THALLOPHYTA: ALGAE

83

produce two sperms. The sperms escape from the antheridium in a mass surrounded by a membrane that soon disappears.

The eggs of Fucus are borne in groups of eight inside the oogonia, which are large oval or globular cells, each of which has a one-celled stalk (Fig. 71B-D). The young oogonium has a single nucleus, the division of which is reductional. Three simultaneous divisions result in the formation of eight free nuclei. Cytoplasmic cleavage follows and an egg is organized around each nucleus. The eggs are extruded from the oogonium in a

A -^""-^ D ' ^^- E

Fig. 71. Sex organs oi Fucus furcatus. .-i, antheridial filament, X 320; B, young oogonium with four nuclei, X160; C, longitudinal section of an older oogonium with eight nearl,\' mature eggs, X160; D, mature oogonium, X160; E, escaped egg of Fucus vesiculosus sur- rounded b>' numerous sperms, X 240. (E, after Thuret.)

group surrounded by a membrane that soon ruptures. In Ascophylhun four eggs are organized in an oogonium, in Pelvetia two, and in Sargassum only one. In all the genera of Fucales, however, eight nuclei always arise in the oogonium, the nonfunctional nuclei either being extruded or degenerating. Thus Fucus represents the primitive condition from which the other genera, by progressive reduction, have been derived.

Depending on the species, the antheridia and oogonia of Fucus may occur in the same conceptacle, in different conceptacles on the same plant, or on different plants. In addition to the sex organs, the conceptacles contain numerous unbranched sterile filaments (paraphyses), some of which often project through the pore. Both the sperms and eggs escape from the conceptacles into the water but only the sperms are motile. The sperms surround the eggs in such vast numbers that they cause them to rotate (Fig. HE). After fertilization has taken place, the zygote sur-

84

PLANT MORPHOLOGY

rounds itself with a cell wall and divides at once to produce a new vege- tative thallus. The reduction of chromosomes occurs when the nucleus of the young antheridium and that of the young oogonium divide. Thus from the four-nucleate stage to maturity the sex organs are haploid, the diploid condition arising at fertilization.

Although Fucus has no alternation of gametophyte and sporophyte plant bodies, there is a brief haploid phase and a prolonged diploid phase. Some botanists interpret the vegetative body of the Fucales as a sporo- phyte, the antheridia as microsporangia, and the sperms as microspores (small zoospores). They interpret the oogonia as megasporangia and the eggs as mega- spores (large aplanospores). Then, to explain the sexual fusion, the microspores and megaspores are said to function di- rectly as gametes. This interpretation implies that a gametophyte generation was once well developed and has become so reduced that it comprises only the haploid nuclei in the gametangia and the gametes themselves.

Sargassum. This is a very large genus whose 250 species are widely distributed throughout tropical and subtropical seas, especially in the Southern Hemisphere. The vegetative body is more highly de- veloped than that of Fucus, having distinct branches, leaf-like blades, and often small stalked air bladders as well (Fig. 72). Sargassum may live either in an attached or a floating condition. Like other rockweeds, it grows chiefly along seacoasts, but frequently plants are torn loose from the rocks and carried for hundreds of miles out to sea. The Sargasso Sea is a vast eddy lying west of the Canary Islands. Here great floating masses of "gulfweed," transported by the Gulf Stream from the West Indies and tropical America, accumulate and propagate themselves by fragmentation of the thallus.

Summary. The Fucales have a coarse, ribbon-like thallus that grows by means of an apical cell. Spores are not formed. The order displays well-developed heterogamy. The sex organs are unicellular, the anther- idia producing numerous biciliate sperms, the oogonia producing one, two, four, or eight nonmotile eggs that escape before fertilization. The sex organs are borne in internal cavities (conceptacles). The Fucales are without a distinct alternation of generations.

Fig. 72. Small portion of a plant of Sargassum, showing differentiation into stem, leaf-like blades, and berry-like air bladders, natural size.

THALLOPHYTA: ALGAE 85

Summary of Phaeophyceae

The Phaeophyceae are algae having in their plastids an excess of carotin and a brown xanthophyll pigment (fucoxanthin) over the chloro- phyll. All of them are multicellular, the thallus being filamentous, plate-like, or massive, often with differentiated tissues. The cells contain a definite nucleus, generally several or many plastids, and a distinct cell wall. Reserve food is stored chiefly as laminarin (a dextrin-like carbo- hydrate) or oil. Except in the Fucales, zoospores are produced or, in the Dictyotales, aplanospores. Gametic reproduction may occur either by isogametes or heterogametes. In the heterogamous forms the eggs may be ciliated but are generally nonciliated. All motile reproductive cells are laterally biciliate, the cilia being unequal in length. No resting cells are formed. Most members exhibit an alternation of generations, the Fucales, with only a diploid plant body, being a notable exception. The gametophyte and sporophyte are either similar or dissimilar vegetatively.

10. RHODOPHYCEAE

Like the Phaeophyceae, the Rhodophyceae, or red algae, are almost all marine in distribution but, as a rule, live in deeper and warmer waters than the brown algae. They include the majority of the seaweeds. Most of the Rhodophyceae are rose red or violet, but some are dark purple, reddish brown, or olive green. In addition to chlorophyll and its associated carotinoids, a red pigment, phycoerythrin, is present in the cells. This more or less obscures the chlorophyll. Many of the Rhodo- phyceae also contain a small amount of phycocyanin, the blue pigment of the Cyanophyceae. Except for several unicellular forms, whose inclusion in the group is doubtful, all the red algae are multicellular. Their bodies are not large, most of them being less than 30 cm. in length, while only a few are as long as 1 m. They are rather varied in form, how- ever, being fdamentous, ribbon-like, or plate-like, but never massive. They are always attached. Some are heavily impregnated with lime. Lime-secreting forms are known as fossils as far back as the Ordovician. The Rhodophyceae are the most highly specialized of all the algae. They are probably not related to any of the higher plants except, perhaps, to some of the fungi. They include about 3,000 species.

There are seven orders of Rhodophyceae. These, together with one or more representative genera, are as follows: (1) Bangiales — Bangia, Porphyra,'Porphyridium; (2 )Nemalionales — Nemalion, Batrachospermum; (3) Gelidiales — Gelidium; (4) Cryptonemiales — Corallina, Lithothamnion; (5) Gigartinales — Plocamium, Gracilaria, Chondrus, Gigartina; (6) Rhody- meniales — Rhodymenia; (7) Ceramiales — Callithamnion, Ceramium, Poly- siphonia, Delessaria.

86

PLANT MORPHOLOGY

Porphyra. Poiyhyra is a typical member of the Baii^iales, the most primitive order of Rhodophyceae. It is widely distributed along rocky seashores, occurring on both coasts of North America. It grows in the intertidal zone on rocks and other algae. The thallus is plate-like and attached by means of a small basal holdfast (Fig. 73A). It is only one or

S^'as).

E

•<^^j

Fig. 73. Porphyra perforata. A, thallus, one-half natural size; B, vertical section of vegetative portion of thallus; C and D, vertical sections of thalli with carpogonia and developing carpospores; E, surface view, showing liberation of carpospores; F, amoeboid carpospores; G, vertical section through portion of a thallus liberating spermatia. {From Gilbert M. Smith.)

two layers of cells in thickness and, in most species, is less than 50 cm. long. The cells lie in a tough gelatinous matrix derived from their walls. They are without apparent cytoplasmic connections (Fig. 73B). Each cell has a nucleus that divides by a primitive type of mitosis. It also has a single large plastid with a central pyrenoid.

Some species of Porphyra are monoecious but most of them are dioe- cious. The antheridia develop directly from the vegetative cells. A cell undergoes repeated divisions in three planes until 64 or sometimes 128 small cells are formed. The walls gelatinize and free the protoplasts, which function as male gametes (Fig. 73G). Such naked, nonmotile male

THALLOPHYTA: ALGAE 87

cells, a feature of all the red algae, are called spermatia. The female sex organs, or carpogonia, also arise from ordinary vegetative cells hut without undergoing division, the protoplast functioning directly as an egg (Fig. 730.

A spermatium, carried by water currents to the carpogonium, enters and fuses with the egg. The zygote divides at once to form a group of spores, usually 8 or 16, that are freed by the breaking down of the surround- ing cell walls (Fig. 7SC-E). These naked cells are carpospores and, like the gametes, are nonciliated. The freed carpospores exhibit an amoeboid movement (Fig. 73F). After coming to rest, each carpospore forms a cell wall and develops into a new thallus. The reduction of chromosomes occurs when the zygote germinates, and so the vegetative plant is haploid.

As compared with the higher orders of Rhodophyceae, the Bangiales have a simple type of nucleus and cells without evident protoplasmic continuity. They display intercalary rather than apical growth. The carpogonium either lacks a trichogyne or has a very short one. The zygote is transformed directly into carpospores. An alternation of generations is not present.

Porphyridium is a unicellular alga whose relationships are uncertain. It forms a reddish gelatinous layer on damp soil and moist walls. It was formerly placed in the Cyanophyceae but, because it has a true nucleus and a distinct plastid, is now included in the Rhodophj^ceae. The cells are spherical and surrounded by a mucilaginous matrix. Fission is the only know^n method of reproduction.

Nemalion. Although showing a considerable advance over Porphyra, this form is much simpler than members of the higher orders. Nemalion is wddely distributed, growing on rocks between the high- and low-tide lines. The thallus, up to 60 cm. in length, consists of a slimy mass of branching filaments that are interwoven to form a worm-like cylinder. This is composed of a central core of slender colorless filaments from which tufts of larger chlorophyllose filaments radiate outward. The cells of the latter have a small nucleus and a large plastid with a con- spicuous pyrenoid. As in all the algae except the Bangiales, growth is apical. Furthermore, the vegetative protoplasts are connected by a conspicuous strand of cytoplasm that passes through a pore in the cell wall.

Nemalion is monoecious, the sex organs occurring at the ends of short branches. The antheridium consists of a single small cell that is budded off laterally from an antheridial branch (Fig. 74A). Its protoplast, the spermatium, is discharged into the water. The female organ, called a procarp, consists of two parts, the carpogonium and the trichogyne (Fig. 745). The protoplast of the carpogonium functions as an egg. The trichogyne is a long thread-Uke cell at the upper end of the carpogonium.

88

PLANT MORPHOLOGY

Its nucleus degenerates. The nonmotile male cell, or spermatium, is carried by water currents. After coming in contact with the trichogyne, it becomes binucleate. Both of the male nuclei may enter the trichogyne but only one passes into the carpogonium, where it fuses with the egg nucleus. The other male nucleus does not function.

Fig. 74. Nemalion Inhricitm, X 700. A, portion of plant with four antheridial branches consisting of groups of small cells, each producing a single spermatium; B, a carpogonial branch, terminating in a carpogonium with its slender trichogyne to which a spermatium is attached; C, a cjstocarp, composed of fertile filaments that cut off terminal carpospores.

Following fertilization, many short filaments, called gonimohlasts, arise from the carpogonium and at the tip of each a carpospore is organized. After a carpospore is shed, another may be cut off from the cell behind it. It is only in the Bangiales that carpospores are produced by direct division of the zygote. In Nemalion the carpogonium, fertile filaments, and carpospores collectively form the cystocarp (Fig. 74C). The carpospores, upon being set free as naked, nonmotile cells, develop into sexual plants.

THALLOPHYTA: ALGAE

89

The reduction of chromosomes occurs just after fertihzation, when the fusion nucleus in the carpogonium divides. Thus the gonimoblasts, carpospores, and sexual plants are hapioid. There is no true alternation of generations.

Batrachospermum. This is a widely distributed fresh-water alga, growing in streams attached to rocks along the bottom. The plants are blue-green, olive green, violet, or reddish. The variation in color is primarily a result of differences in light intensity, plants growing in shallow water being greener than those in deeper water. Bafrachospcrnmm is related to Nemalion but differs from it in several respects. The vegetative body consists of long branching filaments of unUmited growth bearing whorls of dwarf branching fila- ments of limited growth (Fig. 75). The long filaments consist of an axial row of cells which, in the older portions of the body, is covered by a layer of small- celled filaments that form a sheath around it. The cells of the sheath arise from the basal cells of the dwarf fila- ments. Growth occurs by means of an apical cell.

The sex organs are borne at the ends of some of the dwarf filaments and re- semble those of Nemalion (Fig. 76). After coming in contact with the trichogyne, the spermatium remains uninucleate instead of becoming binucleate. Moreover, following fer- tilization, the cells at the base of the carpogonium send out loose fila- ments that grow up around and invest the cystocarp while the carpo- spores are being produced.

The germinating carpospore gives rise to a branching filamentous body that is much simpler than the gamete-producing plant. This plant, which represents a juvenile stage in the life cycle, may multiply by mono- spores, which are formed singly within sporangia at the ends of short lateral branches. Finally a special branch appears that becomes a gamete-producing plant. As in Nemalion, the chromosome number is reduced one-half when the fusion nucleus divides in the fertilized carpogonium. Conseciuently, the juvenile plant is not a sporophyte and there is no alternation of generations.

Polysiphonia. Polysiphonia is a widespread genus of about 150 species. It is abundant along the Atlantic coast of North America but is less com-

FiG. 75. Small portion of the vegetative body of Batrachospermum, showing dwarf filaments arising in whorls from the cylindrical main axis, X400.

90

PLANT MORPHOLOGY

mon along the Pacific coast. It grows in tide pools on rocks and on other algae. It is a more highl}^ developed but more typical red alga than any of the others that have been discussed. The plant body, reaching a length of 25 cm., is filamentous and polysiphonous, being made up of an

F G H

Fig. 76. Batrachosperrrmm. A, antheridial branch with globular antheridia, one of which has liberated its protoplast; B, young carpogonial branch, the terminal cell forming the carpogonium and trichogyne; C, mature procarp with nucleated carpogonium and swollen trichogyne; D, later stage, showing spermatium united with tricliogyne and male nucleus fusing with carpogonial nucleus; E, completed fusion of male and female nuclei; F, develop- ment of gonimoblasts from carpogonium, a sterile branch arising on the left; G, further development of gonimoblasts and sterile filaments; H, formation of carpospores; A to F, X960; G and H, X720. {After Kylin.)

axial row of elongated cells surrounded and completely covered by several rows of smaller peripheral cells that are cut off from the central cells by longitudinal divisions. Growth occurs by means of an apical cell.

The reproductive features of Polysiphofiia are complex. Nonmotile

THALLOPHYTA: ALGAE

91

A C

Fig. 77. Polysiphonia. ^, portion of plant bearing tetraspores, X 160; B, portion of male plant bearing clusters of antheridia, X200; C, portion of female plant with cystocarp con- taining carpospores; also a single carpospore, X80.

spores are formed in groups of four in a one-celled sporangium (Fig. 77 A). They are called tetras'pores. Chromosome reduction occurs in connection with the formation of the tetraspores, and so each is haploid. Upon germination, two tetraspores from each sporangium produce male plants and two produce female plants. These sexual plants are like the tetrasporic ones in general appearance.

The antheridia occur in dense clusters on special lateral branches of the male plants (Fig. 775). In their formation, a number of cells arise laterally from the cells of the axial filament, giving rise to a simple mono- siphonous branch. Each cell of this branch develops two-celled lateral branches. An oblique division of the terminal cell of each branch produces a unicellular antheridium (Fig. 78). The antheridium discharges its protoplast, which functions as a nonmotile male cell, or spermatium. Other antheridia may then be budded off the same terminal cell.

Besides the carpogonium and trichogyne, the procarp includes several other cells as well. It arises from a large 'pericentral cell that first produces a row of four cells, the terminal one becoming the

Fig. 78. Diagram of antheridial branch of Polyalphonia, showing three stages in the development of an antheridium (o), whose protoplast functions as a male cell. {After Yamanouchi.)

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PLANT MORPHOLOGY

carpogonium (Fig. 79.4, B). The nucleus of the carpogonium divides into two nuclei, one of which passes into the trichogyne and finally dis- integrates, while the other remains in the carpogonium and functions as an egg nucleus. The pericentral cell also gives rise to a group of auxiliary cells, one of which crowds in between the pericentral cell and the carpo- gonium (Fig. 79C). The entire structure comprises the procarp.

The free-floating spermatium, coming in contact with the trichogyne, remains uninucleate. The male nucleus enters the trichogyne, passes

Fig. 79. Diagrams showing development of procarp of PoJysiphonia. A, early stage: B, later stage, the pericentral cell (p) having produced four cells, the terminal one forming the carpogonium (c) and trichogyne (0 ; C, mature stage, a group of auxiliary cells having developed from the pericentral cell. (After Yamanouchi.)

into the carpogonium, and fuses with the female nucleus. A passageway to the pericentral cell now is opened through the intervening auxiliary cell and the fusion nucleus passes through. Then all the cells of the procarp unite and the fusion nucleus divides to form many nuclei. Lobes into which these nuclei pass are put out from the procarp and then the carpospores are cut off. The whole structure comprises the cystocarp (Fig. 77C). The usual envelope of sterile cells is formed around it.

After escaping from the cystocarp, a carpospore gives rise to a tetra- sporic plant. A stalked sporangium is produced laterally from an axial cell, pushing through the cortical cells. The tetraspores give rise to sexual plants. Polysi'phonia displays an isomorphic alternation of generations. The sexual plants, with haploid cells, are gametophytes. The tetrasporic plant, with diploid cells, is a sporophyte. The sporophyte generation, however, beginning with the zygote, includes also the cysto-

THALLOPHYTA: ALGAE 93

carp and carpospores. The gametophyte generation begins with the tetraspore.

Summary. In the Rhodophyceae both chlorophyll, with its associated carotinoids, and a red pigment (phycoerythrin) are present in the plastids. With only a few possible exceptions, all members are multicellular, the thallus being most commonly filamentous but often plate-like. The cells contain a definite nucleus (sometimes more than one), one or more plastids, and a cell wall that is often gelatinous. Reserve food is stored chiefly as "fioridean starch." Reproduction occurs by means of aplano- spores and heterogametes. The female organ is a carpogonium. Carpo- spores arise from the zygote, either directly (in the Bangiales) or indi- rectly (in the other Rhodophyceae). Except in the Bangiales and Nemalionales, both carpospores and tetraspores are produced, the latter by a sporophyte. All reproductive cells are nonciliated. No resting cells are formed. A distinct alternation of generations is a feature of all red algae except the Bangiales and Nemalionales. The gametophyte and sporophyte are similar vegetatively.

COMPARISON OF THE CLASSES OF ALGAE

The most important distinguishing characters of the ten classes of algae are as follows:

Cyanophyceae. Cells containing, in addition to chlorophyll and carotinoids, a blue pigment (phycocyanin) and frequently a red pigment (phycoerythrin) also. Pigments not confined to definite plastids. Reserve food stored as glycogen. Plant body unicellular, generally colonial. Cells without a well-organized nucleus. Cell walls usually forming abundant mucilage. Reproduction by fission, never by zoo- spores or gametes No ciliated cells.

Euglenophyceae. Cells with green plastids containing an excess of chlorophyll over the carotinoids; frequently colorless. Reserve food stored as paramylon. Unicellular and usually solitary; sometimes colonial. Cell walls almost always absent. Free-swimming or, when colonial, attached. Reproduction by fission, rarely by isogametes (?). Motile cells with one or two ciha attached anteriorly, equal or unequal.

Chrysophyceae. Cells with golden brown plastids containing chloro- phyll and an excess of carotinoids; sometimes colorless. Food stored as oil or leucosin. Unicellular and often colonial, rarely multicellular. Cell walls almost always absent. Free-swimming or sometimes attached. Reproduction by fission, and sometimes by zoospores, rarely by isoga- metes (?). Motile cells with one or two cilia attached anteriorly, equal or unequal.

