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A TEXT-BOOK OF BOTANY
MACMILLAN AND CO., LIMITED
LONDON • BOMBAY • CALCUTTA MELBOURNE
THE MACMILLAN COMPANY
NEW YORK • BOSTON • CHICAGO ATLANTA • SAN FRANCISCO
THE MACMILLAN CO. OF CANADA, LTD.
TORONTO
TEXT-BOOK OF BOTANY
BY
DR. EDUARD STRASBURGER
PROFESSOR IN THE UNIVERSITY OF BONN
DR. FRITZ NOLL
PROFESSOR IN THE AGRICULTURAL ACADEMY OF POPPELSDORF AND IN THE UNIVERSITY OF BONN
DR. HEINRICH SCHENCK
PROFESSOR IN THE TECHNICAL ACADEMY OF DARMSTADT
DR. GEORGE KARSTEN
PROFESSOR IN THE UNIVERSITY OF BONN
THIRD ENGLISH EDITION REVISED WITH THE EIGHTH GERMAN EDITION BY
W. H. LANG, M.B., D.Sc.
LECTURER IN BOTANY, UNIVERSITY OF GLASGOW
WITH 779 ILLUSTRATIONS, IN PART COLOURED
MACMILLAN AND CO., LIMITED ST. MARTIN'S STREET, LONDON
1908
First English Edition, 1898 Second English Edition, 1903 Third English Edition, 1008
•J.
PREFATORY NOTE
THE first edition of the English translation of this text-book was the work of Dr. H. 0. Porter, Assistant Instructor of Botany, University of Pennsylvania. The proofs of this edition were revised by Professor Seward, M.A., F.K.S. The second English edition was based upon Dr. Porter's translation, which was revised with the fifth German edition. The present edition has been similarly revised throughout with the eighth German edition. Such extensive changes, including the substitution of a completely new section on Phanerogamia, have however been made in the latter that it seemed advisable to give in outline the history of the English translation instead of retaining Dr. Porter's name on the title page.
The official plants mentioned under the Natural Orders are those of the British Pharmacoposia instead of those official in Germany, Switzerland, and Austria, which are given in the original. In making this alteration I have consulted Materia Medica and Therapeutics, by J. Mitchell Bruce, M.A., LL.D., M.D.
The fact that a considerable portion of the original has been rewritten, has necessitated the preparation of a new and extended index. I am indebted to my friend, Mr. F. Tidd Pratt, for assistance in this.
WILLIAM H. LANG.
GLASGOW, February 1908.
/ \ ^ - — —^ v ^
^Vo°s^x<l
L ' B R A R y
^•^.
CONTENTS
PAGE
INTRODUCTION 1
PART I. GENERAL BOTANY
SECTION I.' MORPHOLOGY I. EXTERNAL MORPHOLOGY
The development of form in the plant kingdom . . . .11
The shoot ......... 18
Metamorphosis of the shoot . . . . . . .23
Leaves ......... 30
Metamorphosis of leaves . . . . . . .43
The root .......... 45
Metamorphosis of the root . . . . . . .48
Members of independent origin . . . . . . .49
II. INTERNAL MORPHOLOGY (Histology and Anatomy)
The cell ... 52
Protoplasm ......... 54
Cell wall 63
Inclusions of the protoplasm . . . . . . .73
Cell sap ......... 79
Ontogeny of the cell ........ 80
Cell fusions . . . . . . . .93
Tissues .......... 97
Primary tissues ......... 100
Secondary tissues . . . . . . . .127
Regeneration . . . . . . . . .151
Phylogeny of the internal structure ...... 152
Ontogeny of the internal structure ...... 154
Structural deviations 165
via
BOTANY
SECTION II. PHYSIOLOGY
Physical and vital properties. General conditions of life The stability of the plant body ....
Nutrition ......
Respiration ......
Growth .......
The phenomena of movement . . . .
Reproduction ......
PA01
172 178 186 239 247 263 296
PART II. SPECIAL BOTANY
A
SECTION I. CRYPTOGAMS
/'(^THALLOPHYTA ......
Bacteria . . . . .
Cyanophyceae .......
Flagellata .......
Myxomycetes .......
Peridineae .......
Conjugatae .......
Diatomeae . .
Heterocoutae .......
Chlorophyceae ......
Characeae .......
Phaeophyceae .......
Rhodophyceae .......
Phycomycetes .......
' Eumycetes .......
O Lichcnes .......
/ - fl 6BRYOPHYTA .......
Hepaticae .......
__ Musci ........
( /^PTERIDOPHYTA . .
Filicinae . . . . . ...
Eusporangiatae ......
Leptosporangiatae ....
Equisetinae .......
Lycopodinae .......
FOSSIL CRYPTOGAMS ......
SECTION II. PHANEROGAMIA
TRANSITION FROM THE CRYPTOGAMS TO THE PHANEROGAMS TABLE OF CLASSES, ORDERS, AND FAMILIES OF PHANEROGAMS GYMNOSPERMAE .......
Development of the sexual generation Cycadinae . . . . .
329 331 337 339 340 342 344 347 351 353 363 365 373 378 386 415 421 425 431 438 443 443 445 455 458 465
•'in
478 480 480
484
CONTENTS ix
PACK
Ginkgoinae ....... . 488
Coniferae ......... 489
Gnetinae ......... 498
FOSSIL GYMNOSPEKMS ... . 500
(£• ANGIOSPEKMAE ......... 501
The angiospermic flower ....... 501
Inflorescence ......... 509
Sexual generation ........ 513
Fruit .......... 519
tffG MONOCOTYLAK ... . . 521
Helobiae ....... .522
Glumiflorae . . . . . . . . .524
Spadicittorae . . . . . . . . . 530
Enantioblastae . . . 536
Liliiflorae . .536
Scitamineae .... . 546
Gynandrae .... .548
Cff DICOTYLAE . .651
Choripetalae ... .552
Piperinae .... . 552
Juglandiflorae . . . 554
Saliciflorae ... . 555
Querciflcrae ..... . 556
Urticinae . . • 563
Ceutrospermae . . . 566
Polycarpicae . .571
Rhoeadinae ... . 585
Insectivorae . . . 591
Saxifraginae • 591
Rosinorae . • 593
Leguminosae . . • 599
Gruinales . . • 610
Tricoccae ... .615
Sapindinae ... • 618
Frangulinae ... 621
Columniferae . • • 623
Cistiflorae . . . 627
Passiflorinae ... • 630 Opuntinae .