Dinophyceae. Cells with yellow-brown plastids containing chloro- phyll and an excess of carotinoids, or colorless, storing food as starch or

94 PLANT MORPHOLOGY

oil. I^nicelhilar and mostly solitary, rarely multicellular. Sometimes naked but usually with sculptured cell walls. Nearly all free-swimming. Keproduction by fission, and sometimes by zoospores, rarely by isoga- metes (?). Motile cells generally with two laterally attached cilia, one lying in a transverse groove.

Xanthophyceae. Cells with yellow-green plastids containing a larger proportion of carotinoids than chlorophyll. Reserve food stored as oil or leucosin. Unicellular (and uninucleate), (;oenocytic, or multicellular. Cell walls often absent, when present usually consisting of two over- lapping halves. Reproduction by fission or by spores and isogametes. Motile cells with two unecjual cilia attached anteriorly.

Bacillariophyceae. Cells with golden-brown plastids containing an excess of carotinoids over chlorophyll and storing food mainly as oil. Unicellular and either solitary or colonial. Cell w^all consisting of two overlapping valves, highly silicified. Reproduction by fission, auxo- spores, and isogametes. Motile cells rare.

Chlorophyceae. Cells with plastids containing a greater proportion of chlorophyll than carotinoids. Reserve food usually stored as starch. Unicellular (and uninucleate), coenocytic, or multicellular. Unicellular forms solitary or colonial. Reproduction by fission or by spores and either isogametes or heterogametes. Motile cells generally with two or four cilia attached anteriorly.

Charophyceae. Cells with plastids containing a greater proportion of chlorophyll than carotinoids; usually storing food as starch. Multi- cellular. Reproduction by heterogametes formed in complex multi- cellular sex organs of a distinctive type. Sperms with two equal cilia attached anteriorly.

Phaeophyceae. Cells with plastids containing chlorophyll and an excess of carotin and a brown pigment (fucoxanthin). Reserve food occurring chiefly as laminarin or oil. Multicellular. Reproduction by spores and either isogametes or heterogametes. Motile cells laterally biciliate, the cilia of unequal length.

Bhodophyceae. Cells with plastids containing chlorophyll, carotinoids, and a red pigment (phycoerythrin). Reserve food stored chiefly as "floridean starch." With rare exceptions multicellular. Reproduction by spores and heterogametes, these never ciliated.

GENERAL CONCLUSIONS

The algae constitute the simplest and oldest group of green plants. Their bodies are adapted, both in structure and function, to live in water. Although knowledge is lacking concerning the nature of the first plants to have Uved on the earth, they must have been aquatic and may have been similar to some of the existing blue-green algae. These plants are

THALLOPHYTA: ALGAE 95

unicellular, have a very primitive cell structure, and reproduce by fission. Because they live in a variety of habitats, including hot springs, they may have lived on the earth before conditions were favorable for the existence of other green plants.

The algae are not a homogeneous assemblage l)ut embrace a number of groups representing divergent lines of descent, all of which probably have had a common origin. Advanced students of the algae try to trace these lines of evolution, but we shall be concerned mainly with certain general tendencies and with the progress that the group as a whole has made.

Development of Multicellular Bodies. The unicellular plant body obviously represents the simplest condition of structural organization and, necessarily, also the oldest. It is characteristic of all the blue-green algae, flagellates, dinoflagellates, diatoms, and many of the green algae. Unicellular plants may be either solitary or colonial, the latter condition having arisen from the tendency of cells, following division, to remain together for a while before separating. In the evolution of the algae, close association of cells in a colony may have led to a dependence of the cells on one another, with the resultant establishment of a multicellular body. It is significant that, among the algae, no sharp distinction exists between highly organized colonies and simple multicellular plants. This intergradation strongly indicates that multicellular plants have been derived from unicellular ones through the formation of colonies.

Although the multicellular bodies of algae are very diverse in form, they may be referred to three main types: filamentous, plate-like, and massive. The filamentous type is most common, probably because it seems best adapted to aquatic life. In such a body all the cells are in direct contact with water and the absorbing surface is very large. Thus the absorption of gases is greatly facilitated. The massive type of body, as exemplified by many brown algae, is adapted to withstand the buffeting action of waves and water currents along rocky seacoasts. In the simple multi- cellular algae growth is intercalary, every cell having the power of divi- sion. In branching forms growth is often limited to definite regions, such as the terminal cell of each branch. In many brown and most red algae growth occurs by means of an apical cell that cuts off a series of posterior segments.

An important evolutionary tendency exhibited by the algae is for certain cells to become structurally differentiated in response to special functions. It occurs in both colonial and multicellular forms. A simple expression of this tendency is seen in those filamentous algae having the basal cell modified as a holdfast. In many branching filaments the cells of the branches are smaller than those of the main filament. The forma- tion of sporangia and gametangia represents a specialization of certain

96 PLANT MORPHOLOGY

cells for reproduction. Differentiation becomes marked among the brown algae, especially in the Laminariales and Fucales, where the body- consists of distinct vegetative organs within which simple tissues may be formed. A highly differentiated vegetative body is also characteristic of the Charophyceae and many marine Siphonocladiales and Siphonales.

Asexual Reproduction. In reproduction, as in vegetative structure, the algae show a progressive advance. Most unicellular forms increase in number by fission, which is merely reproduction by cell division and is obviously the most primitive method in the plant kingdom. Among multicellular forms cell division results in growth and, to make repro- duction possible by other means than fragmentation, cells must be Ub- erated from the parent. The spores of algae are merely detached cells with the capacity of directly producing a new plant. They result not only in a multiplication of individuals but in their widespread distribu- tion. Spores may be formed from a cell with or without previous division of its protoplast. The commonest kind of spores in the algae are zoo- spores— naked cells ' with cilia. Nonmotile spores with a cell wall (aplanospores and akinetes) are generally formed in response to adverse environmental conditions, to which they are very resistant. Obviously they have been derived from zoospores that have lost the power of loco- motion. The same may be true of the nonmotile spores of the red algae. Fission and spores produced by a haploid plant body are a means of accompHshing vegetative or asexual reproduction because no reduction of chromosomes is involved. This is the only kind present in the blue- green algae, flagellates, dinoflagellates, many diatoms, and a few green

algae.

Like vegetative spores, the spores produced by two successive divisions of a diploid cell, involving a reduction of chromosomes, are usually regarded as asexual. In reality, however, they belong to the sexual life cycle, since meiosis is always a necessary conseciuence of a previous gametic union. Although such spores are functionally equivalent to the zoospores and aplanospores produced by a haploid plant body, they are not homologous with them, and should be designated as meiospores. Meiospores are produced by the zygote in such green algae as Ulothrix and Oedogonium, and by the sporophyte in all algae with an alternation of generations.

In most green algae any ordinary vegetative cell is capable of producing spores. In nearly all the brown and red algae, however, spores are not borne in transformed vegetative cells but in sporangia, which are cells specialized for reproduction. Sporangia differ from ordinary vegetative cells in size or shape and sometimes are restricted to definite parts of the

body.

Sexual Reproduction. Sexual reproduction is accomplished by gametes and represents a distinct advance over reproduction by vegeta-

THALLOPHYTA: ALGAE 97

tive spores. Its essential feature is the fusion of two cells to form a zygote. It seems certain that originally gametes were derived from vegetative zoospores that had become too small to form a new plant directly. This is shown by the fact that in Ulothrix and many other isogamous algae zoospores and gametes intergrade, the smaller spores often germinating but producing dwarf filaments. The derivation of gametes from zoospores is shown also by the striking similarity between them in form and in the number and arrangement of their cilia. Any peculiarity in the spores is duplicated in one or both of the gametes, as in Oedogonium, the brown algae, etc. With few exceptions (notably the Conjiigales, Charophyceae, and Fucales), gametic reproduction has not replaced reproduction by vegetative spores but is supplementary to it. In nearly all the green algae the zygote is a resting cell, accjuiring a heavy wall and carrying the plant through a period of severe conditions into the next growing season. In fact, the formation of gametes is often induced by the advent of such conditions. In the brown and red algae the zygote germinates at once.

Originality, in the evolution of the algae, both of the fusing gametes were ciliated and of the same size. This condition of isogamy is retained by the yellow-green algae, diatoms, and many of the simpler green and brown algae. In such forms as Pandorina, one of the pairing gametes is slightly larger and less active than the other. In Cutleria both gametes are ciliated, but the female gamete is considerably larger than the male. In Dictyota and Fucus the female gamete (egg) is increased in size still more and, although extruded into the water, is nonciliated, only the male gamete (sperm) being motile. Finally, in Oedogonium and many other algae, the large nonmotile egg is not liberated but is fertilized within the oogonium by the small motile sperm. Thus the evolution of heterogamy from isogamy has been gradual.

After sexual reproduction had become established, one gamete retained its motility and small size, while the other sacrificed its motility for an increased nutritive capacity. The advantage of heterogamy lies in the greater amount of reserve food that comes to be stored in the zygote. This advantage is reflected by the occurrence of heterogamy in many green algae, most brown algae, all stoneworts, and all red algae, as well as in all plants above the thallophyte level. It should be emphasized that heterogamy has arisen independently in the various groups of algae where it occurs.

The production of gametes in ordinary vegetative cells is characteristic of Ulothrix, Oedogonium, and most other green algae. A more advanced condition is seen in Vaucheria, the Charophyceae, and nearly all the brown and red algae, where the gametes are borne in gametangia or sex organs, which are specialized for reproduction, a function lost by the other cells of the body. This tendency parallels the production of spores

98 PLANT MORPHOLOGY

in sporangia. The sporangia remain unicellular but the gametangiaof some algae have become multicellular by the formation of cross walls, as in Ectocarpus and Cidleria. In Diciyota the antheridia are multicel- lular and the oogonia are unicellular.

Alternation of Generations. In nearly all the green algae the vegeta- tive plant, of which there is but one kind, gives rise to gametes and is haploid. Here the diploid condition, which always results from fertiliza- tion, is restricted to the zygote itself, since the reduction of chromosomes takes place when it germinates. This reduction always involves the formation of four haploid nuclei. In Oedogonium each of the four zoo- spores (meiospores) coming from the zygote contains one of these nuclei. In Spirogyra three of the nuclei degenerate and the zygote gives rise directly to a haploid vegetative body. In Coleochaete four haploid cells are formed by the zygote, but each divides one, two, or three more times before zoospores are organized. In the two lower orders of red algae (Bangiales and Nemalionales) an analogous condition exists in the forma- tion of carpospores.

In some of the algae, notably in the diatoms, Acetahularia, C odium, Bryopsis, and the Fucales, there is only one kind of vegetative body and it is diploid, the reduction of chromosomes occurring in connection with the formation of gametes, or in several nuclear divisions immediately preceding their formation. This is also the condition in animals.

Some botanists recognize an alternation of generations wherever there is a diploid and a haploid phase in the life history, even though one or the other is represented by only one cell — in other words, wherever there is sexual reproduction. Such a broad use of the term makes it almost meaningless. In algae displaying a true alternation of generations, a more or less prolonged growth phase intervenes between fertilization and meiosis, as well as between meiosis and fertilization. Here the life history involves two distinct and independent vegetative bodies, a haploid body (gametophyte) producing gametes and a diploid body (sporophyte) producing spores. The gametophyte arises from a spore, the sporophyte from a zygote. The reduction of chromosomes occurs when the spores are formed. A true alternation of generations is found in only a very few green algae, such as Ulva and Cladophora, in all brown algae except the Fucales, and in all red algae except the Bangiales and Nemalionales. The alternation may be isomorphic, with both genera- tions alike vegetatively, as in Ulva, Cladophora, Ectocarpus, Dictyota, and Polysiphonia, or it may be heteromorphic, with both generations unlike vegetatively, as in Cutleria and Laminaria. In all algae possessing a true alternation of generations both gametophyte and sporophyte are free-living; one is never dependent upon the other.

Interrelationships. It is not possible to arrange the classes of algae in such a way as to indicate their true relationship. The secjuence in

THALLOPHYTA: ALGAE 99

which these groups have been presented is merely one denoting an ever- increasing complexity in vegetative and reproductive structures. It does not denote descent of one group from the one preceding it in the series, although in some cases this may be true. Each group merely stands for a different degree of progress from what was originally a primitive condition.

The most important evidence concerning the interrelationships of plant groups is derived from paleobotany. The dearth of fossils belong- ing to groups below the pteridophyte level is so great, however, that practically all conclusions regarding phylogeny must be based on the comparative structure and development of existing plants. This means that such conclusions, even though well substantiated, are largely speculative.

The Cyanophyceae are the most primitive group of autotrophic plants. Except for the presence of chlorophyll, they are strikingly like the bac- teria. Which of these groups appeared first on the earth is very uncer- tain, but is unimportant in connection with the present discussion. Both groups are at a very low level of structural organization. The classes consisting mainl}^ of flagellates show a considerable advance over the Cyanophyceae by their well-organized nuclei, definite plastids, and ciliated cells. In the absence of transitional forms, any direct connection between the Cyanophyceae and flagellates is difficult to visualize. It is easier to think of the flagellates as having arisen directly from the bac- teria. A direct relationship between the Cyanophyceae and any of the higher algal classes is also unlikely, although there is some evidence of this in the case of the Rhodophyceae. Ciliated cells are not present in either group; some members of each have both phycocyanin and phyco- erythrin; and a few primitive Rhodophyceae have a nuclear structure but little advanced over that of the Cyanophyceae. It is primarily the absence of ciliated cells that would seem to preclude the possibility of a relationship between either group and the flagellates.

That the Xanthophyceae and Chlorophyceae have arisen independ- ently from a flagellate ancestry is strongly indicated by the presence of naked, free-swimming reproductive cells in the life history and by the occurrence of intermediate forms. The derivation of the Bacillario- phyceae and Phaeophyceae directly from flagellate ancestors is less evi- dent. The only connection between the diatoms and flagellates is the presence of ciliated reproductive cells in a few diatoms. The origin of the Phaeophyceae is obscure because the group is without unicellular members. Yet their motile reproductive cells suggest that they may have arisen from brown, laterally biciliate flagellates. There is also the possibility of a direct connection between the Phaeophyceae and Chloro- phyceae. The Charophyceae are an isolated group, yet seem to repre- sent a specialized offshoot from the Chlorophyceae.

CHAPTER IV THALLOPHYTA: FUNGI

Fungi are dependent (heterotrophic) thallophytes. Lacking chloro- phyll, they are unable to carry on photosynthesis and hence must obtain their food from an external source. Many are saprophytes, living on dead organic matter; others are parasites, obtaining nourishment from the bodies of living plants or animals, the organism attacked being the host. At least some of the fungi may have evolved directly from the algae through loss of power to carry on photosynthesis. Because of their relation to the decomposition of organic matter and to disease, fungi are of tremendous economic importance. The fungi comprise the five classes Schizomycetes, Myxomycetes, Phycomycetes, Ascomycetes, and Basidiomycetes. To these might be added the class Lichenes.

1. SCHIZOMYCETES

The Schizomycetes, or bacteria, are similar in many respects to the Cyanophyceae, differing from them chiefly in their smaller size and lack of chlorophyll. In fact the two groups are often combined into a single group, the Schizophyta. The bacteria are at once the smallest and simplest of all known organisms, unless the viruses are to be considered as living. They are also the most widely distributed, occurring under all conditions where life may exist — in fresh and salt water, in soil, in the air, and in the living and dead bodies of other organisms. Like the Cyanophyceae, they are a very ancient group and must have been among the first forms of life to have existed on the earth. The bacteria com- prise about 1,500 species. Some common genera are Streptococcus, Micrococcus, Sarcina, Bacterium, Bacillus, Pseudomonas, Microspira, Spirilhim, Cladothrix, and Beggiatoa.

Structure and Reproduction. Like the blue-green algae, bacteria are unicellular plants that reproduce by fission. Their cells are of three general types: spherical (coccus) forms, rod-shaped {bacillus) forms, and curved or spiral (spirillum) forms (Fig. 80). Some are nonmotile, while others bear cilia, by means of which they move rapidly. The cilia may cover the entire cell or may be restricted to one or both ends, where they occur either singly or in tufts. The rod-shaped types average about 2.5ii in length,! while many of the spherical forms are only about 0.5^ in diameter.

' The unit of microscopic measurement is the viicron, abbreviated ju- It is one- thousandth of a millimeter (0.001 mm.), approximately equivalent to 1/25,000 inch.

100

THALLOPHYTA: FUNGI 101

The cells of bacteria are so simple that they might almost be said to be structureless. A mass of homogeneous protoplasm is surrounded by a thin cell wall, generally composed chiefly of chitin, a nitrogenous sub- stance, whereas the cell walls of green plants are composed mainly of cellulose, a carbohydrate. Commonly the cell wall becomes mucilagi- nous, forming a slimy sheath or capsule. There is no organized nucleus but merely some scattered granules of a chromatin-like material that can be revealed by staining. In some bacteria these granules are aggre- gated to form a distinct central group. Other granules may also be present; these generally represent reserve food.

8B 0®^)

i

D '^ E F

Fig. 80. Group of common bacteria, X 1,500. A, Sarcina lutea; B, Bacillus subtilis; C, Bacillus typhosus; D, Spirillum cholerae; E, Streptococcus pyrogenes; F, Spirillum undulatum,.

In some bacteria the two cells separate following division, while in others they remain together in colonies. Spirillum forms are nearly always solitary. In the coccus forms the colonies may be cubical, plate-like, chain-like, filamentous, or irregular. In the bacillus forms the divisions occur only in one plane, and so the colonies are always filamentous. In Beggiatoa, a sulphur bacterium, the filaments are as highly organized as those of Oscillatoria. In Cladothrix, an iron bac- terium, the filaments exhibit false branching.

As in the Cyanophyceae, cell division takes place by the formation of an inward-growing cell wall. Under favorable circumstances, cell divi- sion in many bacteria may occur as frequently as every 20 minutes, so that, in the course of 24 hours, a single cell may give rise to billions. Such a rate of multiplication is soon checked, however, by the exhaustion of the food supply or by the accumulation of poisonous waste products of metabolism. Although all bacteria are active only in the presence of moisture and other favorable conditions, if these fail, many bacteria can pass into a resting stage and remain inactive for a long time. Bacteria on dust particles in the air are in a dormant state and can resist desicca-

102 PLANT MORPHOLOGY

tion and great extremes of temperature. In some bacilli the resting cell becomes an endospore. Here the protoplast rounds up inside the cell cavity and invests itself with a new cell wall, the old wall eventually disappearing. Endospores are extremely resistant. With the return of favorable conditions, they again become active vegetative cells. Thus "spore formation" in bacteria does not result in reproduction but merely in survival during a period of stress.

Although the bacteria are said to be without sexual reproduction, there is some evidence that it may occur at least in certain bacteria, since there seems to be Mendelian segregation resulting from mixtures of different types.

Activities. Most bacteria are either saprophytic or parasitic and in both cases food is absorbed through the cell wall. Some can live either as saprophytes or as parasites, while a few can make their own food without the aid of chlorophyll or light. Aerobic bacteria require free oxygen in respiration, while anaerobic bacteria obtain oxygen from organic compounds.

Most diseases of animals and many diseases of plants are caused by parasitic (pathogenic) bacteria, the disease itself being merely a response on the part of the host to the presence of the parasite. A disease mani- fests itself by symptoms, which are abnormalities in structure or function. Well-known human diseases of bacterial origin are typhoid fever, tuber- culosis, diphtheria, pneumonia, cholera, and tetanus. Some bacterial diseases of plants are pear blight, cabbage rot, cucurbit wilt, and crown gall. The disease may be caused by direct attack of the bacteria on the host tissues, by the liberation of toxins, or by both.

The decomposition of dead organic matter is accomplished chiefly by saprophytic bacteria. They break up organic compounds into sim- pler substances through a series of intermediate steps, a succession of different bacteria being involved. The ultimate products of decom- position are such simple substances as water, carbon dioxide, ammonia, hydrogen sulphide, etc. Bacteria of decay cause fermentation and putrefaction. They play an important part in the economy of nature by returning to the air and soil substances that may again be used by other organisms.

All plants require nitrogen in order to synthesize proteins, but only the nitrogen-fixing bacteria and a few other forms are able to use the nitrogen of the air directly. Practically all green plants are dependent for nitrogen upon its compovnids, particularly nitrates. Some of the nitrogen-fixing bacteria, such as Clostridium and Azotobacter, live free in the soil and are saprophytic on organic matter, while Rhizobium is para- sitic in the roots of various Leguminosae, such as clover, alfalfa, peas, beans, etc. These bacteria combine atmospheric nitrogen with oxygen

THALLOPHYTA: FUXGI 103

and other elements, particularly potassium, sodium, or calcium, and form nitrates, which may later be utilized by green plants. The root of the legume responds to the presence of these parasitic bacteria by forming local enlargements called tubercles or nodules.

Nitrifying bacteria also live in the soil but form nitrates in a different way. The decomposition of organic matter by bacteria of decay yields ammonia (NH^). This is oxidized, first to nitrites (NO2 compounds) by Nitrosomonas, and then to nitrates (NO3 compounds) by Nitrohacter. An interesting fact about these bacteria is that, although lacking chloro- phyll, they are able to synthesize carbohydrate food from water and carbon dioxide (or carbonates). They derive their energy, not from sunlight, but from the oxidations that they carry on. With respect to their nutrition, these bacteria, like green plants, are autotrophic, even though they do not carry on photosynthesis. The process by which they make their own food is sometimes called chemosynthesis. Such autotrophic bacteria may have preceded all other forms of life on the earth. In addition to the nitrifying bacteria there are other kinds that are autotrophic. They oxidize sulphur, hydrogen sulphide, free hydro- gen, methane, or iron compounds. Beggiatoa is a colorless filamentous form that oxidizes hydrogen sulphide (H2S) to form water and sulphur, storing the sulphur as yellow granules inside its cells. It is found in sulphur springs. Certain iron bacteria oxidize ferrous iron compounds to ferric hydroxide (FeOHs), which accumulates to form a kind of iron ore. These bacteria are common in bogs.

Denitrifying bacteria live in the soil, especially where poorly drained. They convert nitrogen salts into gaseous nitrogen. This escapes into the air and so causes a loss of soil fertility.

Myxobacteria. The myxobacteria are a group of peculiar organisms that live as saprophytes on animal refuse. Their cells resemble those of true bacteria but form remarkable complex colonies held together by mucilage. Some of the myxobacteria form stalked sporangia that are often brightly colored. Some exhibit slow creeping movements. In these respects the group resembles the myxomycetes.

Summary. The Schizomycetes are the simplest of all plants. All of them are unicellular, the cells being either solitary or in colonies. A defi- nite cell wall is present, generally composed of chitin rather than of cellulose, and usually breaks down to form mucilage. The protoplast shows Httle organization, a nucleus being represented only by scattered granules of chromatin. Reproduction occurs by fission. Some bacteria move by means of cilia, while others are nonmotile. In some species a resting cell (endospore) may form inside a vegetative cell, becoming invested with a new cell wall. The Schizomycetes are closely related to the Cyanophyceae, differing from them chiefly in the lack of chloro-

104 PLANT MORPHOLOGY

phyll, presence of cilia in some members, and character of the rest- ing cell.

2. MYXOMYCETES

The Myxomycetes, or slime molds, are peculiar organisms that, like the flagellates, are claimed by both botanists and zoologists, the latter calling them Mycetozoa (fungus-animals). They are a widely dis- tributed group, living in damp, shady places as saprophytes on humus, decaying wood, bark, fallen leaves, etc. All lack chlorophyll. The

^,:Mm

<■-)■:■' !»

't-

I

Fig. 81. Plasmodium of Didymitim, a slime mold, X30. {From Gilbert M. Smith.)

Myxomycetes number over 400 species. Some of the common genera are Lycogala, Stemonitis, Fuligo, Arcyria, and Trichia.

Plant Body. The vegetative body of a myxomycete is a Plasmodium, which is a naked mass of multinucleate protoplasm (Fig. 81). The nuclei, like those of the higher plants, are well organized. The Plas- modium is without definite form and may attain a diameter of several centimeters, or even a meter in some myxomycetes. The Plasmodium moves by the formation of pseudopodia and engulfs solid particles of food as it passes over them, digesting them within food vacuoles. In these respects it resembles an amoeba. It also absorbs food material in solu- tion through the plasma membrane. Depending on the species, the Plasmodium may be white or some shade of yellow, orange, red, brown, or violet. The Plasmodium tends to move toward moisture but shows an avoiding reaction to light, appearing at the surface of its substratum only at night. In times of drought it retracts itself into a waxy mass and

THA LLOPH VTA: F UNGl

105

hardens, forming a sclerotium. In this condition the organism may remain dormant for months, or sometimes even for years, becoming active again in the presence of water.

Reproduction. Although the myxomycetes are animal-hke in their vegetative state, their reproductive features are distinctly plant-like. When reproduction is to occur, the entire Plasmodium comes to the

B

^^'^\'^~Pi^Z ~^

F G

Fig. 82. Group of common slime molds, showing sporangia arising from the Plasmodium. j4, Hemitrichia ovata, XlO; B, Craterum leucocephalum, XIO; C, Arcyria incarnata, X5; D, Stemonitis herbatica, X2; E, Diachea leucopoda, XIO; F, Lycogala epidendrum, XI; G, Fuligo septica, X J'^ .

surface of its substratum and contracts into a cushion-like mass. As this hardens, it forms one or more sporangia that are usually brown or yellow (Fig. 82). In some myxomycetes the entire Plasmodium may be con- verted into a single giant sporangium, called an aethalium, but, more commonly, a number of small separate sporangia are formed. These may be either sessile or stalked. Throughout the various genera the sporangia exhibit much diversity in form, but are commonly spherical, oval, or cjdindrical. The sporangium contains many nuclei and the remains of the Plasmodium, the latter usually forming a network of tough strands known as the capillitium (Fig. 83). In the meshes of this

106 PLANT MORPHOLOGY

network innumerable spores are formed, each one being uninucleate. The spores have cellulose walls and are scattered by the wind. In their dispersal the wall erf the sporangium ruptures irregularly at the apex and the capillitium performs hygroscopic movements.

The myxomycetes display considerable variation with respect to the development of the Plasmodium from a spore. Commonly the proto- plast escapes from the spore wall and becomes a zoospore, developing one long cilium and one very short one, both anteriorly attached. Some- times two to eight zoospores are produced. The zoospore may ingest

food and undergo multiplication by fission. After a period of free swimming, the cilia are retracted and the protoplast becomes amoe- boid. These amoeboid cells, called myx- amoebae, may also take in food and divide repeatedly, or they may pass into a resting stage. Finally, however, they fuse in pairs. Then, instead of forming resting zygotes, a number come together to form a multi- nucleate Plasmodium. In the fusion of the small amoeboid cells in pairs, the two nuclei Fig. 83. Portion of capii- Unite, but there are no subsequent nuclear litium of stemonitis with fusions. Consequently the nuclei of the plas-

spores in its meshes, X 250. i • i • i

modmm are diploid. They undergo repeated divisions as the Plasmodium increases in size. Reduction of chromosomes to the haploid number occurs just prior to the formation of spores in the sporangium.

Summary. The Myxomycetes combine features found in both plants and animals. The body is a naked mass of multinucleate protoplasm (a Plasmodium) that displays amoeboid movements and engulfs solid food particles. In a quiescent state it gives rise to sporangia containing numerous walled spores from which eventuall}^, although not directly, a new Plasmodium is formed. Sexual reproduction occurs by a fusion of similar amoeboid gametes. Certain aspects of the life history suggest a relationship to the flagellates. Any possible connection with the true fungi is very uncertain.

Other Slime Fungi. In addition to the Myxomycetes, or slime molds, there are two other groups of slime fungi that deserve some attention. These are the Acrasieae and the Labyrinthuleae. Many mycologists include all three groups in a separate assemblage, the Myxothallophyta, and place them outside and below the fungi. They have certain impor- tant characters in common: simple, naked, nucleated, amoeboid cells resembling protozoans but plant-like in their reproduction by the forma- tion of spores. The interrelationships of the three groups of slime fungi

THALLOPHYTA: FUNGI

107

are not well understood, but are not assumed to be close. They have proliably been derived from protozoan ancestors and have evolved along independent lines.

Acrasieae. These simple forms are saprophytes on soil, decaying wood, and animal refuse. The vegetative body is a myxamoeba, a naked cell with a nucleus and a contractile vacuole. It reproduces by fission and in the presence of unfavorable conditions may encyst. Eventually, a number of myxamoebae come together without fusing to form a pseudo- plasmodium in which each m3^\amoeba retains its individuality. Not

S HRS

Fig. 84. Three-dimensional graph showing the development of the fruiting body of Dictyostelium discoideum in height, time, and position. A, aggregation of a mass of individual myxamoebae; B to D, formation of the pseudoplasmodium; E toG, migration of pseudoplasmodium; H to N, formation of fruiting body with disk, stalk, and spherical spore mass. {From J. T. Bonner.)

only is a multinucleate plasmodium lacking, but no zoospores are pro- duced. The pseudoplasmodium assumes a definite form, usually elongat- ing and varying in length from several tenths of a millimeter to a milli- meter or more.

The subsequent behavior of the pseudoplasmodium is remarkable in that it migrates over the substratum, apparently by a gliding of the amoebae over one another. After coming to rest, the pseudoplasmodium is transformed into a fruiting body consisting of a basal disk, a vertical stalk, and a terminal spherical region that is converted into a mass of spores (Fig. 84). In some species the fertile region consists of a series of spherical spore masses arranged at the ends of whorled branches. All these complex changes are accomplished by movements of individual myxamoebae to their proper place in the fruiting body, where each becomes the type of cell appropriate for its position, such as a disk cell, stalk cell, or spore. The spores have a cell wall. Upon germination, each spore gives rise to a myxamoeba.

108 PLANT MORPHOLOGY

Labyrinthuleae. In this little-known group the vegetative cell is spindle-shaped with tufts of pseudopodia at the ends. When the cells come in contact, their pseudopodia generally fuse, the union of numerous cells producing a net-like structure called a net-plasmodium. The individual cells, retaining their identity, appear to glide along the threads of the net in limited movements. During this stage they feed, increase in size, and undergo repeated division. In dividing, the cells become constricted at the middle and then separate, but are held together by a protoplasmic strand. At the close of the vegetative stage, the cells collect into sessile or stalked masses and become encysted. In some species the spores have cell walls, in others not. Later the spores germi- nate, freeing one to four spindle-shaped cells with polar pseudopodia.

3. PHYCOMYCETES

The Phycomycetes, or alga-like fungi, comprise the first group of "true fungi" (Eumycetes), as the higher fungi are often called in contrast to the bacteria and myxomycetes. All the true fungi have a definite nucleus and nearly all have a characteristic plant body called a mycelium. This is composed of branching filaments, each branch being a hypha. The hyphae may be either loosely or compactly interwoven. With few excep- tions, the Phycomycetes are characterized by an absence of cross walls in the mycelium, and so, as in Vaucheria, the plant body is a coenocyte. Their spores are borne in indefinite numbers within a sporangium. The origin of the Phycomycetes is not clear. They may have evolved either from colorless flagellates or, through loss of chlorophyll, from the Chloro- phyceae, a group which they resemble in both vegetative and reproduc- tive features. A number of Phycomycetes cause diseases of economic plants, such as cranberry gall, brown rot of lemon, downy mildew of grape, and late blight of potato. The group is a relatively small one, numbering about 1,000 species. These are included in seven main orders: Chytridiales, Monoblepharidales, Plasmodiophorales, Sapro- legniales, Peronosporales, Mucorales, and Entomophthorales.

1. Chytridiales

The Chytridiales are the simplest of the Phycomycetes. Nearly all of them are parasitic, many living on fresh-water algae and others attacking seed plants growing in moist situations. The order includes about 65 genera and 300 species, the best-known forms being Chytridium, Olpidium, and Synchytrium.

Chytridium. A common species of Chytridium attacks the green alga, Oedogonium. A uniciliate zoospore comes in contact with an oogonium of the host, loses its cihum, and sends into the host cell a tube through which food is absorbed. This tube represents a weakly developed myce-

THALLOPHYTA: FUNGI

109

Hum. The external part of the fundus then becomes transformed into a sporangium, its protoplast undergoing cleavage into many zoospores (Fig. S5A). A zoospore may penetrate a zygote of the host and, by the secretion of a thick wall, become a resting spore. When the zygote germinates, the resting spore of the fungus sends out tubes that give rise to terminal sporangia.

Olpidium. This fungus grows on many different hosts, some of which are fresh-water algae. One species, Olpidium brassicae, attacks young

A B

Fig. 85. Chytridiales. A, sporangium of Chytridium olla attached to zygote of Oedo- gonium; B, sporangia of Olpidium brassicae in root of cabbage seedling; also two zoospores of same to the left. {A, after Campbell; B, after Woronin.)

cabbage plants. A uniciliate zoospore comes to rest on the host, with- draws its cilium, and secretes a cell wall. It sends a short tube into the host and the protoplast enters one of the cells. At first the protoplast is naked and amoeboid. It enlarges and becomes multinucleate, finally occupying the whole cell cavity. Then it forms a cell wall and becomes a sporangium. A tube is sent to the surface of the host and numerous uniciliate zoospores escape through it (Fig. 855). Sexual reproduction occurs by means of isogametes that are formed like the zoospores but escape and fuse in pairs. The zygote sends a short tube into a host cell, after which its protoplast enters, enlarges, and secretes a thick wall. After resting over the winter, it gives rise to a number of uniciliate zoospores.

Synchytrium. This form attacks the epidermal cells of various seed plants, such as cranberry, primrose, hog peanut, filaree, and many others. A disease called cranberry gall is caused by Synchytrium vaccinii, while

no

PLANT MORPHOLOGY

the destructive l)lack wart of the potato is caused by Stjnchytrium endo- bioticum. A uniciHate zoospore comes in contact with a young epidermal cell of the host and enters it. Without forming a cell wall, the protoplast of the fungus enlarges and lives symbiotically with the protoplast of the epidermal cell, not killing it but causing it and the adjacent cells of the

C D

Fig. 86. Stages in the development of the sporangia of Synchytrium decipiens. A, greatly enlarged fungous protoplast in leaf of host after having destroyed one of its epidermal cells; B, division of large nucleus of fungus to form many small free nuclei; C, cleavage of proto- plast into many small uninucleate cells; D, separation of small cells to form sporangia, each of which has become multinucleate; E, enlarged portion of same; A to D, X125; E, X500.

host to enlarge. A small gall or blister forms on the surface of the host, this serving as a means by which an infected plant can be recognized. Blisters usually appear both on the leaves and stems.

Finally, the infected epidermal cell dies. Then the fungus secretes a wall about itself and goes into a resting stage (Fig. 86^). Later its nucleus undergoes repeated divisions and progressive cleavage of the cytoplasm from the surface inward results in the formation of many protoplasts, each of which secretes a wall (Fig. 865, C). These cells may be multinucleate when formed but, if uninucleate, they soon become multinucleate by additional free-nuclear divisions (Fig. 86Z), E). Each

THALLOPHYTA: FUNGI

ni

of these cells becomes a sporangium and, when conditions are favorable, gives rise to a number of naked zoospores (usually 8 to 12) that escape. Frequently, however, the resting cell arising from a vegetative protoplast divides to form a number of gametangia rather than sporangia. Each of these produces many isogametes that, after escaping, fuse in pairs. The zygote invades a host cell and then goes into a resting stage, forming a thick wall. Later it gives rise to many zoospores. Both the zoospores and gametes of Synchytnum are uniciliate.

Summary. Most of the Chytridiales are unicellular fungi with either no mycelium or a ver}^ poorly developed one. Generally all or most of the vegetative body develops into a sporangium or gametangium. Reproduction occurs by uniciliate zoospores or isogametes. Because they possess the simplest tj^pe of sexual reproduction known among the fungi, the Chytridiales are regarded by some mycologists as primitive forms, while others consider them to be degenerate Phycomycetes.

2. Monoblepharidales

The Monoblepharidales are a very small order containing 2 genera: Monoblepharis, with 6 species, and Monoblepharella, with 2 species.

ABC

Fig. 87. Monoblepharis sphaerica. A, end of hypha with young oogonium and a young antheridium just below it; B, sperms escaping and approaching the mature oogonium with its single egg; C, zygote with empty antheridium below it. (After Cornu.)

Monoblepharis is a saprophyte on decaying aquatic vegetation. It has a well-developed mycelium that produces sporangia and sex organs. The sporangia are terminal club-shaped cells containing many vmiciliate zoospores. Sexual reproduction is heterogamous. The oogonium is a globular cell, cut off by a wall commonly at the end of a hypha (Fig. 87). Its protoplast rounds up and becomes a large uninucleate egg. The antheridium usually arises immediately below the oogonium as a short

112

PLANT MORPHOLOGY

slender branch that is cut off by a basal wall. It gives rise to a number of uniciliate sperms that escape and swim in the water. A sperm enters the oiigoniiiin through a terminal pore and unites with the egg. The ]\Ion()l)lophari(hiles are remarkal)le in being the only fungi with swimming sperms. According to the species, the zygote may mature either inside or outside the oiigonium. It becomes a thick-walled resting cell, later producing a new mycelium.

3. Plasmodiophorales

This order comprises 8 genera and 23 species, of which the best known is Plasmodiophora brassicae, a parasite attacking cabbages and other crucifers and causing a disease known as clubroot. Another member of the group, Spongospora subterranea, is responsible for a disease of potatoes

A B

Fig. 88. Section of a portion of a cabbage root, showing two stages in the development of Plasmodiophora brassicae within the cells, X250. A, plasmodium completely filling a cell; B, spore formation.

called powdery scab. The Plasmodiophorales, once regarded as para- sitic myxomycetes, are now generally considered to belong to the lower phj^comycetes.

When cabbages are attacked by Plasmodiophora, the root undergoes a marked enlargement. The cells of the root are invaded by biciliate zoospores. The cilia, attached anteriorly, are of unequal length.^ The zoospores lose their cilia and become amoeboid, migrating directly through the cell walls of the host. An amoeboid cell (myxamoeba) gives rise to a multinucleate plasmodium (Fig. 88A). This soon undergoes cleavage into many uninucleate, walled cells, each of which is said to form four or eight biciliate isogametes that pair and fuse. The amoeboid zygote enlarges, becomes multinucleate, and migrates into another cell of the root, which it finally fills. The diploid nuclei of the young plas- modium continue to divide until the two reduction divisions have occurred. Then a number of small spores are formed, each with a cell

1 Until recently it was thought that the zoospores were uniciliate and, chiefly on this basis, Plasmodiophora and related forms were classified as a family under the Chytridiales.

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wall (Fig. 88B). These are liberated by decay of the host. Upon germination, each gives rise to a zoospore.