Thymelaeinae . • 631
Myrtiflorae . . .631
Umbelliflorae . 635 Hysterophyta
SYMPETALAE . - 645
Pentacyclicae . 645
Ericinae . . 645
Diospyrinae . . • 646
Primulinae . . • 647 Tetracyclicae .
Contortae . • 649
I/
x BOTANY
PAOI
Tubiflorae ...... . 656
Personatae ......... 661
Rubiinae . . . . . . . . .668
Campanulinae ........ 675
Aggregatae ......... 679
FOSSIL ANGIOSPERMS ........ 688
INDKX OF LITERATURE. ....... 691
SYSTEMATIC INDEX OF OFFICIAL AND POISONOUS PLANTS . . . 717
INDEX ........ 721
INTRODUCTION
IT is customary to place all living beings in either the animal or vegetable kingdoms, but in reality a sharp boundary line between animals and plants first becomes possible when they exhibit a complicated structure. In those of more simple organisation all distinctions disappear, and it becomes difficult to define the exact limits of Botany and Zoology. This, in fact, could scarcely be otherwise, as all the processes of life, in both the animal and vegetable kingdoms, are dependent on the same substance, protoplasm. With more complicated organisation, the specific differences increase, and the characteristics distinguishing animal from vegetable life become more obvious. For the present, it must be confessed, the recognition of an organism, as an animal or a plant, is dependent upon its corre- spondence with an abstract idea of what a plant or animal should be, based on certain points of agreement between the members of each class. A satisfactory basis for the separation of all living organisms into the categories of animals or plants can only be obtained when it is shown that all organisms distinguished as animals are in reality genetically connected, and that a similar connection exists between all plants. The proof of this can only be arrived at through the THEORY OF EVOLUTION.
From the study of the fossil remains and impressions of animals and plants, it has been established that in former epochs forms of life differing from those of the present age existed on the earth. It is also generally assumed that all living animals and plants have been derived by gradual modification from previously existing forms. This leads to the further conclusion that those organisms possess- ing closely similar structure, which are united as species in a genus, are in reality related to one another. It is also probable that the union of corresponding genera into one family and of families into higher groups serves to give expression to a real relationship existing between them.
The presumable origin of a living organism from others previously existing has been distinguished by HAECKEL (x) as its phylogenetic
2 BOTANY
development or PHYLOGENY. He termed the series of changes passed through by a living being in attaining its mature condition, its ontogenetic development or ONTOGENY. The supposition, that the successive steps in the ontogenetic development of an organism correspond to those of its phylogenetic development, and that the ontogeny of an organism is accordingly a more or less compk-te repetition of its phylogeny, was asserted by FRITZ MULLER (2), who based his conclusions on the results of comparative research.
The idea of the gradual evolution of higher organisms from lower was familiar to the Greek philosophers, but a scientific basis was first given to this hypothesis in the last century. Through the work of CHARLES DARWIN (3) in particular, who accumulated evidence for a reconsideration of the whole problem of organic evolution, the belief in the immutability of species has been overturned.
DARWIN is the author of the THEORY OF SELECTION. In drawing his conclusions, he proceeded from the variability of living organisms, as shown by the fact that the offspring neither exactly resemble their parents nor each other. Further, he called attention to the constant over-production of offspring, the majority of which must inevitably be destroyed. If this were not so, and all the embryos produced by a single pair attained their full development, they would alone, in a few generations, completely cover the whole surface of the earth. On account of insufficient space for all, the different claimants are engaged in an uninterrupted struggle, in which the victory is gained by those that, for any reason, have an advantage. Through this " struggle for existence " a selective process goes on among the characters appearing in individual variations, and those which under the conditions of life are in any way advantageous tend to be preserved. In this manner DARWIN arrived at the supposition of a process of NATURAL SELECTION, which is the essential of his theory. Newly developed peculiarities arising from individual vari- ability must be inherited in order to become permanent characteristics of a later generation. DARWIN sought in the experience of breeders evidence that such characters are inherited. The breeder selects indi- viduals presenting any desired characters for the purpose of breeding, and has thus formed the races of domesticated animals and cultivated plants. These have often departed so widely from their wild ancestral forms that the latter are not certainly known. Just as in artificial selection, natural selection, although unconsciously, ac- complishes this result. As individual peculiarities may be developed by careful breeding and rendered permanent, so by natural selection those qualities which are advantageous in the struggle for existence become more pronounced and are finally confirmed by heredity. By the continued operation of natural selection, organisms must result, which are, in the highest degree, fitted and adapted to their environment. In this way the theory of selection seeks to explain as
INTRODUCTION
due to natural causes that adaptability to the environment which is such a striking characteristic of organic life. That the transitional forms in this process of phylogenetic development no longer exist, is accounted for in the theory of natural selection by the assumption that the struggle for existence must necessarily have been most severe between similar organisms. For similar organisms have similar necessities, and the new and better-equipped forms must ultimately prevail over the original less specialised organisms and exterminate them.
Since the publication of Darwin's works many investigators have laboured to advance and make clear our views on phylogeny. Difficulties in applying the results of artificial selection to the natural process became evident, for one main condition of successful artificial selection, the isolation of the organisms from which breeding is taking place, is not fulfilled under natural conditions. Of late years HUGO DE VRIES has endeavoured to obtain an insight into the laws of phylogenetic development by systematic cultivation of particular plants. It would appear from such cultures (4) that the starting- point for the origin of new species is not afforded by the " fluctuating variations," which continually occur, but by more marked variations which have been termed " mutations " ; these mutations appeal- suddenly and are strongly inherited. On the other hand it may be said that a sharp line cannot be drawn between mutations and fluctuating variations. DE VRIES tended to assume the existence of a development of the organic world due to original innate capabilities of the living substance, and not dependent on selection. The origin of the large subdivisions of the animal and vegetable kingdoms, the "archetypes," would be due to this sort of evolution (5). The organisms have been, and are still, continually influenced by the environment, and by their reaction to external conditions have become more or less directly adapted. In this way striking re- semblances in external form have arisen between organisms living under similar conditions although belonging to different archetypes (6). Natural selection exercises a constant influence on the process and tends to render species distinct by removing the less advantageous variations.
If the higher organisms have been evolved from the lower, a sharp distinction between plants and animais is excluded. For the characters which are distinctive of animals and plants have appeared in the course of the phylogenetic development of organisms, and were at first wanting. The simplest organisms which now exist are in all probability similar to those which formed the starting-point of this development. The walls which surround the cells com- posing the plant body, and the green chroma tophores within the latter, have been cited as decisive indications of the vegetable character of an organism. Surrounded by firm walls, the living
4 BOTANY
substance becomes more isolated, and, consequently, independence of action in plants, as compared with animals, is diminished. By means of the green colouring matter, plants have the power of producing their own nutritive substances from certain constituents of the air and water, and from the salts contained in the soil, and are thus able to exist independently ; while animals are dependent, directly or indirectly, for their nourishment, and so for their very existence, on plants. Almost all the other differences which dis- tinguish plants from animals may be traced to the structure of plants, or to the manner in which they obtain their food. Another characteristic of plants is the unlimited duration of their ontogenetic development, which is continuous, at the growing points, during their whole life. That none of these criteria are alone sufficient for distinguishing plants from animals is evident from the fact that all the Fungi are devoid of green pigment, and, like animals, are dependent on substances produced by green plants for their nourishment. On the borderland of the two kingdoms, where all other distinctions are wanting, phylogenetic resemblances, accord- ing as they may indicate a probable relationship with plants or animals, serve as a guide in determining the position of an organism.
While it is thus impossible to distinguish sharply the two great groups of living organisms from one another, a distinction between them and lifeless bodies is readily recognised. Living organisms are endowed with the quality of IRRITABILITY, in which all lifeless bodies are deficient. External or internal stimuli influence living organisms to an activity, which is manifested in accordance with the requirements and conditions of their internal structure. Even in the smallest known organisms the manifestations of life are occasioned by a similar sensitiveness to external or internal stimuli. It is, therefore, probable that the simplest living beings must have possessed essentially simpler properties than any organisms now known, which would enable us to connect them with non-living substances. The substance which serves as a basis for all develop- ment must be supposed to have had an inorganic origin. So far as i- actually known, however, all living organisms have arisen only from similar organisms. So far as experience has shown, spontaneous generation is unknown. In the olden times it was a common supposition, which ARISTOTLE himself held, that even highly organised animals and plants could originate from sand and mud. In the same degree that knowledge of the actual development of living organisms was extended, the previously accepted cases of spontaneous generation became more and more restricted, and were finally limited to intestinal worms which could not otherwise, it was thought, be accounted for, and to microscopic organisms, the origin of which also was not understood. Now, for such organisms the
INTRODUCTION
possibility of a spontaneous generation has been disproved by more modern investigations ; the history of the development of intestinal worms is known, and the germs of minute organisms have been found to exist everywhere. SCHWANN and PASTEUR have been pioneers in this work, and have shown that it is possible to hinder the development of the lower organisms, in places where it is customary to find them, by destroying all existing germs and at the same time preventing the entrance of new ones. It is due to the results obtained by these men in their investigations on spontaneous generation that we are now able to preserve food in a scientific manner. The germs previously existing in the substance to be conserved are destroyed by heat, while, by a proper mode of sealing, the entrance of new germs is rendered impossible, and the decomposition, which their presence would occasion, is accordingly prevented.
All known living organisms have been derived from other living organisms. But the idea of the origin of living from dead substances has on the other hand derived important support from the progress of chemical research. In the early decades of the last century it was customary to draw a distinct line of separation between organic and inorganic chemistry, and to assume that the substances dealt with by organic chemistry could only be produced by the vital action of organisms. The laws governing inorganic chemistry appeared to have no reference to organic chemistry, the formation of organic substance being due to a special force, the li vital force." In 1828 WOHLER obtained urea from ammonium cyanate, and thus for the first time produced an organic compound from an inorganic substance. In 1845 KOLBE completely synthesised trichloracetic acid, and in 1850 BERTHKLOT synthesised alcohol and formic acid. The former substance had been synthetically prepared by HENNEL in 1828, but BERTHELOT was the first to recognise its identity with the substance formed in alcoholic fermentation. By these results the former distinction between organic and inorganic chemistry was destroyed. Organic chemistry has become the chemistry of carbon compounds.
In some such way it is possible that living matter originated from non-living at some period in the evolution of the earth when the conditions for its formation existed. In order that the organic world should have developed from the first living matter, one of the original properties of the latter must have been a capability of continued existence among its surroundings. It must have been capable of variation and of retaining the new characters appearing in this way, of growth, i.e. the increase of itself at the cost of foreign substances, and of reproduction, i.e. multiplication by separation into a number of parts. Some observers have recently described the origin of microscopic structures which behave similarly
6 BOTA X Y
to living beings in bouillon anil other organic culture^ media when exposed to the action of radium (7).
Botany, or the science of plants, may be divided into a general and a special part. In the general part, the structure and functions of plants as such will be considered ; in the special part, the particular structure and functions of the separate orders of plants will l)e discussed.
The study of the structure of plants is called MORPHOLOGY ; that of their functions PHYSIOLOGY. In the general part, morphology and physiology will be treated separately; in the special part, conjointly.