4. Saprolegniales

The Saprolegniales, or water molds, are an order of aquatic fungi, usually occurring in ponds and streams. Most of them are saprophytic on plant or animal remains lying in the water, while a few are parasitic. Many also occur on damp soil. The order includes 20 genera and about 120 species, representative members being Saprolegnia and Achlya.

/

\

A B

Fig. 89. Saprolegnia. A, a sporangium and three escaped zoospores, X350; B, an oogonium with many eggs and with two antheridia in contact with it, X350.

Saprolegnia. This common water mold usually lives on dead insects, fishes, tadpoles, and other aquatic animals. Sometimes it attacks living fishes and fish eggs. The vegetative body consists of a delicate, branching, coenocytic mycelium that penetrates the food supply. Some of the hyphae form terminal sporangia, each of which is a slender elon- gated cell, cut off by a basal wall, and giving rise to many uninucleate zoospores (Fig. 89A). These are developed by progressive cleavage of the cytoplasm within the sporangium. The zoospores escape singly into the water through a terminal pore in the sporangial wall. In Achlya they escape as a mass.

The zoospores of Saprolegnia are oval and have two equal cilia attached apically. After swimming for a while, they become quiescent, form a cell wall, and go into a dormant stage. After about 24 hours, the proto- plasts escape and again become motile, but this time the spores are kidney-shaped and laterally biciliate. Finally they settle down and, on a suitable substratum, each produces a new mycelium. The occur-

114 PLANT MORPHOLOGY

rence of two types of zoospores is very puzzling and its significance has never been satisfactorily explained.

Saprolegnia is heterogamoiis, forming sex organs on special branches of the mycelium (Fig. 89B). The oogonium is a spherical cell that pro- duces several eggs, sometimes many, rarely only one. At first they are multinucleate but, by degeneration of the extra nuclei, become uni- nucleate. The antheridium is a slender curved tube that arises just below the oogonium or, in most species, from an adjacent hypha. Each oogonium may be surrounded by several antheridia. Both kinds of sex organs are cut off from the vegetative mycelium by a basal wall. No sperms are organized. Instead, the tip of the antheridium comes in contact with the oogonium and sends into one or more of the eggs a fer- tilization tube through which some of the cytoplasm and a male nucleus pass. This nucleus unites with the egg nucleus, resulting in fertilization. The zygote secretes a heavy wall and usually remains dormant for several months, finally producing a new mycelium. In Achlya it has been shown that the reduction of chromosomes occurs when the zygote germinates. In some species of Saprolegnia the antheridia are nonfunctional, while in others antheridia are not even formed. Nevertheless the eggs become thick-walled and later germinate, thus developing by parthenogenesis.

In Achlya, which is dioecious, the appearance of sex organs is caused by hormone-like substances. These are secreted into the water by the male and female plants and stimulate production of sex organs of the opposite sex. A hormone produced by the male plants causes oogonia to appear on the female plants, while a hormone produced by the female plants results in the appearance of antheridia on the male plants.

Summary. The Saprolegniales are chiefly saprophytes. They are aquatic fungi Avith a well-developed mycelium. They produce biciliate zoospores in persistent sporangia. All of them are heterogamous. The oogonium contains one or more eggs that are fertilized by a male nucleus coming from the antheridium through a fertilization tube. In most members the entire oogonial protoplast enters into the formation of eggs. The absence of swimming sperms in an exclusively aquatic order is a noteworthy feature.

5. Peronosporales

The Peronosporales, or downy mildews, are mostly parasites that attack various seed plants, the mycelium living within the intercellular spaces of the host. The order includes about 12 genera and 150 species, representative members being Pythium, Albugo, Phytophthora, Plasmo- para, and Peronospora.

Albugo. This fungus lives as a parasite on a number of different seed plants. A common species, Albugo Candida, attacks various members of

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the Cruciferae, such as radish, turnip, mustard, and shepherd's-purse, causing a disease known as white rust of crucifers. The rayceHum, which may hve in almost any part of the host, ramifies throughout the inter- cellular spaces and sends short button-Uke haustoria into the living cells. Here and there beneath the epidermis the mycelium gives rise to com- pact clusters of erect sporangiophores from the ends of which thin-walled,

A C

Fig. 90. Sporangia and sex organs of Albugo. A, cross section of a small portion of the stem of shepherd's-purse, showing sporangiophores of Albngo Candida arising beneath the epidermis and giving rise to multinucleate sporangia, X 600; B, sex organs of Albugo Candida, a fertilizing tube from an antheridium penetrating an oogonium with a single nucleus in the ooplasm, X750; C, oogonium of Albugo portulacae with multinucleate ooplasm, X500.

globular, multinucleate sporangia are cut off in chains (Fig. 90.4). These push up the epidermis and form a white blister on the surface of the host. These blisters may appear on the leaves, stems, floral parts, or fruits. Finally, the epidermis is ruptured and the sporangia are carried by the wind to uninfected hosts. Here they give rise to 12 or more laterally biciliate zoospores that escape, swim about for a while, encyst, and finally produce new mycelia. When a spore germinates, it produces a hypha that enters the host through a stoma.

The sex organs of Albugo Candida, appearing later in the season than the sporangia, are formed on the mycelium deep within the host tissues.

116 PLANT MORPHOLOGY

The oogonium is a globular multinucleate cell, cut off by a cross wall from the swollen end of a hypha (Fig. 905). The cytoplasm becomes differ- entiated into a peripheral zone, the periplasm, and a central denser region, the ooplasm, which becomes the egg. At first both regions are multinucleate, but later all nuclei degenerate except a single nucleus in the ooplasm. The antheridium, appearing on a separate hypha, is a slender multinucleate cell. After coming in contact with the oogonium, it sends into it a fertilizing tube that extends into the egg, where a male nucleus and a small amount of cytoplasm are discharged. Following fusion of the male and female nuclei, the periplasm is used up in the formation of a heavy wall around the zygote. The zygote is finally freed by decay of the host tissues and, after undergoing a period of rest, gives rise to more than one hundred biciliate zoospores, each of which may, under appropriate conditions, produce a new mycelium.

Albugo bliti, a species common on the pigweed {Amaranthus) , differs from Albugo Candida in several respects. Periplasm and ooplasm are differentiated but the latter remains multinucleate. The entire contents of the antheridium are discharged into the egg and each male nucleus fuses with a female nucleus. In Albugo portulacae, which lives on the common purslane (Portulaca), multinucleate pairing and fusing also occur

(Fig. 90C).

Other Downy Mildews. The Peronosporales include genera that bear sex organs like those of Albugo, but differ in the way their sporangia and spores are formed. Some of these are of considerable economic importance. A species of Pythium is frequently the cause of a disease of seedlings known as damping-off. It is particularly common in green- houses and other warm, moist places. Pythium is intermediate between the Saprolegniales and Peronosporales in that it produces zoospores in both permanent and detachable sporangia.

Phyfophthora infestans causes a serious potato disease called late blight, while another species, Phytophthora cifrophtJiora, is responsible for the brown rot of lemon. Plasmopara viticola causes downy mildew of the grape, a very destructive disease. In both Phytophthora and Plasmopara the internal mycelium sends erect sporangiophores to the surface of the host (Fig. 91). Instead of forming blisters, as in Albugo, the sporangio- phores push out through the stomata and bear solitary terminal sporangia on branches. The sporangia, which are shed without opening, are carried by the wind to uninfected hosts, where each produces several biciliate zoospores. These form a new mycelium within the leaf.

Peronospora is a large genus of about 60 species, some of which are parasitic on various garden vegetables, such as cabbage, spinach, onion, pea, etc. It is of interest in that, in many species, no zoospores are pro- duced, the detachable sporangia giving rise to new mycelia directly.

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Summary. The Peronosporales are almost all internal parasites on seed plants. They have a well-developed mycelium and small multi- nucleate sporangia that, with few exceptions, are borne on erect spo- rangiophores. The sporangia are almost always detachable and, after dispersal by the wind, give rise to biciliate zoospores or, in some cases, to a new mycelium directly. All members are heterogamous. The oogonium produces only one egg, in the formation of which the outer portion of the oogonial protoplast is not included. The male nucleus reaches the egg through a fertilization tube developed by the antheridium.

6. Mucorales

The Mucorales are the black molds, most of which are terrestrial saprophytes living on decaying vegetable and animal matter. There are about 30 genera and 400 species, common representatives of the group being Rhizopus, Mucor, and Pilobolus. The largest genus is Mucor, with about 50 species.

Rhizopus. The common black mold that grows on moist stale bread is Rhizopus nigricans. It also occurs on fruits, vege- tables, jelly, and other decaying organic matter. The mycelium consists of a white fluffy mass of profusely branched coenocytic hyphae. These grow horizontally over the substratum, sending into it tufts of short

root-like haustoria through which food is absorbed (Fig. 92.4). Erect un- branched sporangiophores arise in clusters from the mycelium at places where the haustoria are formed. Each sporangiophore produces a large, globular, terminal sporangium. In its development, the tip of the spo- rangiophore enlarges as additional cytoplasm and nuclei pass into it (Fig. 93.4). Soon the peripheral part of the enlarging sporangium becomes denser than the central portion and a line of vacuoles appears between them (Fig. 93B). These two regions are then separated by a cleavage furrow, arising from below, and finality by a dome-shaped wall. This projects into the sporangium to form a columella (Fig. 92B).

The portion of the sporangium lying between the columella and the outer wall now undergoes a process of progressive cleavage, whereby it becomes divided into numerous small, multinucleate protoplasts by

Fig. 91. Plasmopara viticola on the stem of grape. Sporangio- phores bearing numerous spo- rangia are emerging through a stoma, X200.

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PLANT MORPHOLOGY

furrows that start at the surface and grow inward (Fig. 9ZC-E) . Finally, each protoplast secretes a cell wall and becomes a minute, black, multi- nucleate spore (Fig. 93F). The spores, produced in enormous numbers, are liberated into the air by rupture of the sporangial wall. Upon reach- ing a suitable supply of food, they give rise to new mycelia. The replace- ment of zoospores by aerial spores is a notable feature of the Mucorales.

Fig. 92. Rhizopus nigricans. A, horizontal branch of mycelium producing haustoria and sporangia, X 15; B, a mature sporangium, showing central columella, X 150; C, D, E, stages in sexual reproduction, resulting in the formation of a heavy-walled zygote, X 150.

Sexual reproduction occurs in Rhizopus only under special conditions (Fig. 92C-E'). A short lateral branch is put out by each of two hyphae lying parallel to each other. Their tips come in contact, enlarge, and from each a multinucleate cell is cut off by a cell wall. Although ordi- narily of the same size, often one cell is slightly larger than the other. Finally, the wall between the cells is dissolved and their contents fuse to form a zygote. Many of the nuclei become associated in pairs and fuse, the others disintegrating. The zygote enlarges and becomes a thick- walled resting cell. The two conjugating cells are usually interpreted as gametangia and their contents as large compound isogametes. It has been observed in other Mucorales, but not in Rhizopus, that the zygote.

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119

Fig. 93. Development of the .sporangium of Rhizopus nigricans. A, young sporangium; B, appearance of small vacuoles between outer and inner parts of sporangium; C, enlarge- ment and fusion of vacuoles to form columella cleft; appearance of cleavage furrows at outer surface; D, enlarged view, showing early cleavage furrows and scattered nuclei and vacuoles; E, sporangium completely cut off from columella; cleavage further advanced; F, mature sporangium. (After Swingle.)

120 PLANT MORPHOLOGY

upon germination, gives rise to a short hypha bearing a terminal sporan- gium. This contains many aerial spores. Meiosis occurs during the first two divisions of the fusion nucleus in the zygote.

Although all the mycelia of Rhizopus appear to be ahke, gametic repro- duction does not take place unless two sexually differentiated myceha, designated as plus and minus strains, come together. This may happen very infreciuently for, when a mycelium produces spores, all the resulting mycelia belong to the same strain and conjugation does not take place between them. Molds with sexually differentiated strains are said to be heterothaUic, while those without such differentiation are homothaUic. In homothallic species conjugation may take place between any two hyphae, even those of the same mycelium. In some of the heterothaUic Muco- rales, when a sporangium is formed at the end of a hypha arising from the zygote, a segregation of strains occurs, so that some of the spores in the sporangium produce plus myceha and others minus myceha. In other heterothaUic species this sporangium contains spores of one kind or the other, but not both kinds. In Rhizopus nigricans it is not known where the segregation of strains takes place.

Pilobolus, which lives on barnyard refuse, is an interesting mold with a peculiar method of spore dispersal. As the sporangium ripens, the por- tion of the sporangiophore just below it enlarges and becomes very turgid. Finally it bursts suddenly, shooting out the entire sporangium with considerable force, sometimes to a distance of 2 m., and always toward the brightest source of light.

Summary. The Mucorales are largely saprophytic fungi with a well- developed mycelium. They produce no zoospores, asexual reproduction occurring by aerial spores borne in sporangia. Sexual reproduction is isogamous, conjugation occurring between the entire contents of two multinucleate gametangia.

7. Entomophthorales

The Entomophthorales constitute a small group of fungi, most of which are parasitic on insects. The order includes 6 genera and about 50 species, the best-known genera being Empusa and Entomophthora. A common species, Empusa muscae, attacks the housefly. The mycelium, which is feebly developed, penetrates the body of the host and eventually kills it. Then it sends out numerous sporangiophores, from each of which a single multinucleate sporangium is cut off (Fig. 94). This is forcibly discharged into the air and, upon coming in contact with an unin- fected fly, produces a new mycelium. Although it becomes detached and functions directly as a spore, the sporangium of Empusa corresponds to the sporangium of the Mucorales. In Entomophthora the sporangiophores

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are branched and the sporangia uninucleate. Sexual reproduction seems to be absent in Emqmsa muscae but, in several other species, as in the Mucorales, it occurs by the conjugation of multinucleate protoplasts, each representing the whole contents of a gametangium.

c@

©■)

\®(

"t;

B

Fig. 94. Development of the sporangium of Empusa muscae, X600. A, hyphal body elongating to form a sporangiophore; B, migration of nuclei to apex; C, formation of multinucleate sporangium at tip of sporangiophore.

4. ASCOMYCETES

The Ascomycetes, or sac fungi, constitute the largest group of fungi. They differ from the Phycomycetes in having a septate mycelium, that is, one divided by cross walls into cells. They are also characterized by the production of spores in a sac-like structure called an ascus. This is a cell that at first contains two nuclei. These fuse and the fusion nucleus typically gives rise to eight nuclei by three successive divisions, the first two of which are reductional. From these haploid nuclei, eight walled ascospores are then organized. In all except the lowest orders, the asci are enclosed by a definite fruiting body, the ascocarp, composed of interwoven hyphae. The relationships of the Ascomycetes are obscure. They maj^ have been derived either from the Phycomycetes or from the Rhodophy- ceae. The group is of immense economic interest, many members causing serious plant diseases, such as peach leaf curl, brown rot of stone fruits, black knot of plum, apple scab, and bitter rot of apple. There are about 25,000 species of Ascomycetes. These are included in nine main orders: Protoascales, Protodiscales, Plectascales, Perisporiales, Pezizales, Helvel- lales, Tuberales, Pyrenomycetales, and Laboulbeniales.

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1. Protoascales

The Protoascales include the yeasts and other simple forms, most of which are regarded as degenerate Ascomycetes. They number about 500 species. These are mainly saprophytes but some are parasites on animals. A few of the saprophytic; forms have a mycelium. In the yeasts, which are unicellular fungi, a mycelium ordinarily is not developed. Yeasts are of economic value in breadmaking and in the preparation of alcoholic beverages. The best-known genus is Saccharomyces. Some yeasts repro- duce by fission but most of them reproduce by budding (Fig. 95). A bud

Fig. 95. Saccharomyces cerevisiae. Cells in the living condition, showing reproduction by budding, X 1,500.

arises as a small outgrowth, usually at one end of the cell. The nucleus divides to form two nuclei, one of which goes into the bud. The bud enlarges and becomes abstricted from the parent cell. It may either separate at once or remain attached and produce another bud. In this way short chains may be formed.

In many yeasts, under conditions unfavorable for vegetative activity, the contents of any cell may divide to form four or, in some species, eight thick-walled spores, thus becoming a simple ascus. In some yeasts a con- jugation of two cells precedes the formation of ascospores. The develop- ment of an ascus directly from the zygote is a feature occurring only in the Protoascales.

There is considerable variation in the life history of different yeasts, and even in the same yeast under different environmental conditions. Thus the ascospores may enlarge to form vegetative cells that undergo a long period of multiplication, or they may conjugate at once. The zygote may become an ascus directly, or may give rise to vegetative cells that later become asci. Under unfavorable conditions, vegetative multiplication may be omitted. If no conjugation occurs, the ascospores are formed by parthenogenesis.

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Yeasts present three different types of life cycles. The first may be illustrated by Schizosaccharomyces odosporus, a fission yeast. Here the vegetative cells are haploid, and eight spores arise in the cell formed by the conjugation of two cells (Fig. 96). The zygote is the only diploid cell in the life history, meiosis occurring when its nucleus divides. In the second type of life cycle, the vegetative cells are diploid. Two ascospores unite and the zygote, without undergoing meiosis, gives rise to vegetative cells that multiply and finally produce ascospores. Meiosis occurs when the spores are formed, and so they are the only haploid cells in the life history.

E F G H

Fig. 96. Schizosaccharomyces octosporiis. A to D, conjugation of two cells, the two nuclei uniting to form a single nucleus; E to G, three successive divisions of the fusion nucleus to form eight nuclei; H, formation of eight ascospores. (After Guilliermond.)

The third type of life cycle, represented by Saccharomyces cerevisiae, is more complicated. Here the vegetative cells are either haploid or diploid. When two haploid cells conjugate, the zygote gives rise to a large number of diploid vegetative cells by budding. Meiosis occurs when one of these cells forms four ascospores. The spores give rise to haploid vegetative cells that multiply by budding. These are smaller than the diploid vegetative cells.

Yeasts live in sugar solutions and are the principal agents in causing alcoholic fermentation. They use as food only a small part of the sugar that they absorb. The rest is broken down into carbon dioxide, ethyl alcohol, and small amounts of other substances. This process of fermen- tation is accomplished by the production of an enzyme called zymase. It is most active in the absence of free oxygen and serves as a means of releasing energy when the ordinary type of respiration cannot be carried on.

2. Protodiscales

The Protodiscales, numbering less than 100 species, are internal para- sites attacking seed plants, especially trees. The only genus is Taphrina. A common species, Taphrina deformans, causes a disease of peaches

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PLANT MORPHOLOGY

known as peach leaf curl, while Taphrina pruni produces a disease of the domestic plum called plum pockets, in which the fruit becomes shriveled. Taphrina cerasi attacks branches of the cherry, causing brush-like deform- ities known as witches'-brooms.

The mycelium of Taphrina grows in the intercellular spaces of the host and sends to the surface groups of asci that arise just beneath the cuticle (Fig. 97). Each ascus contains eight ascospores. The asci are crowded to form a layer, called the hymenium, but are Avithout accom- panying sterile hyphae. Moreover, an ascocarp is not developed and

Fig. 97. Taphrina deformans. Cross section of portion of peach leaf, showing layer of asci and ascogenous cells on the surface, X500.

there is no formation of sex organs. The cells of the mycelium are binucleate. The two nuclei in the young ascus fuse, three successive divisions result in the formation of eight free nuclei, and from these the eight ascospores are organized. Upon germination, the ascospores, which are haploid, may give rise to one or more uninucleate cells by a process that resembles budding in yeasts. In some species these cells, which are called conidia, are formed while the ascospores are still within the asci. The ascospores, or the conidia produced by them, infect new host plants, a hypha penetrating the cuticle and pushing its way between the epidermal cells. The germinating spore may become binucleate by divi- sion of its nucleus, or a pair of ascospores or conidia may conjugate, a nucleus passing from one to the other. The binucleate condition is then transmitted to the cells of the vegetative mycelium.

3. Plectascales

The Plectascales include the blue and green molds, saprophytes that are abundant everywhere, occurring on bread, cheese, jelly, fruits, vege- tables, meat, leather, etc. The order includes over 30 genera and 800 species. The two commonest genera are Aspergillus and Penicillium, the latter numbering over 500 species. One species, Penicillium notatum,

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125

produces a substance, called penicillin, that has remarkable germicidal properties. It has recently come into prominence as a valuable agent in the treatment of many infections and diseases caused by certain bac- teria, particularly cocci. Its great advantage over many other drugs lies

Fig. 98. Branching conidiophores of PeniciUium producing chains of conidia, X800.

B E

Fig. 99. Aspergillus niger. A to E, successive stages in the development of a conidiophore and its conidia, as seen in optical section, X400.

in its almost complete nontoxicity to the human body. Substances like penicillin are called antibiotics.^

The mycelium of the Plectascales produces special branches, called conidiophores, that cut off chains of spores, or conidia, enormous numbers

1 Most antibiotics, including streptomycin, aureomycin, and Chloromycetin, are derived from actinomycetes, a group of organisms of which some are mold-like and others bacteria-like. They are variously classified with the Fungi Imperfecti, the bacteria, or as a distinct group of fungi. Some are parasites but most are saprophytes prevalent in the soil.

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PLANT MORPHOLOGY

of which are liberated into the air. Upon coming in contact with a suit- able food supply, the conidia produce new mycelia. In Penicillium the conidia arise from the ends of branched conidiophores (Fig. 98). In Aspergillus the conidia are abstricted from the ends of short hyphae that radiate from the enlarged tip of a conidiophore (Fig. 99).

A B C

Fig. 100. Development of the ascocarp of Aspergillus. A, sex organs; B, sterile hyphae enclosing the sex organs; C and D, later stages, showing the development of asci. (From a Turtox classroom chart.)

Fig. 101. Section through a mature ascocarp of Aspergillus, showing the completely enclosed asci, X 500.

The sex organs are represented by two short, spirally twisted filaments, the contents of which appear to fuse (Fig. 100). Then ascogenous hyphae, bearing numerous small asci, arise from one of the filaments. These are intermixed with and surrounded by sterile hyphae, those on the outside forming a minute, globular, closed ascocarp. A fruiting body of this type is known as a cleistothecium. There is no definite hymenium in the

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Plectascales, the asci being irregularly scattered throughout the mass of sterile hyphae (Fig. 101).

4. Perisporiales

The Perisporiales, or powdery mildews, are superficial parasites attack- ing many kinds of seed plants, such as grape, lilac, willow, rose, squash, bean, pea, apple, grasses, and numerous others. They number about 500 species. Common genera are Sphaerotheca, Erysiphe, Uncinula, Podosphaera, Microsphaera, and Phyllactinia. The mycelium lives on

Fig. 102. Erysiphe graminis growing on surface of grass leaf, showing haustoria in epidermis of host and conidia in various stages of development, X500.

the surface of the leaves, forming whitish patches. Short haustorial branches are sent into the epidermal cells and through them food is absorbed. During the summer the mycelium produces erect conidio- phores, which give rise to chains of conidia (Fig. 102). These are very abundant and result in a rapid spread of the fungus to uninfected hosts. In the autumn closed ascocarps (cleistothecia) appear. They are minute, spherical, dark brown or black bodies with long appendages that, in some genera, are branched at the tip (Fig. 103). Inside the ascocarps are the asci, each usually with eight ascospores. The ascocarps, scattered by the wind, survive the winter. During the next season the ascospores pro- duce new mycelia.

The character of the appendages produced by the ascocarps is impor- tant in distinguishing genera from one another. Thus in Sphaerotheca and Erysiphe the tips of the appendages are undivided, while in Podo- sphaera and Microsphaera they are dichotomously divided. In Uncimda the tips of the appendages are hooked or curved, while in Phyllactinia they are straight but the appendages are swollen at the base so as to form an enlarged plate.

128 PLANT MORPHOLOGY

The sex organs arise from uninucleate cells formed at the tips of special branches of the mycelium, all the (;ells of which are uninucleate (Fig. 104). The antheridium, slightly smaller than the oogonium (ascogo- nium), comes in contact with it. The intervening cell wall is dissolved and the male nucleus passes over to fuse with the female nucleus. Ster- ile hyphae, arising from the cell beneath the oogonium, form a closed ascocarp. Following fertilization, the fusion nucleus gives rise to three

Fig. 103. Ascocarp of Microsphaera alni with characteristic appendages, crushed slightly so that three asci, each with eight ascospores, have appeared, X 250.

to five (often more) free nuclei and then transverse walls come in, form- ing a short row of cells. All of these are uninucleate except the penul- timate cell, which is binucleate. In Sphacrofheca and Podosphaera this cell directly forms a solitary ascus in which the two nuclei fuse, while in the other genera it gives rise either to a row of cells, each of which becomes an ascus, or to ascogenous hyphae that, in turn, produce the asci. Although in Sphacrotheca and Podosphaera the ascocarp has only one ascus, in the other genera it contains a basal layer of several parallel asci. The development of the ascus takes place in the regular way, except that it frequently contains less than eight ascospores. Eight nuclei are formed as usual, but some are not organized into spores. The asci are generally not intermixed with sterile hyphae.

If the male and female nuclei actually fuse in the oogonium, the fusion

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in the young ascus involves two diploid nuclei, necessitating a double reduction of chromosomes in the two meiotic divisions that immediately follow. This beha\ior has been disputed by some investigators, who assert that the only nuclear fusion occurs in the young ascus and involves haploid nuclei, some claiming that the male and female nuclei remain

F G

Fig. 104. Sphaerothera castagnei. A, antheridial and oogonial branches in contact; B, antheridial branch cut off by a wall; C, antheridial cell separated from stalk cell; D, union of male and female nuclei in oogonium; E, oogonium with zygote nucleus and two layers of investing hyphae derived from cell just below; F, multicellular ascogonium, the penulti- mate cell, with two nuclei, becoming the ascus; G, young ascus with fusion nucleus and two ascogonial cells below it. {After Harper.)

distinct in the oogonium, others that the antheridium is nonfunctional and a male nucleus does not enter the oogonium. If these views are cor- rect, the fusion nucleus in the young ascus is diploid and divides meiot- ically in the usual way.

5. Pezizales

The Pezizales, or cup fungi, grow mostly on decaying wood or humus, but some are parasitic on seed plants. They are a large order of approxi- mately 5,000 species. The principal genera include Pyronema, Peziza, Ascoholus, Lachnea, and Sclerotinia.

Pyronema. This is a saprophyte on soil, especially where it has been burned over. The mycelium grows as a white fluffy layer on the surface.

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It bears well-developed sex organs. The female organ resembles the procarp of Nemalion. It consists of a globular, multinucleate basal por- tion, the ascogonium, and an elongated curved cell, the trichogyne, arising from its upper end (Fig. 105.1). The antheridium, which is terminal, club-shaped, and multinucleate, arises from an adjacent hypha. It comes in contact with the tip of the trichogyne, whose nuclei degenerate, and discharges its contents into it. The wall at the base of the trichogyne

A B

Fig. 105. Pyronema conftuens. A, ascogonium and trichogyne with antheridium in con- tact with its tip and discharging nuclei into it. Antheridium is curved around trichogyne and appears in section to be cut in two. B, somewhat diagrammatic section of a young ascocarp, involving two ascogonia from which ascogenous hyphae and paraphyses have arisen. Asci are shown in various stages of development. (After Harper.)

disappears and the male nuclei migrate into the ascogonium, where multi- nucleate pairing of male and female nuclei occurs. The nuclei do not fuse, however, until an ascus is formed.

Following fertilization, the ascogonium is cut off from the trichogyne by a new wall and branching ascogenous hyphae arise from it (Fig. 1055). These give rise to asci. Sterile hyphae (paraphyses) grow up from the mycelium and intermingle with the asci, the entire group of fertile and sterile hyphae becoming surrounded by a fleshy ascocarp. Ordinarily several sets of sex organs enter into the formation of a single ascocarp. The ascocarp of Pyronema is disk-shaped, red or yellow, and only 2 or 3 mm. in diameter. The asci and paraphyses form a definite layer, the hymenium, that covers its upper surface. A broadly open ascocarp is called an apothecium, a type of fruiting body that is characteristic of the Pezizales.

The origin of the asci is somewhat complex (Fig. 106). The paired

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nuclei of the ascogonium pass into the ascogenous hyphae, where they multiply. The members of each pair remain together as walls are formed. The terminal cell of a branch that is to become an ascus bends back to form a hook and its two nuclei divide simultaneously. Three

Fig. 106. Origin of the ascus in Pyronema confluens. A, hook formation at tip of ascog- enous hypha; B, simultaneous division of nuclei; C, formation of uninucleate terminal and basal cells and of binvicleate penultimate cell; D, fusion of nuclei in penultimate cell to form an ascus; also migration of nucleus of basal cell into terminal cell; E, same stage except that nucleus of terminal cell has migrated into basal cell; F, later stage showing development of hook from basal cell; G, development of three hooks and an ascus from binucleate tip of an ascogenous hypha. (After Claussen.)

cells are now cut off by walls. The terminal and basal cells are uninu- cleate but the middle one (the penultimate cell) has two nuclei of opposite sex, these being the descendants of a male and female nucleus that came from the fertilized ascogonium. The two nuclei may now fuse and the middle cell become an ascus, or the nucleus from the terminal cell may migrate into the basal cell and another hook may be formed. This

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behavior may be repeated a number of times. Each cell in which a nuclear fusion occurs may become an ascus, the fusion nucleus under-

FiG. 107. Ascocarps of Peziza growing on decaying wood, natural size.

going the usual three successive divisions to produce eight ascospore nuclei. The significance of hook formation, which occurs in many

Ascomycetes, is a puzzle.

Peziza. Peziza is one of the best-known cup fungi, including about 150 species. It is a common saprophyte on rich humus or on decaying wood. The mycelium, which is ex- tensive and much branched, penetrates the substratum and gives rise on the surface to smooth, fleshy, cup-like ascocarps 1 to 5 cm. or more in diameter (Fig. 107). These are generally bright red, brown, or gray. As in Pyronema, the ascocarp is lined with a layer of parallel asci and paraphyses, these consti- tuting the hymenium (Fig. 108). Each ascus contains eight ascospores. Upon germination, these produce new mycelia. In Peziza the ascocarp apparently arises directly from the mycelium without any formation of sex organs. Sclerotinia. A parasitic cup fungus, Sclero- tinia fructicola, attacks plums and peaches, causing a disease known as brown rot of stone fruits. The twigs, flowers, and fruits become infected with the mycelium. As the fruit turns brown and decays, great numbers of conidia are formed on the surface. These are cut off in chains from the ends of short conidiophores. The conidia carry the fungus to

Fig. 108. Several mature asci of Peziza, each with eight ascospores, intermixed with paraphyses, X 250. Some young asci are arising below.

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uninfected trees. The fungus is usually carried over the winter on dried diseased fruits, called "mummies," that remain on the tree and furnish a fresh source of conidia the following spring. Brown cup-like ascocarps, which are rare, resemble those of Peziza and may be formed early in the season on mummified fruits lying on the ground.

Summary. The Pezizales are mostly saprophytes but some are para- sites. All have a well-developed mycehum. The asci, accompanied by paraphyses, form a hymenial layer that lines an open, disk-like or cup- like ascocarp, the apothecium. This may be fleshy or leathery and ses- sile or stalked. Some members have well-developed sex organs, the asci arising from the fertilized ascogonium. In other members the asci arise directly from the mycelium, sex organs being absent.

6. Helvellales

The Helvellales are related to the Pezizales, being distinguished from them mainly by the form of the ascocarp, which is also an apothecium but

Fig. 109. Ascocarps of Morchella (A) and Helvella {B), natural size.

is more highly differentiated. The Helvellales are saprophytes that grow chiefly on humus. They number about 300 species. The best-known genera are Morchella and Helvella.

Morchella. The common edible morel (Morchella esculenta) has a much-branched mycelium growing in rich humus soil. On it are formed compact masses of hyphae that develop into fleshy ascocarps of character- istic form. These come to the surface of the soil, where they often attain

134 PLANT MORPHOLOGY

a height of 15 to 20 cm. A mature ascocarp of Morchella is differentiated into a thick hollow stalk and a conical cap (Fig. 109A). The surface of the cap contains numerous depressions lined with a hymenium consisting of parallel asci and paraphyses.

Helvella. The mycelium is subterranean and composed of hyphae with multinucleate cells. It gives rise to fleshy ascocarps that push upward to the surface of the ground, there reaching a height of about 5 cm. These are differentiated into a stout stalk and a saddle-shaped cap, the outer surface of which is covered with a hymenium consisting of parallel asci and paraphyses (Fig. 109B). The asci contain eight ascospores and discharge them into the air with considerable force.

7. Tuberales

The Tuberales are the well-known truffles, esteemed as a gastronomic delicacy. There are nearly 300 species, the representative genus being Tuber. The mycelium is subterranean, especially in woods, some forming the mycorrhiza of forest trees. Truffles occur in California and in various parts of southern and central Europe. Their life history is incompletely known. The ascocarp is fleshy and matures underground. It is more or less globular, its diameter rarely exceeding 8 cm. It is usually open when young but later nearly or completely encloses the asci. The ascocarp is thus a modified apothecium. The hymenium may surround a large cen- tral cavity or it may form irregular folds that divide the cavity into chambers.

8. Pyrenomycetales

The Pyrenomycetales, or black fungi, are a large order of about 450 genera and 14,000 species that are generally segregated into three smaller orders, the Hypocreales, Dothideales, and Sphaeriales. They include saprophytes that live on decaying wood, humus, etc., and parasites that attack various seed plants. Some representative genera are Nectria, Claviceps, Plowrightia, Venhiria, Xylaria and Neurospora.

Nectria. This large genus of about 250 species grows on living or dead wood. It is responsible for several important fungous diseases. One of the most destructive of these, canker of woody plants, is caused by Nectria cinnabarina and Nectria galligena. They attack a great variety of shrubs and trees, but not conifers. The fungus gains entrance through wounds in the stem. The cortex becomes infected and its cells are immediately killed. This results in a wound that gradually enlarges. Sometimes enough cork tissue is developed around the infected area to close the wound, but usually this is not possible and the trunk is finally girdled. During the summer the mycelium produces large, pinkish, disk-like masses, or stromata, that break through the bark and give rise to large

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135

numbers of conidiophores, the conidia being carried by the wind to new hosts. Later in the season small, red, flask-shaped ascocarps, called perithecia, are developed on the stromata (Fig. 110).

Claviceps. The common ergot disease of rye and other grasses is caused by Claviceps purpurea. Its damage to the rye is usually slight, but the eating of diseased grain by animals results in a paralysis and other serious conditions. A drug derived from the fruiting bodies of this fungus, called ergotine, has important uses in medicine. The ovaries of the rye are infected by ascospores in the early summer and become hypertrophied, a

Fig. 110. Stroma of Nectria cinnabarina on bark of Rihes, showing two perithecia with young asci and paraphyses, X75.

mycelium developing within. The formation of conidia soon follows. The conidia are minute cells abstricted from the tips of short conidio- phores. As they are formed, a sweet liquid is exuded from the spikelet. This attracts insects, which carry the conidia to uninfected flowers. Later the mycelium hardens to form a compact sclerotium, which replaces the ovary of the flower. The sclerotia are elongated, slightly curved, purplish bodies that project from the ears of the rye. Many of them eventually fall to the ground, where they pass the winter. In the spring the sclerotium produces several or many globular, stalked stromata, which are compact mycelial masses containing numerous flask-shaped, deeply embedded perithecia (Fig. 111). The entire stroma is cream-colored at first, becoming grayish violet. Each perithecium is fined with a hyme- nium consisting of many asci and paraphyses. The ascospores, which are needle-shaped, are discharged forcibly and dispersed by the wind.

The sex organs of Claviceps are borne on hyphae lying below the surface of the stroma. The ascogonium is broader than the antheridium and both are multinucleate. The contents of the antheridium enters the

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PLANT MORPHOLOGY

ascogonium, which then gives rise to ascogenous hyphae. As mPyronema, the asci arise as a result of hook formation at the tips of the ascogenous hyphae. Their development occurs in the typical manner.

' Plowrightia. This is another parasitic genus, its best-known species, Plowrightia morbosa, causing a destructive disease of the plum and cherry known as black knot. The mycelium passes the winter under the bark of

Fig.

B C

111. Claviceps purpurea. A, stalked stromata arising from a sclerotium, X4; B,

longitudinal section through a stroma, showing the embedded perithecia, X30; C, a perithecium with young asci and paraphyses, X 250.

a branch or twig. In the spring it breaks out on the surface to form an elongated gall or knot consisting of both mycelium and hypertrophied host tissue (Fig. 112). Leaves and fruits are not attacked. The elongated knots, often reaching a length of 12 cm. or more, are developed mostly on one side of the stem, which becomes more or less deformed. In early summer the mycelium within the knot gives rise to innumerable short conidiophores that form a velvety layer on the surface. The conidia, distributed by the wind, spread the fungus to other hosts. Later in the season conidium formation ceases and the knot becomes hard and black, forming a stroma in which hundreds of perithecia appear (Fig. 113). These are small flask-shaped organs, embedded in the stroma, and lined with a hymenium consisting of asci and paraphyses. The ascospores

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mature and are liberated during the following spring. Like the conidia, they directly infect new hosts.

Venturia. Venturia inaequalis is the cause of an apple disease known as apple scab. It affects chiefly the leaves and fruits, producing brown spots that become scaly as a result of cork formation. The mycelium grows between the cuticle and the epidermis. It forms large numbers of

Fig. 112. Galls produced on cherry twigs by the black-knot fungus, Plowrightia morbosa, natural size.

conidiophores that break through to the surface (Fig. 114A). Conidia, abstricted from their tips, spread the fungus during the summer to other apple trees. In the autumn, after the infected leaves fall to the ground, the mycelium becomes saprophytic and produces perithecia in the follow- ing spring (Fig. 1 145). These appear on the lower side deeply embedded within the leaf tissues. Sex organs are produced, but the ascocarp begins to develop before fertilization has occurred. The ascogonium is long, coiled, and multinucleate. It has a trichogyne with which the anther- idium comes in contact. Following fertilization, the ascogonium gives rise to ascogenous hyphae from which asci are developed as a result of hook formation at their tips. The ascocarps (perithecia) are dark brown and flask-shaped when mature, discharging the ascospores forcibly.

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A B

Fig. 113. Plorvriyhtia morhosa. A, section of stroma bearing young perithecia, X50; B, a single peritheciuni with young asci and paraphyses, X 150.

Fig. 114. Venturia inaequalis. A, conidiophores arising on lower side of apple leaf, X 600; B, section of mature perithecium on old apple leaf, X 250.

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139

Xylaria. Xylaria is a large genus of about 200 species. It is a common saprophyte, the myeehum Uving in decaying wood. It produces sclerotia from which black, club-shaped, often branched stromata arise. At first these are covered with a mass of white conidiophores from which small oval conidia are abstricted. Later the stromata produce numerous embedded, flask-shaped perithecia lined with a hymenium (Fig. 115).