PART I GENERAL BOTANY
SECTION I MOEPHOLQGY
GENERAL BOTANY
SECTION 1 MORPHOLOGY
Tin; object of vegetable morphology is the scientific study of the forms of plants. It does not attempt to discover the causes of the variation in the forms, but rather has accomplished its purpose when it succeeds in showing how one form may be derived from another. The only real basis of morphological study is, accordingly, the genea- logical development or phylogeny (p. 2). As phylogenetic develop- ment can only be inferred, and cannot be directly followed, the methods of morphology must also be indirect. They are dependent on the one hand upon ontogeny, i.e. on the study of the development passed through by an organism in attaining its mature condition, and on the other hand upon the comparison of existing organisms with one another and with those that have become extinct. To a certain extent the ontogenetic development of a plant repeats its phylogeny and helps to elucidate the latter, while, by means of com- parative investigation, extreme forms may be connected by inter- mediate links. As, however, the ontogeny of a plant is neither a complete nor invariable repetition of its phylogeny, and as connecting links between extreme forms are often wanting, the results of morphological study are frequently incomplete. Such parts or members of plants which it is reasonable to presume have had a common origin are distinguished as HOMOLOGOUS ; those which, while probably having different origins, yet exercise the same functions, are termed ANALOGOUS. Through the adaptation of different parts to the same function, a similarity in both external form and internal structure often results ; and in this way the correct determination of morpho- logical relationships is rendered difficult. Only homologous parts have the same " morphological value." This homology is determined
10 BOTANY PARTI
by the facts of phylogeny and origin, and not by any correspondence in function. Though the function of any structure does not influence its morphological value, the need of making clear the intimate con- nection between form and function often introduces physiological considerations into morphological questions. When, for phylogenetic reasons, it seems possible to attribute to a number of different members a common origin, such a hypothetical original form is termed the fundamental or primitive form. The various modi- fications which the primitive form has passed through constitute its METAMORPHOSIS. In this way the theory of the metamorphosis of plants, which was once but an ideal conception, acquires an actual significance.
Slightly differentiated structures, which are found at the beginning of a series of progressively differentiating forms, are termed i;n>i MENTARY ; imperfect structures, which have arisen as the result of the deterioration of more perfect forms, are termed REDUCED.
Vegetable morphology includes the study of the external form and the internal structure of plants. The descriptive study of the external form of plants has been termed organography (8). This term will not be used, since by the use of the word " organ," it would seem to have a physiological signification. Morphology takes no recognition of the parts of a plant as organs, but treats of them merely as members of the plant body. On the other hand, one of the most important aims of physiology is to place the external form and the internal structure of the living body in relation to the functions performed by the latter ; physiology also investigates the causes of the organisation. The study of the internal structure of plants is often designated Anatomy or Phytotomy ; but as it usually includes also the study of the more minute internal structure, it resembles rather histology, in the sense in which that term is used by zoologists, and concerns itself to a much less degree with the coarse anatomy of the plant body. In any case, it is the simplest plan to designate the study of the outer form EXTERNAL MORPHOLOGY, and that of the inner structure INTERNAL MORPHOLOGY.
I. EXTERNAL MORPHOLOGY (9)
Plants show a great diversity in the form and arrangement of their members ; it is the task of morphology to determine the points of agreement existing between them. It seeks to do this by deter- mining the common origin of the homologous parts or members.
SECT. I
MORPHOLOGY
11
The Development of Form in the Plant Kingdom
The Thallus (10). — When the body of a plant is not differentiated into separate members, or is composed of members which (though they
may be similar) are not homologous with those of the most highly organised plants, it is termed a THALLUS. When the Fl°- 2---s '"" *?
,, ,, . ,.„. . , txrevisiae. 1, Cells
thallus is differentiated wlthout buds ; -i and into members analogous 3, budding ceils, (x to those of the higher 540-) plants some confusion may arise from the At same names being used for parts which, Commencement of division ; B, since their origin has been distinct, are not
(to the left) shortly aftendi vision ; homoloo-QUS C', a resting stage, (x MO.) » . S-
The simplest form that we can imagine
for an organism is that of a sphere, and this is actually the form of some of the lower plants. A green growth often seen on damp
Fm.i.-Gto«ttif»a
Km. 4. — Bacteria from deposits on teeth, c, Leptothi-ii' hum i//'x; "', the same after treat- ment with iodine; b, Micrococcus ; c, Spin- chaete dentium after treatment with iodine ;
il. *pii-!Uit,ii .tntitiiji-rtiim. (X 800.)
FIG. 3. — Pinnului-iu viridi*. A,
view ; B, lateral view. ( x 540.)
walls consists of an aggregation of the microscopically small spherical bodies of Gloeocapsa poli/dermatica (Fig. 1), an Alga belonging to one of
12
IJOTAXY
the lowest divisions of the vegetable kingdom. The single plants of the Beer-yeast (Saccharomyces cererisiae) are ellipsoidal ; but, from their peculiar manner of growth, by budding, they form lateral outgrowths, and thus often appear constricted (Fig. 2). Cylindrical and also disc -shaped forms are shown by various Algae. The Diatomeae (Fig. 3), in particular, exhibit a great variety of spindle, canoe, helmet, and fan-like shapes ; but they may all be derived from the more simple spherical, discoidal, or cylindrical forms.
1' ".. Plea Lactuca.
young stage, show- ing apex anil base. x 220.)
Fio. 0. — Portion of Cluiln/,: (X 48.)
Flu. ~. — <'lail"*tiiJins ivrtii-illatn.". (After TRINOSHETM, x 30.)
Among the Bacteria, which, as the cause of infectious diseases and of decomposition, have been the object of so much recent investiga- tion, we also meet with spherical, rod-shaped, filamentous, and spirally wound forms (Fig. 4). The next stage in the progressive
SECT. I
MORPHOLOGY
Fio. 8. — Dictyota dichotnmn. (jj nut. size.)
development of external form in the vegetable kingdom is exhibited by such plants as show n DIFFERENTIATION INTO APEX AND P.ASE. The base serves as a point of attachment, while growth is localised at the apex. In this way a growing point is developed at the apex. As an example of such a form, a young plant of the green Alga, Ufoa Laduca (Fig. 5), may be taken. The de- velopment of a more com- plicated external form is represented by the branched filamentous, or ribbon- shaped Algae, in which the origin of new formations is more and more restricted to the apex. An ACROPETAL order of development, in which the youngest lateral
members are always nearest the growing apex, is clearly demon- strated by the branched fila- ments of the common green Alga, Cladophora glomerata (Fig. 6). Still more pronounced is the apical growth in the brown seaweed Cladostephus rerticillatus (Fig. 7). The great variety in the form of the larger Fungi and Lichens, by which they are distinguished as club-, um- brella-, salver-, or bowl-shaped, or as bearded or shrub -like, comes about by the union or intertwining of apically grow- ing filaments. This type of construction is limited to Fungi and Lichens. As the apex itself may undergo successive bifurca- tion, as in the case of Dictyota dichotoma (Fig. 8), it does not always necessarily follow that new members must be formed beneath the original apex. The highest degree of external differentiation among the lower plants is met with in certain groups of red and brown sea-weeds (Rhodophyceae and Phaeophyceae). Many representatives of these
FlO. 9. — Hydi-<ili'ji"tli'nii
(o nut. sixr.)