Neurospora. This is the pink bread mold, a form much used experi- mentally in genetics. The mycelium produces conidia in branched chains. Perithecia are rarely formed. They are dark-colored, pear-shaped, and without paraphyses. Like Rhizopus, Neurospora is heterothallic and sexual reproduction occurs only when a plus and a minus strain come together. The young peri- thecium contains a coiled ascogonium from which trichogynal hyphae grow out. If these come in contact with spermatia, conidia, or hyphae of the opposite strain, the perithecia mature and asci are pro- duced. Two nuclei of opposite sex fuse in the young ascus, the fusion nucleus undergoes three divisions of which the first two are meiotic, and eight ascospor^ are formed in the usual way. Experi- ments have shown that sexual differen- tiation occurs in connection with asco- spore formation, usually during the first meiotic division but sometimes during

the second. As a result, four ascospores in each ascus will produce plus mycelia and four minus mycelia. Other genetic characters behave similarly.

Summary. The Pyrenomycetales include both saprophytes and para- sites. They are characterized by a flask-shaped ascocarp (a perithecium) with a small opening at the top. It is Uned with a hymenium composed of parallel asci and paraphyses. The perithecia may arise singly on the mycelium, in small groups, or may be embedded in a compact mycelial mass, the stroma. Sex organs are present in some members.

Differences in the character of the perithecia and stromata provide a basis for splitting up this large order into three smaller orders.

1. Hypocreales. These forms have soft, bright-colored perithecia with a definite wall. The perithecia may occur singly or in a stroma, which is also bright-colored. They include Nectria and Claviceps.

2. Dothideales. Members of this group have black stromata in which

Fig. 115. Longitudinal section through a perithecium of Xylaria, showing asci arising from the hymenium, X 100.

140 PLANT MORPHOLOGY

the perithecia, lacking independent walls, are developed as stromatal cavities. Plowrightia belongs here.

3. Sphacriales. The Sphaeriales have dark-colored perithecia with a distinct wall. The perithecia may be free or embedded in the substratum or in stromata that are firm, leathery or brittle, and dark-colored. Here belong Venturia, Xylaria, and Neurospora.

9. Laboulbeniales

The Laboulbeniales comprise an order of about 50 genera and 1,200 species. They are parasitic on insects, especially aquatic ones. As a rule the mycelium grows on the surface of the host and is very small, usually less than 1 mm. in length. The Laboulbeniales are of particular interest because their sex organs are remarkably like those of the red algae. The antheridium is unicellular and produces a nonmotile male cell, the spermatium. The ascogonium has a trichogyne and auxihary cells. Ascogynous hyphae arise from the fertilized ascogonium, small perithecia are formed, and the asci bud out from the auxiliary cells. The whole process resembles cystocarp formation in the red algae.

5. BASIDIOMYCETES

The Basidiomycetes, or club fungi, comprise the highest group of fungi. They resemble the Ascomycetes in having a mycelium with cross walls. They are characterized by the production of spores externally on a club- like structure known as a hasidiiOm. This arises from the swollen end of a hypha and may consist of either four cells or one. Four slender branches (sterigmata) arise from the basidium, each forming a hasidiospore at its tip. The young basidium contains a nucleus derived from the fusion of two nuclei. Two successive divisions, which are reductional, result in the formation of fourhaploid nuclei, each passing into one of the basidiospores. In the higher members the basidia are borne on a distinct fruiting body, the hasidiocarp, composed of interwoven hyphae. The Basidiomycetes are related to the Ascomycetes and are generally regarded as having been derived from them. Some are of great economic importance, particularly the smuts, rusts, and mushrooms. The Basidiomycetes number about 20,000 species. They embrace seven principal orders: Ustilaginales, Uredinales, Auriculariales, Tremellales, Exobasidiales, Hymenomycetales, and Gasteromycetales. '^

1. Ustilaginales

The Ustilaginales, or smuts, are parasites that live on various her- baceous seed plants. They attack chiefly floral organs, particularly those of grasses. They are most destructive to oats, less so to wheat and corn. The smuts number about 500 species. The principal genera are Ustilago and Tilletia.

THALLOPHYTA: FUNGI

Ul

Ustilago. The life history of the corn smut (Ustilago zeae) will be described. The mycelium ramifies throughout the stem and leaves of the corn plant and in its vegetative condition does not seem to do much dam- age. It lives in the intercellular spaces, sending short haustoria into the host cells. When flowers appear, some of the ovaries become packed with the mycelium and, as a consequence, become greatly swollen and distorted. Swellings may also appear in other parts of the plant. Later the mycelium divides up into countless numbers of black spores, called

Fig. 116. Ustilago zeae. A, an ear of corn infected with smut, some of the grains of which are greatly enlarged and filled with chlamydospores, one-half natural size; B, a germinating chlamydospore, the four-celled basidium producing basidiospores, X 1,400.

chlamydospores, which form large powdery masses (Fig. 116^4). A chlamydospore is a heavy-walled cell representing merely a transformed cell of the vegetative mycelium.

A chlamydospore may germinate at once but, as a rule, falls to the ground and remains dormant until the following spring. Then it sends out a short filament of three or four cells that lives saprophytically on organic matter in the soil (Fig. 1165). Thin-walled basidiospores are bud(ied off each cell of the filament, often in great numbers. This fila- ment is a basidium but, because of the large number of spores produced, is not a typical one. In some smuts, however, only one spore is budded off each of the four cells of the basidium. The basidiospores infect young corn plants in the spring.

The cells of the vegetative mycelium are binucleate, as are the j^oung chlamydospores. But before the chlamydospore is mature the two nuclei

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PLANT MORPHOLOGY

fuse, thus establishing the diploid condition. The fusion nucleus divides reductionally in the young basidium and four cells are formed by the appearance of transverse walls, thus separating the four haploid nuclei. When a basidiospore is budded off, two nuclei are formed, one of which passes into the spore while the other remains in the basidium. The latter may divide again, if another spore is budded off, and this may be repeated many times. These haploid basidiospores produce on the young corn plant mycelia of limited extent and with uninucleate cells. When two mycelia of opposite sex come together within the host, a union of cells takes place without a fusion of nuclei. The binucleate cells formed in this way give rise to a mycelium that spreads throughout the host, eventually producing chlamydospores.

2. Uredinales

The Uredinales, or rusts, are destructive parasites. They attack a great variety of vascular plants, including ferns, conifers, and angio- sperms, being especially common on grasses. The mycelium lives in the intercellular spaces, particularly of the leaves. There are about 3,000 species of rusts, the most important genera being Puccinia, Uromyces,

Fig. 117. Puccinia graminis. Section through a uredinium on a leaf sheath of wheat, showing uredospores in various stages of development, X200.

Gymnosporangium, Phragmidium, Cronartium, Coleosporium, and Metkrnp- sora. The largest genus, Puccinia, has about 700 species.

Puccinia. The common wheat rust {Puccinia graminis) is the best- known member of the order. Its life history is very complicated, involv- ing two different hosts and several kinds of mycelia and spores, all with a definite relation to one another.

The mycelium that lives on the wheat is an internal parasite, extend-

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ing throughout the entire body of the host. It does not directly kill the host cells, but lives on their food materials, which it absorbs by means of haustoria. During the late spring and early summer numerous spores are produced. They break through the epidermis of the leaves, groups of them, known as uredinia, appearing on the surface as reddish brown streaks or lines (Fig. 117). These spores are called uredospores. Each consists of a stalked binucleate cell with a rather thick cell wall. They are scattered by the wind, directly infecting other wheat plants, and are chiefly responsible for the rapid spread of the disease, especially during a wet season. Successive crops of uredospores may be produced through- out the summer.

Fig. 118. Puccinia graminis. Section through a telium on a leaf sheath of wheat, showing teliospores in various stages of development, X 200.

Later in the season, at harvest time or thereabouts, the same mj^celium that produced the uredospores earlier now gives rise to elongated groups of black spores called teliospores. These groups, known as telia, appear chiefly on the stems and leaf sheaths (Fig. 118). The teliospores are also stalked but are two-celled and heavy-walled. At first each cell has two nuclei, but the members of each pair fuse as the spore matures. The teliospores do not germinate until the next spring, thus carrying the fungus over the winter. Upon germination, one or both cells of the teliospore gives rise to a short filament. This filament is the basidium (Fig. 119.4). It consists of four cells, each of which sends out a short branch, called a sterigma, bearing a small terminal basidiospore. The basidiospores cannot infect wheat plants. They are carried by the wind to leaves of the common barberry {Berberis vulgaris), where they germi- nate and produce an extensive internally parasitic mycelium. It is mainly this species that is susceptible to infection by the basidiospores of wheat rust. Most other barberries are immune.

The mycelium produced by the basidiospores on the barberry develops spermogonia (pycnidia), small flask-shaped organs appearing on the upper

^44 PLANT MORPHOLOGY

side of the leaves (Fig. 119^). In these organs small cells, called sper- matia (pycnospores), are formed by abstriction from the ends of slender hyphae. The spermatia are exuded from the spermogonia in drops of a sweet liquid. This attracts insects that aid in their dissemination. Soon

Fig. 119. Pucciniagraminis. Stages on the barberry. ^, four basidiospores arising from a basidium produced by a teliospore, X300; B, leaf of common barberry with groups of aecia, natural size; C, enlarged view of group of aecia, X 10; D, longitudinal section of aecium with numerous aeciospores arising in chains, X200; E, longitudinal section of spermogonium producing numerous small spermatia, X200. {A, after Chamberlain.)

after the appearance of the spermogonia, the mycelium on the barberry produces larger, cup-like structures that appear in clusters on the lower side of the leaves (Fig. 119S and C). These aecia, or cluster cups, con- tain large numbers of aeciospores, which arise in chains from the bottom

THALLOPHYTA: FUNGI 145

of the cup (Fig. 119/)). The chains consist of alternating spores and sterile cells, the latter disintegrating. The aeciospores cannot infect the barberry. Instead, carried by the wind, they infect wheat plants during the late spring and summer, thus completing the life cycle.

The cells of the mycelium produced by the basidiospores on the bar- berry, as well as the spermatia, are uninucleate, but the aeciospores are binucleate. The binucleate condition appears to arise by the spermatia coming in contact with special receptive hyphae of the opposite sex. These extend from the basal cells of the young aecium to the orifice of the spermogonium, through which they project. A spermatium enters a receptive hypha and passes down into the basal cell, which then becomes binucleate. Each binucleate basal cell gives rise by repeated division to a chain of aeciospores. The binucleate condition is carried over by the aeciospores to the mycelium on the wheat and to the uredospores and young teliospores produced by it.

The fusion of the two nuclei in each cell of the teliospore introduces the uninucleate condition. When the teliospore germinates, the diploid nvicleus in each of its cells undergoes two successive meiotic divisions that result in the formation of four haploid nuclei. Each of the four cells in the basidium receives one of these nuclei, which then passes into a basidiospore. The uninucleate basidiospores, being haploid, produce a haploid mycelium on the barberry. Thus, although there is an alternat- ing haploid and diploid phase in the life history of Puccinia, the latter is not initiated by a nuclear fusion, as in most plants, but by the coming together in the same cell of two nuclei that retain their identity through- out a large number of cell divisions. Eventually the nuclear fusion occurs, but is then followed by the reduction divisions, which mark the beginning of the haploid phase.

Other Rusts. The wheat plant is attacked not onl}^ by Puccinia graminis, its most destructive rust, but by several related species. One of these is Puccinia coronata, whose alternate host is the buckthorn {Rhamnus); another is Puccinia rubigo-vera, which produces the aecial stage on blueweed (Echiuni) and other Boraginaceae. All three species may attack other grasses than wheat, such as barley, oats, rye, and various meadow grasses, producing the same morphological type of mycelium and spores on each kind of grass but a different physiological strain.

Many rusts have a shorter life cycle than Puccinia graminis. All rusts produce teliospores and these always give rise to basidia and basidio- spores, but one or more of the other spore forms may be missing. Thus the aecia may be omitted, the uredospores, the aecia and spermatia, or the aecia, spermatia, and uredospores. If a rust requires two different and unrelated hosts to complete its life cj^cle, it is said to be heteroecious;

146 PLANT MORPHOLOGY

if all stages are passed on the same host, or on closely related hosts, it is autoecious. Gymnosporangium juniperi-virginianae, a heteroecious rust, has no uredospores. It develops the telial stage on the red cedar {Junip- erus virginiana), or related species, and the aecial-spermogonial stage on the apple, pear, and quince (Pyrus). A heteroecious rust of great eco- nomic importance is Cronartium ribicola, the white pine blister rust. Its uredospore-teliospore stage is passed on various species of currants and gooseberries (Ribes), its aecial-spermogonial stage on the white pine {Pinus strobus) and related species. The damage to white pines has been so great that it has resulted in their virtual extinction in many parts of the country. Puccinia asparagi is an autoecious rust, producing uredospores, tehospores, aeciospores, and spermatia on the asparagus. Pxiccinia malvacearuvi, another autoecious rust, has a very short life cycle, producing only teliospores on the hollyhock and other Malvaceae.

3. Auriculariales

The Auriculariales are the ear fungi, an order of about 15 genera and over 100 species. They are chiefly saprophytes growing on bark and decaying wood. The representative genus is Auricularia. The myce- lium produces brightly colored, gelatinous, ear-shaped bodies, each being a basidiocarp. When dry, the basidiocarps become wrinkled and hairy. The inner surface is lined with a hymenium consisting of basidia inter- mixed with paraphyses. As in the Uredinales, the basidia are four-celled and have sterigmata. Each basidium produces four basidiospores. This order may be regarded as transitional between the lower and higher Basidiomycetes.

4. Tremellales

The Tremellales, or trembhng fungi, are somewhat similar to the Auriculariales. They include 18 genera and nearly 100 species, the best- known genus being Tremella. The mycelium lives in decaying wood and bark, producing gelatinous basidiocarps. These are indefinite in form and more or less wavy or folded. The hymenium occurs on the upper surface. The basidia are characteristic, being longitudinally divided into four cells instead of transversely divided. Each basidium bears four basidiospores on long sterigmata.

5. Exobasidiales

The Exobasidiales are internal parasites attacking particularly mem- bers of the Ericaceae, such as blueberries, cranberries, huckleberries, azaleas, etc. There are about 30 species, nearly all belonging to the genus Exobasidium. Galls composed of mycelium and host tissue are produced on stem tips, leaves, and floral organs. The basidia are formed under the epidermis and, when they break through, cover the host with a

THALLOPHYTA: FUNGI

147

whitish bloom. There is no formation of basidiocarps, the basidia arising directly from the mycelium. In this and succeeding orders the basidium is one-celled. The young basidium has two nuclei that fuse, two succes- sive nuclear divisions follow, and four basidiospores are developed, each at the end of a sterigma.

6. Hymenomycetales

This large order of approximately 15,000 species is usually split up into several smaller orders, but here will be regarded as one homogeneous group. Most of the members are saprophytic on humus, bark, decaying wood, etc. Some are parasitic on trees, often causing considerable damage. The Hymeno- mycetales have complex basidiocarps with basidia in a definite hymenial layer that be- comes freely exposed. The basidia are one- celled and bear four basidiospores, each at the end of a slender sterigma.

Families. The families of Hymenomyce- tales are distinguished from one another on the basis of the form of the basidiocarp and the position of the hymenium. The principal families are as follows:

1. Thelephoraceae. These forms produce simple basidiocarps appearing on tree trunks. Some resemble leathery incrustations with the hymenium on the smooth upper surface, while some are bracket-like with the hymenium on the lower surface. Others have the hymenium on the outside of a funnel-like basidiocarp. The representative genus is Thelephora, with about 150 species.

2. Clavariaceae. The coral fungi produce erect, fleshy basidiocarps that are usually branched like coral, the hymenium covering the surface of the branches (Fig. 120). They are commonly white or yellowish, but sometimes are more brightly colored. In some forms the basidiocarps are club-shaped and unbranched, with a complete hymenial covering. The principal genus is Clavaria, wdth about 250 species.

3. Hydnaceae. These are the tooth fungi, the hymenium being borne on tooth-like or spine-like processes that generally point downward. The simpler forms occur as rounded masses or thin sheets of indefinite form. Some are more or less branched. Others have a stalk and an umbrella-like pileus that bears teeth on its lower side. The main genus is Hydnum, with about 150 species.

4. Polyporaceae. The pore fungi bear a number of tubes or grooves lined with a hvmenium. The basidiocarp may be crustaceous, the tubes

Fiu. 120. A coral fungus {Clavaria), natural size.

148

PLANT MORPHOLOGY

Fig. 121. Agaricus campestiis, four-fifths natural size. A, mature basidiocarp, showing pileus, stipe, and annulus; B, view of underside of pileus with stipe removed, showing the radiating gills; C, young basidiocarp before the pileus has expanded; D, young basidiocarp cut in half, showing velum attached to stipe.

opening on its upper side. Ordinarily, however, the basidiocarp is bracket-like or umbrella-like, the tubes opening on its lower side. The texture of the basidiocarp may be leathery, fleshy, or hard and woody. Some of the largest genera are Merulius, Porta, Fomes, Polyporus, Poly- stichuSy and Boletus. Polyporus, the largest genus, has about 500 species.

THALLOPHYTA: FUNGI

149

MeruUus lacrymans is the dry-rot fungus, a species attacking woodwork and structural timbers. It often causes great destruction to wooden buildings.

5. Agaricaceae. This is the large family of gill fungi, a group to which the common mushrooms and toadstools belong. The basidiocarp may be bracket-like but more commonly is umbrella-hke. It is usually fleshy, rarely leathery in texture. In this family the hymenium covers blade- like radiating plates known as gills. Of the numerous genera, a few common ones are Coprinus, Agaricus, Amanita, Lepiota, Hypholoma, Russula, and Marasmius. The largest genus is Marasmius, with about 450 species.

Fig. 122. Coprinus micaceus. A, cross section through a few of the gills, X 10; B, enlarged portion of same, showing four basidia arising from surface of gill, each with four stalked basidiospores, X750.

Agaricus and Other Mushrooms. The common field mushroom {Agaricus canipestris) grows in lawns, fields, and along roadsides. It is the principal species used for food and practically the only one that is cultivated. The mycelium lives on organic matter in the soil. The fleshy basidiocarp arises just below the surface as a "button" composed of interwoven hyphae. Soon a stalk or stipe and a cap-like pileus become differentiated. The gills, which develop on the lower side of the pileus, are covered by a membrane called the velum. This extends from the margin of the pileus to the stipe, becoming ruptured as the pileus expands. In Agaricus and many other mushrooms a portion of the velum remains attached to the stipe, forming an annulus around it (Fig. 121). In Amanita, a genus of poisonous mushrooms, the young basidiocarp is completely enclosed by an outer membrane that ruptures as the stipe elongates, forming a cup or sheath, called the volva, at the base of the stipe.

The hymenium of the Agaricaceae, covering the surface of the gills, consists of innumerable basidia, each of which typically bears four basidio- spores on slender sterigmata (Fig. 122). The cells of the vegetative mycelium are typically binucleate and there are two nuclei in the young

150

PLANT MORPHOLOGY

basidium. These fuse, two successive divisions take place, and the four resulting nuclei pass through the sterigmata into the basidiospores (Fig. V2iiE-G). The reduction of chromosomes occurs when the fusion nucleus divides. The cultivated variety of Agaricus cmnpestris is exceptional in that only two basidiospores are borne on a basidium, each of which

U.'ry.

'■■■&X-*

Fig. 123. Clamp formation and development of the basidium in Armillaria mucida. A, beginning of damp formation in binucleate terminal cell; B, one nucleus passing into the clamp; C, conjugate division of the two nuclei; D, appearance of walls cutting off uni- nucleate clamp and basal cells from young binucleate basidium; E, fusion of clamp and basal cells, the latter sending out another branch; F, basidium with diploid fusion nucleus; G, basidium with four haploid nuclei and the developing sterigmata. {Ajter Kniep.)

receives two of the four haploid nuclei. The mycelium of both the wild and cultivated form is multinucleate and probably unisexual (homo- thallic).