14
BOTANY
classes resemble the higher plants in the formation and arrangement of their members; Hydrolapathum sant/uim-iim (Fig. 9), for example, as is indicated by its name, resembles a species of fittmex, and affords an instructive illustration of the analogy of form existing
Fia. 10. — Kiccin ii<iitmi.< (Nat. slice.')
Fio. 11. — Blasia piisillu. s, Sporogoiiiuin ; r, rhizoids. (x 2.)
between plants phylogenetically widely distinct from one another. The progressive differentiation of the thallus of the Bryophyta (Mosses and Liverworts)" presents an even closer parallel to the
organisation of the higher plants. Within the group of the Liverworts (Hepaticae) are found ribbon-shaped forms and others that show a division into distinct members. Thus the thallus of fiiccia jlnitans (Fig. 10) is ribbon -shaped and dichotomously branched, and its habit or general ap- pearance recalls the Brown Alga Didyota ilichotoma mentioned above (Fig. 8). Blasia pusilla(Fig. 11) has marginal indentations in its ribbon-shaped body. Lastly, Plagio- chila asplenioides (Fig. 12), another Liver- wort, has a distinction of stem-like and leaf-like members which is completely analogous to that exhibited by the most highly organised plants. In spite of this variety in external form, and the high organisation that may be reached, the body of these lower plants is termed a thallus, and they are grouped together as Thallophytes in contrast to the higher plants or Cormophytes.
The Cormus. — All plants, from the Pteridophtya or fern-like plants onwards, may be grouped together as CORMOPHYTES. It may be assumed that they have had a common origin, and that
Ki<:. 1-.- -I'luiiiiK-Iiilit ax/ill .--. S[>orogoiiiuin. (Xat. size.)
MORPHOLOGY 15
the similarity of their organisation is due to true homology. In addition to the distinction of stem and leaf in their shoots, they possess true roots, while even the most highly organised Bryophyta have only filamentous structures (rhizoids) (Fig. 11 r) in place of roots to attach them to the substratum. True roots, on the other hand, which appear for the first time in the Pteridophyta, are, for the most part, cylindrical structures with apical growth. Besides possessing a distinctive internal construction they are distinguished in their external form from the shoot by having a special sheath, the ROOT- CAP or CALYPTRA covering the growing point, and by the absence of leaves. The body of the higher plants provided with stem, root, and leaf is termed a cormus, and the plants are spoken of as cormophytes.
The Metamorphosis of the Primary Members of Cormophytes. — After the differentiation into stem and leaf and the appearance of the roots had taken place, further changes have consisted essentially in a more or less profound modification of these primary members of the cormophytic plant-body. Such changes are spoken of as a metamorphosis (p. 10), and in some cases may be so extensive as to lead to one primary member assuming the characters of another.
The relationships between homologous members, which are often vei'y striking, did not escape the notice of earlier observers. They suggested comparisons, although no real phylogenetie basis for such comparisons existed. Thus, an ideal- istic conception of the form of external members was developed, and finally reached its highest artificial development in GOETHE'S Theory of Metamorphosis ; and its scientific conclusion in the writings of ALEXANDER BIIAUN. As the great variety exhibited in the external appearance of the lower plants precluded any possibility of assigning to them hypothetical primitive forms, the whole terminology of the external morphology of plants has been derived from conceptions applicable only to the Cormophytes. Even to-day, the same terms used in reference to the Cormophytes are applied to parts of the Thallophytes, which are evidently only analogous.
Members of Independent Origin. — Parts which cannot be derived by metamorphosis of the primary members of the cormophytic plant are sometimes met with. Though they are of infrequent occurrence they are of importance as showing that the natural evolutionary process is not to be limited by any formal scheme.. Such structures will be discussed farther on.
Relations of Symmetry
Every section through a part of a plant, made in the direction of its longitudinal axis, is distinguished as a longitudinal section ; those at right angles to it being termed cross or transverse sections. Parts of plants which may be divided by a number of longitudinal planes into like halves are termed either MULTILATERAL, RADIAL, or ACTINOMORPHIC. Such parts are symmetrically constructed around
16
BOTANY
their longitudinal axis. The degree of symmetry peculiar to any leafy shoot will be more apparent from a diagram, that is if the
KIC;. lli. -Diagram showing tin- su-callfl cl> rnssiiti- arrangement <>t' leaves.
. 14. — Diagram showing two-tmoked
alternate arrangement of lea\
leaves which it bears be projected on a plane at right angles to its axis. The radial symmetry of a shoot with opposite leaves is clearly
shown in the adjoining diagram (Fig. 13). I lV A shoot with its leaves arranged alternately
in two rows shows somewhat different rela- tions of symmetry. The diagram of such a shoot (Fig. 14) can only be divided into similar halves by two planes. When such a condition exists, a member or plant is said to be BILATERAL. When, however, a division into two similar halves is only possible in one plane, the degree of sym- metry is indicated by the terms DORSI- VENTRAL or ZYGOMORPHIC; since, while the right and left halves correspond to one another, differences exist between the dorsal and ventral surfaces. Ordinary foliage- leaves exhibit this dorsiventral structure. In the accompanying figure (Fig. 15) such a monosymmetrical, dorsiventral foliage- leaf is diagrammatical ly represented. From the surface view (A) and from the cross- section (/?), in which the distinction between the dorsal and ventral sides is indicated by shading, it is obvious that but one plane of symmetry (s) can be drawn. Dorsiventral members are often ASYMMETRICAL, not being divided by any plane into corresponding halves : the leaves of many kinds of Begonia will
.v
<i. 15. — Diagram of a foliage-leal. .1, Surface view ; li, trausverse section : -. plane of symmetry.
SKUT. I
MORPHOLOGY
17
serve as examples of this. In such cases — and the leaf of the Elm may be mentioned as another striking example — the symmetry of the individual leaf is subordinated to that of the entire plant.
Branch Systems
Thallophytes as well as Cormophytes exhibit systems of branching, resulting either from the formation of new growing points by the bifurcation of a previously existing growing point, or from the develop-
Ma
Fie. 10. — Diagrams of branch systems. A, Dichotoinous branching ; Aa, ci|iial dichotomy ; Ab, scorpioid dichotomy ; Ac, helicoid dichotomy, li, monopodial branching ; lla false dichotomy ; Kb, scorpioid cyme ; Be, helicoid cyme ; .«, s, sympodia.
ment of new growing points in addition to those already present. In this way there arise two systems of branching, the DiCHOTO- MOUS and the MONOPODIAL. By the uniform development of a continuously bifurcating stem, a typical dichotomous system of branching is produced, such as is shown in Dictyota dichotoma (Fig. 8), and is represented diagrammatically in Fig. 16 Ad. In a typically developed example of the monopodial system there may always be distinguished a persisting main axis, the MONOPODIUM, giving rise to lateral branches from which, in turn, other lateral branches are developed. A good example of this form of branching is afforded by a Fir-tree. Where one of the two branches is regularly developed at the expense of the other, the dichotomous system assumes an
c
18 BOTANY PART I
appearance quite different from its typical form. The more vigorous branches may then, apparently, form a main axis, from which the weaker branches seem to spring, just as if they were lateral branches. This mode of branching (Fig. 16 Ab} is illustrated by the Selaginellas. Such an apparent main axis (s, s) is termed, in accordance with its origin, a SYMPODIUM. On the other hand, in the monopodial system two or even several lateral branches may develop more strongly than the main axis, and so simulate true DICHOTOMY or POLYTOMY. Such monopodial forms of branching are referred to as FALSE DICHOTOMY (Fig. 16 Ea) or FALSE POLYTOMY, as the case may be. A good example of false dichotomy may be seen in the Mistletoe (Viscum itHniin). If, however, a lateral branch so exceeds the main axis in development pushing the apex of the latter to one side, that it seems ultimately to become a prolongation of the axis itself, a sympodium is again formed (Fig. 16 Eh). This is what occurs in many of our forest trees, e.g. the Lime and Beech ; in both of these trees the terminal buds of each year's growth die, and the prolongation of the stem, in the following spring, is continued by a strong lateral bud, so that in a short time its sympodial origin is no longer recognisable. In many rhizomes, on the other hand, the sympodial nature of the axis can be .easily distinguished ; as, for example, in the rhizome of Polygonatum multiftorum (Fig. 23), in which, every year, the terminal bud gives rise to an aerial shoot, while an axillary bud provides for the continuance of the axis of the rhizome. In the flower-producing shoots or inflorescences of Phanerogams the different systems of branching assume very numerous forms. These will be more fully described in their proper place. To such inflorescences belong the ventrally coiled dorsiventral shoots which produce new shoots from their convex dorsal surfaces, instead of in their leaf -axils.
The Shoot
The Development of the Shoot. — Under the term shoot a stem and its leaves are collectively included. A stem possesses an apical mode of growth (Fig. 17), and its unprotected growing point is described as naked, in contrast to that of the root with its sheathing root-cap. The apex of the shoot generally terminates in a conical protuberance, called the VEGETATIVE CONE. As it is usually too small to be clearly visible to the unaided eye, it is best seen in magnified median longitudinal sections. So long as the apex of the shoot is still internally undifferentiated, it continues in the embryonic con- dition, and it is from the still embryonal vegetative cone that the leaves take their origin. They first appear in acropetal succession as small, conical protuberances, and attain a larger size the farther removed they are from the apex of the stem. As the leaves usually grow more rapidly than the stem which produces them, they envelop
MORPHOLOGY
19
the move rudimentary leaves, and, overarching the vegetative cone, form a BUD. Buds are therefore merely undeveloped shoots. If they are to remain for a long time undeveloped, as for example is the
PIG. 17. — Apex of u. shoot of a phanerogamic plant, v, Vegetative cone ; /, leaf rudiment ; g, rudiment of an axillary bud. (x 10.)
FH:. 18. — Longitudinal section »f :i bifurcating shoot (;>) of I./ico- podlum alpinimi, showing equal development of the rudimentary shoots, p', p" ; b, leaf rudiments ; c, cortex; /, vascular stninds. (After HF.UKI.MAIKK, x i>0.)
case with winter buds, they are protected in a special manner during their period of rest.
The Origin of New Shoots. — The formation of new growing points by the bifurcation of an older growing point, in a manner similar to that already described for Dictyota dichotoma (Fig. 8), occurs also in the lower thalloid Hepaticae (Riccia fluitans, Fig. 10). Among the Cormophytes this method of producing new shoots is of less frequent occurrence, and is then mainly limited to the Pteridophytes, and is typically shown only in some Lycopodiaceae. In this case, whenever a shoot is in process of bifurcation, two new' vegetative cones are formed by the division of the growing point (Fig. 18). In most of the Lycopodiaceae the new shoots thus formed de- velop unequally ; the weaker becomes pushed to one side and ultimately appears as a lateral branch (Fig. 19). Although a relationship as re- gards position is generally apparent between the origin of leaves and the lateral shoots, in the system of branching resulting from such a bifurcation of the vegetative cone this connection does not exist. In the more highly developed Bryophytes, particularly in the true Mosses, new shoots arise obliquely below the still rudimentary leaves
FIG. 19. — Hifurcating shoot (p) of Lycofniilhitii hin i" datum, showing unequal development of the rudi- mentary shoots, ]>', p" ; b, leaf rudiments. (After HEGKLMAIF.R, x 40.)