In most mushrooms the basidiospores, upon germination, give rise to mycelia of two different sexes. These have uninucleate cells. When two mycelia of opposite sex come together, fusions take place between vegeta- tive cells, resulting in the formation of a binucleate mycelium. Upon this the basidiocarps are produced.

THALLOPHYTA: FUNGI 151

Following the formation of a binnoleate cell by the fusion of two uni- nucleate cells, a short branch arises into which the two nuclei pass. A hook-hke lateral outgrowth, pointing toward the base of the cell, then appears at a point directly opposite the two nuclei (Fig. 123.4, B). After both of these divide, one of the daughter nuclei passes into the hook and a cross wall forms at its base, another wall continuing across the branch (Fig. 123C, D). Thus two nuclei of opposite sex are in the terminal cell, one nucleus being in the lower cell and one in the hook. The tip of the hook now fuses with the lower cell to form a "clamp connection." The nucleus in the hook passes into the lower cell, which thereby becomes binucleate (Fig. 123£'). The terminal cell continues to grow and, at each cell division, a new clamp connection is formed.

A mycelium with clamp connections is characteristic of many Basidio- mycetes, occurring in at least some members of all the orders except the Uredinales. Clamp formation in the Basidiomycetes is thought to corre- spond to hook formation in the Ascomycetes where, however, it is limited to the ascogenous hyphae. It must be remembered that in both groups there are many members without any such formations, the ascus or basidium developing directly from the terminal cell of a hypha. Clamp connections are not present on the mycelium of Agaricus campestris or its cultivated variety.

7. Gasteromycetales

Like the Hymenomycetales, the Gasteromycetales are often broken up into several smaller orders. Nearly all its members are saprophytic on humus, but a few grow on decaying wood. There are about 1,000 species. The very complex basidiocarp entirely encloses the hymenium, remaining closed or opening only after the spores are mature. The basidiocarp is composed of an outer peridium and a central gleha, the latter generally containing many chambers. In the lower forms the chambers are filled with hyphae bearing terminal basidia; in the higher forms the chambers are lined with a definite hymenium. The basidia are one-celled and bear four terminal basidiospores, each at the end of a sterigma.

Families. The principal families of Gasteromycetales, distinguished from one another by the character of the peridium and gleba, are as

follows:

1. Hymenogastraceae. This family is intermediate between the Hyme- nomycetales and the Gasteromycetales. The peridium is simple, being one-layered and rupturing irregularly. The glebal chambers are Uned with basidia borne at the ends of lateral branches of the glebal hyphae. Because the basidiocarps are subterranean, these forms are not commonly seen. The chief genera are Hymenogaster and Rhizopogon.

152

PLANT MORPHOLOGY

2. Sclerodermaceae. In this family the basidiocarp is nearly spherical, with a thick, leathery, one-layered peridium that ruptures at the apex. The gleba is indistinctly chambered. The basidia are borne on lateral branches of the glebal hyphae. There are no sterigmata, the basidio-

spores being sessile. The representa- tive genus is Scleroderma.

3. Lycoperdaceae. These are the familiar puffballs. The globular basid- iocarps are usually less than 8 cm. in diameter but sometimes reach 50 cm. or more. The peridium is two-layered and has no definite dehiscence. In Lycoperdon the outer layer flakes off, the inner one bursting at the apex to liber- ate the spores. In Geaster the outer layer splits into stellate segments that spread out on the ground, the inner one dehiscing by a terminal pore. In this family the gleba is distinctly cham- bered. It is lined with a hymenium and contains a capillitium consisting of fibrous interwoven hyphae that aid in spore dispersal.

4. Nididariaceae. The bird's-nest fungi resemble the puffballs in their younger stages, but at maturity the peridium opens and becomes cup- shaped. The separate glebal chambers, with much-thickened walls, lie at the bottom of the cup like eggs in a nest. The two chief genera are Nidularia and Cyathus.

5. Phallaceae. The stinkhorn fungi are the highest of the Basidiomy-

cetes. Their basidiocarps are extremely complex (Fig. 124). At first they are white and egg-shaped. The peridium is two-layered but the tissue within is differentiated into a hollow sterile axis and an investing, dome-like, chambered gleba. When the basidiocarp is mature, these become the stipe and pileus, respectively. The gleba becomes mucilagi- nous and foul-smelling, attracting carrion flies that distribute the spores. The principal genera are Phallus, Mutinus, and Dictyophora. In Dicty- ophora there is a conspicuous net-like veil that hangs down beneath the pileus and spreads out around the stipe like a skirt.

Fig. 124. A stinkhorn fungus, P/iaiius impudicus, natural size.

THALLOPHYTA: FUNGI 153

FUNGI IMPERFECTI

The Fungi Imperfect! constitute a large assemblage of forms that, because of an incomplete knowledge of their life histories, cannot be assigned to any of the three natural classes of true fungi: the Phycomy- cetes, Ascomycetes, and Basidiomycetes. Generally the only known method of reproduction is by conidia. Zygotes, ascospores, or basidio- spores are unknown. In many cases the unknown stage has apparently been lost from the life history. When a member of this artificial group is found to possess any reproductive stage previously not reported, it is transferred to its proper genus, family, order, and class. Meanwhile it is placed in a "form genus." Many of the imperfect fungi cause important plant diseases, such as potato scab, early blight of potato, flax wilt, and various anthracnose and leaf-spot diseases. Practically all the fungi that cause such human diseases as ringworm and athlete's foot are imperfect fungi.

LICHENES

A lichen is a plant consisting of a unicellular alga and a fungus living together in symbiotic relationship. This association, resulting in a body having a distinctive form and structure, suggests a single plant rather than a composite one. Lichens are commonly regarded as constituting an autonomous group of thallophytes, the Lichenes, which are either made coordinate with the Algae and Fungi, or included with the latter as a distinct class. By those who consider lichens to be merely fungi parasitic upon algae, they are sometimes broken up and distributed among the fungous groups that they most closely resemble.

Lichens are commonly seen growing on rocks, tree trunks, dead wood, and on the ground. They are a widely distributed group of which about 400 genera and 15,000 species are known. A few of the largest genera are Lecidia, Buellia, Lecanora, Parmelia, Physcia, Collema, Stida, Cladonia, Ramalina, and Usnea. Lichens are mostly gray or grayish green, but some are more conspicuously colored. Based on their external form, three general types are recognized: (1) crustose Hchens, w^hich occur as incrustations on rocks and bark; (2) foliose lichens, which are flat, leaf -like, and only partially attached to the substratum; and (3) fridicose hchens, branching forms that hang from trees or grow either erect or prostrate on the ground (Fig. 125).

The greater part of a Hchen is composed of a compact mass of tangled fungous hyphae, among which are numerous algal cells, either scattered irregularly or in a definite layer (Fig. 126). The body is usually differ- entiated into a compact cortical region and a lower region of looser tex- ture, in either of which the algal cells may occur. In some lichens the

154

PLANT MORPHOLOGY

algae live on the surface of the mycelium, closely covering it. With only a few rare exceptions, lichen-forming fungi are ascomycetes belonging either to the Pezizales or to the Pyrenomycetales. In three genera of lichens the fungus is a basidiomycete, the best-known species being Cora pavonia, which is widely distributed throughout Central and South America. The lichen-forming algae are members either of the Cyanophy- ceae or Chlorophyceae, most of the latter belonging to the Chlorococcales.

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•"Sr

Fig. 125. Group of common lithens, natural size. A, a crustose form {Placodium) grow- ing on rock; B, a foliose form {Parmelia) growing on bark; C, a fruticose lichen (Cladonia) which grows erect on the ground; D, a branching form {Usiiea) that hangs from the limbs of trees.

Lichens were once regarded as single plants. In 1868, their dual nature was demonstrated. In 1889, lichens were first synthesized by sowing spores from the fungous element of a lichen among appropriate free-living algae. The developing mycelium was seen to enclose the algae and develop into a lichen. Although the algal symbionts are forms that may exist independently, the fungi are known only as constituents of lichens. Vegetative reproduction takes place mainly by soredia, globular or scale-like bodies composed of a few hyphae closely investing one or more algal cells. They arise as buds on the upper surface of the thallus, become detached, and are scattered by the wind. The algal components multiply

THALLOPHYTA: FUNGI

155

by fission within the lichen body. The fungous components produce ascocarps, generally in abundance (Fig. 127). These are either apothecia or perithecia. Sex organs have been observed in many lichens. The ascogonium is a spirally coiled multicellular filament commonly terminat- ing in a trichogyne. The male cells, or spermatia, are borne on branching hyphae arising within a flask-like chamber, or spermogonium. After

Fig. 126. Cross section through the body of a lichen (Physcia), showing cells of the alga (shaded) surrounded by a mass of interlacing fungous hyphae, X 500.

fertilization, which may not always take place, the ascogonium gives rise to many ascogenous hyphae and paraphyses (Fig. 127 B). At the tips of the ascogenous hyphae typical asci with eight ascospores are formed. An ascospore, in germination, produces hyphae that die unless they come in contact with a suitable alga.

The relation of the two lichen components to each other is important to understand. The fungus lives on the alga as a parasite but does not kill it. In fact, the alga seems to be only slightly injured, merely sacrificing some of the food that it makes. At the same time, however, the alga is benefited in that the fungous body readily absorbs and retains moisture,

156

PL A N T M ORPHOLOG Y

without which the alga could not live. The fungus derives food from the alga while the alga obtains moisture from the fungus. This reciprocal relation makes it possible for many lichens to live in dry exposed situa- tions where neither the alga nor the fungus could live alone. Thus the relation between the two lichen components is one of mutual advantage.

w^

V0K

■'?,7'..:^'T

vi^

;;i''l.-

^•- ^-i-.-j^-' \;'V/( ,•''-- " -v-'.< .'.ii^

>f

Fig. 127. .4, longitudinal section through an apothecium of Physcia, showing hymenium and embedded algal cells, X60; B, enlarged view of hymeniuni, showing asci and para- physes, X500.

COMPARISON OF THE CLASSES OF FUNGI

The chief distinguishing characters of the five classes of fungi are as follows :

Schizomycetes. Plant body unicellular, solitary or colonial, ciliated or nonciliated. Cells without a definite nucleus. Cell walls usually forming mucilage. Reproduction by fission.

Myxomycetes. Plant body a naked amoeboid mass of multinucleate protoplasm (a plasmodium). Asexual reproduction by small uninucleate spores, each with a cell wall and usually borne within sporangia of definite form. Sexual reproduction by amoeboid isogametes.

THALLOPHYTA: FUNGI 157

Phycomycetes. Plant body typically a nonseptate multinucleate mycelium. Asexual reproduction by spores formed by cleavage and borne in indefinite numbers in sporangia. Lower members with zoo- spores, higher members with aerial spores. Sexual reproduction isoga- mous or heterogamous. Heterogamous forms with well-developed sex organs.

Ascomycetes. Plant body typically a septate mycelium. Spores borne usually in groups of eight in a sac-like structure, the ascus, their nuclei arising by three successive divisions of a fusion nucleus. Zoospores wanting. Sex organs reduced, obscure, or entirely absent.

Basidiomycetes. Plant body a septate mycelium. Spores borne usually in groups of four on a club-like structure, the basidium, their nuclei arising b}^ two successive divisions of a fusion nucleus. Zoospores wanting. Sex organs not present.

GENERAL CONCLUSIONS

The fungi are a heterogeneous assemblage of thallophytes of diverse origin held together by a physiological character — the absence of chloro- phyll. Two classes, the Schizomycetes and Myxomycetes, stand apart from each other and from the three classes of "true fungi" (Eumycetes). In their unicellular organization, cell structure, and reproduction the Schizomycetes resemble the Cyanophyceae much more closely than they resemble any of the other fungi. The Myxomycetes, with their naked Plasmodia, highly developed sporangia, and amoeboid isogametes, exhibit similarities to some of the Protozoa, on the one hand, and to some of the lower Phycomycetes (Plasmodiophorales) on the other.

Some botanists believe that the "true fungi" are a monophyletic group that have arisen from colorless flagellates and have subsequently differen- tiated into the three existing classes of Phycomycetes, Ascomycetes, and Basidiomycetes. According to this theory, no direct relationship exists between the algae and fungi, their resemblances being a result of parallel evolution along two independent lines. Other botanists believe that the "true fungi" have been derived from the algae through loss of chlorophyll, their origin having been either monophyletic or polyphyletic. According to this theory, the Phycomycetes have evolved from the Chlorophyceae, the Ascomycetes from either the Phycomycetes or the Rhodophyceae, and the Basidiomycetes from the Ascomycetes.

Vegetative Body. The characteristic plant body of the fungi' is a mycelium, made up of branching hyphae that may be either nonseptate and coenocytic (Phycomycetes) or septate (Ascomycetes and Basidiomy- cetes) . Only a few forms are unicellular. The hyphae elongate by apical

1 In the following discussion the term fungi will be limited to the three classes of "true fungi."

158 PLANT MORPHOLOGY

growth. They may be either loosely or compactly arranged. Some- times they are aggregated to form root-like strands or a compact resting body (sclerotium) . In the development of fruit bodies in the higher fungi — ascocarps and basidiocarps— masses of hyphae become interwoven to form a pseudoparenchymatous structure, but no tissue is formed by cells dividing in three planes. In the lower Phycomycetes the cell wall con- sists largely of cellulose, but in the other fungi its composition is altered by the presence of chitin and other substances, such as fatty acids. Within the cells of the mycelium are one, two, or many nuclei embedded in the cytoplasm. Sugars and glycogen represent the reserve carbo- hydrates, no starch being present. Varying amount of fats may also

occur.

Spore Reproduction. The Phycomycetes produce spores in sporangia, either zoospores in the lower orders or aerial spores in the higher orders. The spores are formed in indefinite numbers by cleavage. After escaping, they germinate into a mycelium. The entire sporangium may be persist- ent, as in the Saprolegniales and Mucorales, or detachable, as in most of the Peronosporales. In many of the Ascomycetes and Basidiomycetes the detachable sporangia are replaced by conidia, which function as spores and produce a new mycelium directly. Many conidia, as well as certain other spores, multiply by budding, like the vegetative cells of the

yeasts.

Many fungi produce resting spores that are thick-walled and resistant to adverse conditions. Often the same species has two or more different kinds of spores, as in the rusts. Ascospores, which are characteristic of the Ascomycetes, arise by free-cell formation. They are borne internally in an ascus, usually in groups of eight, while basidiospores, characteristic of the Basidiomycetes, are produced externally on a basidium, usually in fours. The formation of ascospores and basidiospores is related to the sexual process.

Gametic Reproduction. In the Phycomycetes sexual reproduction is alga-like. The Chytridiales and Plasmodiophorales produce free- swimming isogametes that fuse in pairs to produce a zygote. Among the heterogamous Phycomycetes (Monoblepharidales, Saprolegniales, and Peronosporales), all of which have well-developed antheridia and oogonia, only the Monoblepharidales have swimming sperms; in the two other orders a male nucleus reaches the egg by passing through a fertihzation tube. The gametes are nearly always formed within special cells, the gametangia or sex organs. In the higher Phycomycetes (Mucorales and Entomophthorales) the gametangia are not differentiated as antheridia and oogonia, but the entire contents of two gametangia conjugate to form a zygote.

The Ascomycetes show^ various stages in the degeneration of the sex

THALLOPHYTA: FUNGI 159

organs. Where these are well developed, the oogonium (ascogonium) often resembles that of the red algae. The zygote may develop directly into an ascus or, more commonly, may give rise to many ascogenous hyphae that, in turn, produce the asci. The Basidiomycetes have no sex organs (unless the spermogonia of the rusts are so regarded), but fusions between vegetative cells are common. In the Ascomycetes and Basidiomycetes the sexual nuclei come together without immediately fusing. The nuclear fusion, which takes place in the ascus or basidium, is followed at once by the production of ascospores or basidiospores, respectively.

CHAPTER V BRYOPHYTA

The bryoph>'tes. numbering about 20,000 species, form a well-defined division comprising the two classes Hepaticae (liverworts) and :Musci (mosses). They are small, rather inconspicuous, green plants nearly all of which five on land in moist, shaded places. The bryophj^es doubtless have been derived from aquatic ancestors, probably from some group of green algae, but it is uncertain whether they have given rise to any of the higher plant groups. Nevertheless, the bryophnes represent a general condition of structural organization through which the higher plants may have passed in the course of their evolution. Although abundant mois- ture is necessary for vigorous vegetative gro^^'th, some forms live in dry situations and endure considerable desiccation during long rainless periods. A few hver worts and mosses live in fresh water, but the aquatic habit in the bry ophites, as in the higher groups, has undoubtedly been secondarily acquired.

A well-defined alternation of generations is an established feature of all bryophj^es, the gametophj-te and sporoph>i:e always being morphologi- cally dissimilar. The gametoph^-te, arising from the spore, is the haploid generation, producing sperms and eggs. The sporophyte, arising from the zygote, is the diploid generation. It produces spores, the reduction in chromosome number taking place in connection ^^-ith their formation, as in all the higher plants. S\^-imming spores are entirely ehminated. In the green algae the zygote is liberated into the water and is nearly always a resting cell, while in the bryophytes and all higher groups it germinates at once, without escaping, to produce an embryo sporophyte. In the bryoph>i:es the gametophyte, or haploid generation, is always an independent indi\'idual, while the sporophyte. or diploid generation, is entirely or largely dependent on it for its nutrition. Although the garnet oph^-te is thalloid in some of the liverworts, in most bryophytes it is dift'erentiated into stem and leaves. Growth takes place through the activity of an apical cell. The sex organs, antheridia and archegonia, are always multicellular and provided with an outer sterile jacket. Throughout the algae the gametangia are prevailingly unicellular but, where multicellular, all their cells produce gametes (except in the Charo- phyceae). The antheridium is a stalked, spherical or club-shaped organ consisting of a mass of spermatogenous tissue enclosed by a jacket of

160

BRYOPHYTA 161

sterile cells. It gives rise to numerous small biciliate sperms, two of which arise from each sperm mother cell. The presence of swimming sperms, universal among bryophytes and pteridophytes, represents the retention of a primitive algal character.

The archegonium is a very characteristic organ of bryophytes and pteridophytes. Although corresponding to the oogonium of the algae, it is much more highly developed. The archegonium is usually stalked and flask-shaped. It is composed of an axial row of cells surrounded by a sterile jacket. The axial row consists of an egg — the basal and largest cell of the series — and a variable number of canal cells, which disorganize and become mucilaginous prior to fertilization. The fertilized egg gives rise to an embryo that develops within the archegonium, the basal portion of which enlarges to form a protective covering, the calyptra.

In all bryophytes the sporophyte is without differentiation into stem

and leaves and is w^ithout a direct connection with the soil. In nearly

all bryophj^tes the sporophyte consists of a ba.sal absorbing organ {foot),

a stalk (seta), and a terminal spore-producing portion (capsule). The

capsule is a sporangium. All bryophytes are homosporous, the spores of a

given species being alike in size and form. On germination, the spore

produces either the main gametophyte directly or, more commonly, a

filamentous protonema from which the main gametophyte sooner or later

arises.

1. HEPATICAE

The liverworts are primitive land plants, most of them growing in the presence of abundant moisture on soil, rocks, and tree trunks. With very few exceptions, the gametophyte is dorsiventral. It may be thalloid, but more commonly is leafy, the leaves being nearly always without a midrib. Unicellular unbranched rhizoids maintain a connection with the substratum. The Hepaticae are widely distributed but are more numer- ous in the tropics than elsewhere. A few fossil forms are known from the Upper Carboniferous of England. There are about 6,000 species of liverworts, nearly all being included in four principal orders, the r^Iarchan- tiales, Sphaerocarpales, Jungermanniales, and Anthocerotales.

1. Marchantiales

The Marchantiales are a well-defined order of about 30 genera and 400 species. They range from arctic to tropical regions and are well represented in the Temperate Zones. In the tropics they occur chiefly between altitudes of 900 and 1,500 m. Nearly all of them are terrestrial, growing mainly on damp soil or rocks. Some common genera of Mar- chantiales, all of widespread distribution, are Riccia, RehouUa, Asterella, Conocephalum, and Marchantia. The largest genus is Riccia, wth over 100 species.

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PLANT MORPHOLOGY

Gametophyte. The Marchantiales are characterized by a flat, dorsi- ventral, thalloid gametophyte — with few exceptions ribbon-hke and nearly always rather fleshy. It branches either dichotomously from the apex or, less commonly, by means of adventitious outgrowths arising apically or ventrally. In Riccia the thallus is small and, as a result of repeated dichotomy, often grows in the form of a fan or rosette (Fig. 128). In all the Marchantiales growth takes place by means of an apical cell situated in an apical notch. It is of the cuneate (wedge-shaped) type,

Fig. 128. Dorsal view of the gametophyte of Riccia nutans, showing sporophytes in the grooves and scales arising from the ventral surface, X 3.

Fig. 129. Reboulia hemisphaerica. A, longitudinal section of portion of growing region of thallus with apical cell and developing air chambers, X 160; B, portion of upper region of thallus, showing air pore and air chambers, X85.

cutting off segments on four sides — above and below as well as left and right (Fig. 129A).

The gametophyte is of simple external form but exhibits a high degree of internal differentiation, nearly always consisting of (1) an upper epidermal layer; (2) a loose, green, dorsal region having one or more layers of air chambers; (3) a compact, colorless, ventral region. The epidermis, usually colorless or pale green and often with slightly thickened walls, nearly always contains numerous air pores that communicate with the air chambers. Air pores and air chambers are not developed in a few genera {e.g., Dumortiera and Monoclea), their absence being a result of reduction.

BRYOPHYTA

163

In most species of Riccia the dorsal region is composed of erect rows of cells separated by very narrow, vertical air chambers, but in some species it is spongy, consisting of a loose network of large irregular air chambers separated by one-layered partitions that extend in all directions (Fig. 137A). The uppermost cells form a rather ill-defined epidermis. In Riccia nutans simple air pores are present, but in the other species air pores are either rudimentary or wanting. Simple air pores consist of a single tier of cells surrounding a small central opening, the cells being in

Fig. 130. Section through the thallus of Marchantia polymorpha, showing epidermis with an air pore that leads to an air chamber with green filaments, X200. Rhizoids are shown below.

several concentric circles (only one circle in Riccia natans). Simple air pores occur on the thallus of most of the genera of Marchantiales (Fig. 1295). In Marchantia and a few related forms, however, the thallus bears compound air pores. These are barrel-shaped, consisting of four or five superimposed layers of cells and having both an upper and a lower opening (Fig. 130). Conocephalum, Marchantia, and many other genera have a single layer of air chambers from the floor of which special chloro- phyllose filaments arise. Rehoulia and Asterella have several layers of air chambers without green filaments (Fig. 1295). In many forms the limits of the air chambers are plainly visible on the dorsal surface of the thallus as polygonal areas, an air pore occurring in the center of each.

In practically all the Marchantiales the lower surface of the thallus bears numerous rhizoids and scales. The rhizoids are of two kinds, smooth and tuberculate. The former have smooth walls, the latter peg-like thicken-

164

PLANT MORPHOLOGY

ings that project into the lumen. In Riccia rhizoids are usually abun- dant, but frequently ventral scales are rudimentary or absent. ^ In nearly all the Marchantiales the ventral scales are arranged in two longitudinal rows; in Marchantia they are in four or more rows.

Throughout the order decay of the older parts of the thallus results in the isolation of branches, each of which forms a new plant. In some species vegetative propagation occurs by the formation of adventitious branches that become detached. In two genera, Lunularia and Marchan- tia, multicellular gemmae are produced. These are flat, stalked, discoid bodies that arise in groups on the dorsal side of the thallus inside cupules.

Fig. 131. Male (A) and female (B) plants of Marchantia polymorpha, natural size.

In Lunularia the cupules are crescentic, while in Marchantia they are cup-shaped. The gemmae arise from the floor of the cupule. Each gemma has two notches, one on either side, and in each notch is an apical cell. Upon separation from the cupule, a single gemma gives rise to two new thalli.

Sex Organs. The sex organs of the Marchantiales are invariably dorsal in origin, arising either directly on the thallus itself or on a more or less specialized receptacle. Both kinds of sex organs arise in acropetal succession from segments of an apical cell. According to the species, the antheridia and archegonia occur on the same plant or on separate plants. In Riccia each branch of the thallus has a median dorsal groove extending backward from the growing apex; in this groove the sex organs are borne. Although generally scattered irregularly, they sometimes tend to be segregated into separate groups. The sex organs arise singly just behind

' When Riccia nutans floats on the surface of quiet water, it has numerous large scales and few or no rhizoids. When it grows on muddy banks and flats, it has many rhizoids and few scales.

BRYOPHYTA

165

the apical cell and soon become sunken in the thallus by upgrowth of the surrounding tissues, each coming to lie in an individual pit.

In the other Marchantiales the antheridia are similarly sunken in pits but the archegonia are not. The antheridia may be borne in irregular median groups on the dorsal side of the thallus, as in some species of Asterella, but more commonly they occur on a definite receptacle. This may be cushion-like and sessile, asmReboidiasindConocephalum, or raised above the thallus on a stalk, as in Marchantia. The antheridial recepta- cle of Marchantia has a number of marginal growing points, from each

Fig. 132. Male structures of Marchantia polymorpha. A, longitudinal section through young male receptacle, showing embedded antheridia, X40; S, nearly mature antheridium, X200; C, a single sperm, more highly magnified.

of which an acropetal series of antheridia extends toward the center, the antheridia being sunken in the upper surface of the receptacle (Figs. 13L4 and 132A).

In Rehoulia, Asterella, Conocephalum, and many other genera the archegonia are borne on a stalked receptacle that, with few exceptions, is terminal in position and represents a specialized upright branch of the thallus. Unlike the antheridia, the archegonia are not embedded in pits. The female receptacle is commonly hemispherical or conical and more or less lobed. Each lobe represents a separate growing point back of which either one or several archegonia arise. As the receptacle grows, the archegonia are carried to a position on its lower side close to the stalk. In Marchantia the archegonial receptacle reaches its greatest degree of specialization. It does not have lobes, but consists of a number of rays alternating with groups of archegonia (Figs. 13 IB and 133.4). The archegonia hang with the necks downward.

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PLANT MORPHOLOGY

Air chiimbers and air pores are developed on both the male and female receptacles. In Reboulia and Asterella the air pores on the female recep- tacle are compound, while those occurring elsewhere on the plant are simple. In Conocephalum the air pores are compound on both the male

Fig. 133. Female structures of Marchantia polymorpha. A, longitudinal section through young female receptacle, showing a row of archegonia, X40; B, mature archegonium with egg ready for fertilization, X300; C, young embryo lying within the archegonium, X300.

and female receptacles, but are simple on the thallus. In Marchantia the air pores are everywhere compound.

The mature antheridia of the Marchantiales are club-shaped structures with a short stalk (Fig. 1325). The chamber in which each lies com- municates with the surface of the thallus or receptacle by a pore through which the sperms escape. The antheridium arises from a single super- ficial initial cell that becomes papillate and then divides transversely (Fig. 134^). The outer segment undergoes several additional transverse

BRYOPHYTA

167

divisions, resulting in the formation of about four superimposed cells (Fig. 1345-Z)). In each of these vertical walls appear at right angles to each other and later, by the formation of periclinal walls in the upper part of the antheridium, an outer layer of sterile cells is cut off from a cen- tral group of spermatogenous cells (Fig. 134£'~//). The lower portion of

Fig. 134. Early stages in the development of the antheridium of Marchantia polymorpha, X750. A, division of initial into an inner and outer cell; B, C, D, formation of a filament of four cells from the outer cell; E and F, appearance of vertical walls; G, appearance of periclinal walls; H, later stage, showing sterile jacket surrounding spermatogenous cells, with stalk below.

the antheridium forms the stalk. By continued division, the spermatog- enous cells give rise to many small, cubical, sperm mother cells, each of which produces two biciliate sperms (Fig. 132B, C).

The archegonium also arises from a single superficial initial that becomes papillate and divides transversely (Fig. ISoA). Three vertical walls now appear in the outer segment, these being arranged in such a way that a middle cell and three peripheral cells are formed (Fig. 1355, H). The middle cell is the primary axial cell, the peripheral ones the primary wall cells. The primary axial cell, by a transverse division, gives rise to a cover cell and a central cell (Fig. 135C). The archegonium now grows in

1G8

PLANT MORPHOLOGY

\e\wt\\ the central cell dividing to form a primary neck canal cell and a primary ventral cell (Fig. 135D). As a result of additional transverse divisions, the primary neck canal cell gives rise to a vertical row of neck canal cells, most commonly either four or eight in number, while the primary ventral cell divides transversely to form the ventral canal cell

and egg (Fig. ISSS-C). , « , u a

By this time the archegonium has become distmctly flask-shaped, the slender neck being sharply marked off from the bulbous venter. In

E ^-"T^ F ^-1 G

Fig 135. Development of the archegonium of Marchaidia polymorpha, X600. A, division of initial into an inner and outer cell; B, appearance of three vertical walls in the outer cell; C, formation of cover cell and central cell from the primary axial cell; D, forma- tion of primary neck canal cell and primary ventral cell from the central cell; E and F, later stages, with two and four neck canal cells; G, nearly mature archegonium, with egg and ventral'canal cell derived from the primary ventral cell; H, cross section of very young archegonium, showing primary axial cell surrounded by primary wall cells ; I, later stage, .showing six neck cells surrounding a neck canal cell.

all the Marchantiales the neck consists of six vertical rows of jacket cells surrounding the canal (Fig. 135/). The canal cells disorganize, forming a mass of mucilage through which the sperms can swim (Fig. 133B). The egg is fertilized within the venter of the archegonium, which enlarges to form the calyptra, the embryo developing within (Fig. 133C). In all the Marchantiales except Riccia, an involucre arises around the arche- gonia. In Asterella, Marchantia, and several other genera an additional envelope, the pseudoperianth, arises after fertilization and generally becomes very conspicuous (Fig. 139).

Sporophyte. Riccia displays the simplest sporophyte among the Bryophyta. In its development, the fertilized egg divides by a trans- verse wall, resulting in two cells approximately equal in size (Fig. 136).

BRYOPHYTA

169

Each of these now divides by a vertical wall, followed by another at right angles to it. Additional divisions, without definite sequence, take place in all three planes. Then periclinal walls cut off an outer layer, the amphithecium, from a central group of cells, the endothecium. As the embryo continues to grow, the entire central group becomes sporogenous, while the outer layer remains sterile. After the sporogenous cells have divided for the last time, they separate and round off to become spore mother cells (Fig. 137). Each of these then enlarges and undergoes two

ABC

Fig. 136. Development of the embryo of Riccia natans, X400. .*i, two-celled stage; B, four-celled stage; C, later stage, showing differentiation into amphithecium and endo- thecium.

consecutive divisions during which the number of chromosomes is reduced one-half, and a tetrad of cells is formed. The walls thicken and the four members of the tetrad separate as mature spores. During the early development of the sporophyte, the venter of the archegonium becomes two-layered and forms the calyptra (Fig. 136). The sterile jacket of the sporophyte and the inner layer of the calyptra break down before the spores have ripened, leaving them enclosed within the outer laj^er of the calyptra. Riccia has no spore-dispersing mechanism, the spores being liberated by progressive decay of the thallus.

In practically all the other Marchantiales the sporophyte consists of a foot, seta, and a capsule containing both spores and elaters. As in Riccia, the first division of the fertilized egg is transverse. In some genera, such as Reboulia, Asterella, Conocephalum, and others, the next two divisions are also transverse, resulting in a filament of four superimposed cells (Fig. 138). Then vertical walls come in and, with the formation of

170

PLANT MORPHOLOGY

A D

Fig. 137. Sporophyte of Riccia natatis. A, longitudinal section of nearly mature sporo- phyte embedded in the gametophyte, showing spore tetrads enclosed within the calyptra; B, spore mother cell; C, tetrad; D, mature spore; A, X 100; B, C, D, X500.

C D E

Fig. 138. Development of the embryo of Cryptomitrium tenerum, X400. A, two-celled stage; B, division of lower cell; C, four-celled stage, the two lower cells giving rise to the foot and seta, the two upper cells to the capsule; D, eight-celled stage; E, older stage, showing differentiation of sporogenous tissue in the capsule. {After Haupt.)

BRYOPHYTA

171

additional walls in the upper part of the embryo, the foot, seta, and cap- sule are differentiated. In other genera, such as Marchantia, the first division of the fertilized egg is followed by the appearance of two vertical walls at right angles to each other in both the upper and lower segments, thus forming octants, as in Riccia. Additional walls in all three planes produce a globular embryo rather than an elongated one (F'ig. 139),

An early formation of periclinal walls in the capsular region cuts off the amphithecium from the endothecium, the former forming the capsule

B C

Fig. 139. Development of the embryo of Marchantia polymorpha. A, four-celled stage, X320; B, slightly later stage, X320; C, older embryo, showing the foot (/), seta (s), capsule (c) with sporogenous tissue differentiated, pseudoperianth (p), and calyptra (a), X200.

wall and the latter the sporogenous tissue (Fig. 138£'). In Riccia the sporogenous tissue is derived equally from both halves of the embryo. In practically all the other genera, whether the embryo is of the filamentous or of the octant type, apparently only the upper half contributes to the sporogenous tissue, the lower half giving rise to the foot and seta. The foot anchors the sporophyte and absorbs nourishment. The seta elon- gates, especially after the spores ripen, pushing the capsule through the calyptra.

In most genera, except Riccia, some of the potentially sporogenous cells of the young capsule give rise to elaters, while the others directly become spore mother cells. In Marchantia, however, the sporogenous ceils greatly elongate, some remaining undivided to form elaters, the others dividing transversely a number of times to form vertical rows of spore

172

PLANT MORPHOLOGY

mother cells (Fig. 140). As in Riccia, tetrads are formed and the walls of the spores thicken. The elaters are long, slender cells, pointed at each end, their walls developing spiral thickenings as the protoplasm disappears (Fig. 140D). Elaters are hygroscopic and perform squirming movements that assist in the liberation of the spores. The capsule wall ■ one layer of cells thick except in the apical region. In RebouUa,

IS

0

M /"^Z^

Fig. 140. Sporophyte of Marchantia polymorpha. A, longitudinal section of nearly mature sporophyte, showing the foot (/), seta (s), capsule (c), and ruptured calyptra (a); B, two rows of spore mother cells and portion of an undeveloped elater; C, a row of spore tetrads; D, three mature spores and the end of an elater; A, XQO; B, C, D, X600.

Asterella, and related forms local thickenings are not formed on the cells of the capsule wall and dehiscence takes place by means of an apical lid. In nearly all the other genera, however, the cells of the capsule wall bear annular thickenings, dehiscence occurring by irregular clefts.

Summary. The Marchantiales are a group in which the gametophyte, while remaining simple in form, has achieved a high degree of structural complexity. In all members of the order the gametophyte is thalloid and grows by means of a cuneate apical cell. It is nearly always differentiated into an upper epidermis with air pores, a dorsal photosynthetic region

BRYOPHYTA 173

with air chambers, and a compact, colorless ventral region. The gameto- phyte reaches an extreme of complexity in forms with compound air pores and air chambers having green filaments. It bears both smooth and tuberculate rhizoids. In the lower members the sex organs are borne directly on the thallus, sunken in the dorsal surface, but throughout the group there is a marked tendency to restrict and specialize the regions producing sex organs, resulting in the development of complex receptacles. The female receptacles, and sometimes the male as well, are stalked. The antheridia, when mature, are elongated organs lying in a deep chamber. Their early development is characterized by a series of trans- verse divisions. The neck of the archegonium shows six cells in cross section.

In the lower members the sporophyte is a spherical, undifferentiated spore case, all the inner cells forming spores. In the higher members the sporophyte is elongated and differentiated into a foot, seta, and capsule. The capsule contains both spores and sterile cells, the latter practically always developed as elaters. Thus, throughout the group, there is a marked tendency to divert potentially sporogenous tissue to functions other than spore production. The seta is comparatively short. The cap- sule is spherical or nearly so, its wall being composed of a single layer of cells (the apex usually thicker). Dehiscence, lacking in Riccia, nearly always occurs by irregular clefts or an apical lid. The Marchantiales are a group in which a complex gametophyte is combined with a relatively simple sporophyte.

2. Sphaerocarpales

The Sphaerocarpales comprise a small order of 3 genera and 25 species. Sphaerocarpus is a widely distributed genus but Geothallus, represented by a single species, has been found only near San Diego, California. Both of these forms grow on moist earth. Riella is an aquatic form occurring in Europe, Africa, California, and western Texas.

Gametophyte. The gametophyte of the Sphaerocarpales displays none of the internal differentiation seen in the Marchantiales. It consists of a simple plate-like thallus that differs somewhat among the three genera. In Sphaerocarpus the thallus is small, flat, and often orbicular, with an entire or more or less lobed margin. It has a broad indistinct midrib, several layers of cells in thickness, that merges gradually into the one- layered wings (Fig. 141). In Geothallus the thallus is larger and consists of an elongated thickened axis giving rise to crowded leaf-like outgrowths on either side, these mostly one layer of cells thick. A large portion of the axis becomes converted into a fleshy tuber that lives over into the next growing season. In both Sphaerocarpus and Geothallus the thallus may be either simple or dichotomously branched. The lower surface lacks

174 PLANT MORPHOLOGY

scales but bears numerous colorless rhizoids of the smooth-walled type. Growth of the thallus results from the activity of a cuueate apical cell.

In general appearance Riella is unlike any other liverwort. It is a sub- merged aquatic, usually growing erect in standing water. It has a stem- Uke axis that bears a dorsal leaf-like wing or, in the Algerian Riella bialata, two wings. The wing is mostly one layer of cells thick. It is frequently undulate and sometimes spirally twisted. The axis is commonly several

times dichotomous. It produces rhizoids near the base.

Sex Organs. The antheridia and archegonia are borne directly on the thallus, each enclosed in a special involucre that is open above. They arise in acropetal succession from dorsal segments of the apical cell. V^^^^ In Riella both kinds of sex organs

''^ B may occur on the same plant, al-

FiG. 141. Female (A) and male (B) though generally, as in ^Sp/merocarpMS gametophytes of Sphaerocarpus caii- ^ Qeothallus, they are borne on

fornicus, X6. {From Gilbert M . Smith.) > ''

separate plants. In Sphaerocarpus the sex organs are closely crowded on the dorsal sur- face of the thallus. The male plants, often purplish, are minute and much smaller than the female plants (Fig. 141). The antheridial involu- cres are flask-shaped and each contains an ovoid short-stalked antherid- ium. In development, two transverse walls appear in the outer cell arising from a transverse division of the papillate initial (Fig. 142 A, B). In the upper two segments vertical walls are formed at right angles to each other