20 BOTANY PART i
at some distance from the growing point. In the Phanerogams new shoots generally arise in the axils of the leaves. In the accom- panying illustration of a longitudinal section of a phanerogamic shoot (Fig. 17) the rudiment of a shoot (g) is just appearing in the axil of the third uppermost leaf ; in the axils of the next older leaves the conical protuberances of the embryonic leaves are already beginning to appear on the still rudimentary shoot. Shoots thus produced in the axils of leaves are termed AXILLARY SHOOTS. The leaf, in the axil of which a shoot develops, is called its SUH- TENDING LEAF. An axillary shoot is usually situated in a line with the middle of its subtending leaf, although it sometimes becomes pushed to one side. As a rule, only one shoot develops in the axil of a leaf, yet there are instances where it is followed by additional or ACCESSORY SHOOTS, which either stand over one another (serial buds), as in Lonicera, Gleditschia, Gymnoclcuhis, or side by side (collateral buds), as in many Liliaceae, e.g. species of AUnnn and Muscari.
Although in the vegetative regions, i.e. the regions in which merely vegetative organs are produced, the rudiments of the new shoots of phanerogamic plants make their appearance much later than those of the leaves, in the generative or flower-producing regions the formation of the shoots follows directly upon that of their sub- tending leaves, or the shoots may even precede the leaves. In this last case the subtending leaves are usually either poorly developed or completely suppressed, as in the inflorescence of the Cruciferae.
The bud forming the end of a shoot is called the terminal bud, while those borne on the sides of the shoot are the lateral buds.
Shoots developing in predetermined positions on young parts of the plant are designated NORMAL, in contrast to ADVENTITIOUS SHOOTS, which are produced irregularly from the old or young portions of a plant. Such adventitious shoots frequently spring from old stems, also from the roots of herbaceous plants (Brassica oleracea. Anemone sylvestris, Convolvulus arrensis, Eumex Acetosella), or of bushes (J,'n!iiis, Rosa, Corylus), or of trees (Populus, Ulmus, Rolinia). The)' may even develop from leaves, as in Cardamim pratensis, Nasturtium officinale, and a number of Ferns. An injury to a plant will frequently induce the formation of adventitious shoots, and they frequently arise from the cut surface of stumps of trees. Gardeners often make use of pieces of stems, rhizomes, or even leaves as cuttings from which to produce new plants.
Leaves and also normal shoots, which make their appearance as out- growths from the portions of the parent shoot, still in an embryonic condition, have an external or EXOGENOUS origin. Adventitious shoots, on the other hand, which arise from the older parts of stems or roots, are almost always ENDOGENOUS. They must penetrate the outer portions of their parent shoot before becoming visible. Adven-
SECT, i MORPHOLOGY 21
titious shoots formed on leaves, however, arise, like normal shoots, exogenously.
Buds are formed in the marginal indentations of the fleshy leaves of species of Bryophyllum (Crassulaceae). Although arising from the leaf these buds must properly be regarded as "normal," and as forming part of the normal ontogeny of the plant, since they arise in pre-determined positions from young tissue. In the strict sense of the term only those buds can be called adventitious which are produced in casual positions from tissues which in their production enter into renewed activity, e.g. the buds which arise at the base of isolated leaves of Beyonia when these are laid on damp soil. The concept of " normal b'uds" is notwithstanding usually taken in a narrower sense and limited to buds which arise on the axis of the shoot in the normal course of development. Examples, however, occur which make it difficult to draw a sharp distinction in this latter sense between normal and adventitious shoot-formation (n).
While, as a rule, new leaves arise beneath the apex of a stem, which continues to grow, exceptions to this occur, especially in floral structures ; the apical cone may be used up in the formation of a terminal leaf-rudiment. In this way many of the stamens and carpels of Phanerogams, which are described as terminal, arise.
In some shoots of limited growth such as the inflorescences of many Phanerogams (Vallisneria) new lateral members arise in an inverted order proceeding from the apex towards the base. The introduction of new lateral members between those already present is also met with in the floral region (12).
It was mentioned above that normal shoots arose from the embryonic tissue of the growing point of the parent shoot. When they are apparent at a greater distance from the apex (Fig. 17) it can usually be shown that embryonic substance has been reserved at the proper points for their formation. The growing points of adventitious shoots are for the most part derived from embryonic tissues which have persisted iu the older portions of the plant and are capable of increase. They can, however, also arise from older tissues, owing to the capability of the latter to return to the embryonic condition and produce new growing points.
Further Development of the Shoot. — The processes of develop- ment, which result in the production of new members at the apex of a shoot, are followed by their increase in size and further growth. This growth is usually introduced by the vigorous elongation of the segments, which is spoken of as growth in length, while at the same time the buds expand. The zone of most rapid growth in length is at some distance from the growing point. The growth in length and consequent elongation of the shoot is in some cases so slight that the leaves remain close together, and leave no free spaces on the stem, thus forming so-called SHORT SHOOTS. The same plant may bear ordinary elongated shoots and short shoots. The duration of life of the latter is usually shorter, and they fre- quently do not branch, and take no part in the permanent branch system of the tree. This is seen in the Larch, in which the short shoots with crowded needle-shaped leaves are borne on elongated shoots. The stem of a shoot, as contrasted with the leaves,
22
BOTANY
PART I
is often spoken of as the AXIS ; the portions of the stem axis between the insertions of the leaves are termed the INTERNODES, and the parts of the axis, from which the leaves arise, the NODES. When the base of the leaf encircles the stem, or when several leaves take their origin at the same node, the nodes often be- come strongly marked (Labiatae).
In some cases the growth in length of a shoot continues for a longer time at certain intermediate points by means of INTERCALARY <;ROYVTH. Such regions of intercalary growth are generally situated at the base of the inter- nodes, as in the case of the Grasses. A dis- placement from the position originally occupied by the members of a shoot frequently results from intercalary growth. A bud may thus, for example, become pushed out of the axil
Kic;. -JO. — Xaiiiolus I'/'Icraiidi, . ., i , T i e j Ai i_
each axillary shoot (a) of lts. subtending leaf, and so apparently have bearing its subtending leaf its origin higher on the stem; or a subtending
fruit*"'1 iSt'si"""1'8 '" * leaf' in the COU1>Se °* itS growth* maJ Can7 its
axillary bud along with it, so that the shoot
which afterwards develops seems to spring directly from its subtend- ing leaf ; or, finally, the subtending leaf may become attached to its axillary shoot, and growing out with it, may thus appear to spring from it (Fig. 20).
Iu the annual growth in length of a shoot of a tree, i.e. the increment formed during one vegetative period, tlio loAvest internodes which intervene between the bud- scales are very short. By means of the closely crowded scars of the bud-scales the limits of the shoots formed in successive years can be determined.
Resting Buds. — As a means of protection against loss of water in our latitudes, the buds of shrubs and trees are usually invested, in winter, with scale-like leaves or BUD-SCAL1> (Fig. 21). These are rendered still more effec- tive as protective structures by hairy outgrowths and excretions of resin and gum, and also by the occurrence of spaces filled with air between the scales. Not infrequently the subtending
^ J &
leaf takes part in the protection of its axillary
bud, the base of the leaf-stalk, after the leaf
itself has fallen, remaining on the shoot as a
cap-like covering for the winter bud. The buds of tropical plants,
which have to withstand a dry period, are similarly protected ; but
I.- tin
,
Fin. 21.— Winter bu<is of the
Beech (*•,.,"'••>•
*™. Bud--««to* (Nat.
MORPHOLOGY
23
where the rainfall is evenly distributed throughout the year buds develop no such means of protection.
In many deciduous trees, such as the Willow, the terminal buds of the year's growth regularly die. In nearly all trees many buds, iisually the first-formed buds of each year's shoot, seem able to remain dormant during many years without losing their vitality : these are termed DORMANT BUDS. In the case of the Oak or Beech such latent buds can endure for hundreds of years ; in the meantime, by the elongation of their connection with the stem, they continue on its surface. Often it is these, rather than adventitious buds, which give rise to the new growths formed on older parts of stems. It may sometimes happen that the latent buds lose their connection with the woody parts of their parent stem, but nevertheless grow in thickness, and develop their own wood ; they then form remarkable spherical growths within the bark, which may attain the size of a hen's egg and can be easily separated from the surrounding bark. Such globular shoots are frequently found in Beech and Olive trees.
i,
FIG. 23.— Rhizome of Polygonatum tnultiflorum. a, Bud of next year's aerial shoot ; b, scar of this year's, and c, d, e, scars of three preceding years' aerial shoots ; »•, roots. (^ nat. size.)
FHJ. 22.— Shoot of Dentaria bulbi fera, bearing bulbils, br. , (Nat. size.)
The Metamorphosis of the Bud. — The BULBILS and GEM»LE> which become separated from the parent plant and serve as a means of reproduction, are special forms of modified buds. To facilitate the storage of reserve nutritive substances they have usually the form of small tuberous bodies. Many plants owe their specific name to the fact that they produce such bulbils, as, for example, Lilium bulbiferum and Dentaria bulbifera (Fig. 22).
The Metamorphosis of Subterranean Shoots. — Shoots that live underground undergo characteristic modifications, and are then termed
24
BOTANY
PART I
ROOT-STOCKS or RHIZOMES. By means of such subterranean shoots many herbaceous perennial plants are enabled to persist through the winter. A rhizome develops only reduced leaves in the form of larger or smaller, sometimes scarcely visible, scales. By the presence of such scale leaves, with their axillary buds, and by the absence of a root-cap, as well as by its internal structure, a rhizome may be dis- tinguished from a root. Rhizomes usually produce numerous roots ; but in a few cases these are wanting and the rhizome itself functions as a root. Rhizomes often attain a considerable thickness and store
Fio. 24. — Rhizome of Coralllorrhiza in- nata. a, Floral shoot ; 6, rudiments of new rhizome branches. (After SHACHT, nat. size.)
zk
FIG. 25. — Longitudinal section of Tulip bulb, Ttdipa Gesnerwna. zk, Muililioi stem ;i zs, scale leaves ; v, terminal bud ; k, rudimentof a younj; bulb ; »•, roots. (Nat. si/c.)
up nutritive material for the formation of aerial shoots. In the illustration on the preceding page (Fig. 23) is shown the root-stock of Solomon's Seal (Polygonatum multiflorum). At d and c are seen the scars of the aerial shoots of the two preceding years ; and at b may be seen the base of the stem growing at the time the rhizome was taken from the ground, while at a is shown the bud of the next year's aerial growth. The rhizome of Coralliorrhiza innata, a sapro- phytic Orchid, which grows in soil rich in humus, affords a good example of a root-stock functioning as a root (Fig. 24). BULBS, also, belong to the class of metamorphosed shoots. They represent a shortened shoot with a flattened, discoid stem (Fig. 25 zk), the fleshy
MORPHOLOGY 25
thickened scale leaves (zs) of which are filled with reserve food material. The aerial growth of a bulb develops from its axis, while new bulbs are formed from buds (k) in the axils of the scale leaves. Another form of underground shoot, allied to bulbs and connected with them by transitional forms, is distinguished as a TUBER. The axis of a typical tuber, in contrast to that of a bulb, is fleshy and swollen, serving as a reservoir of reserve material, while the leaves are thin and scaly. Of such tubers those of the Meadow Saffron (Colchicum autumnale) or of Crocus sativus are good examples. In the Meadow Saffron new tubers arise from axillary buds near the base of the modified shoot, but in the Crocus from buds near the apex. In consequence of this, in the one case the new tubers appear to grow out of the side, and in the other to spring from the top of the old tubers. The tubers of the Potato (Fig. 26) or of the Jerusalem Artichoke (Helianthus tuberosus) are also subterranean shoots with swollen axis and reduced leaves. They are formed from the ends of branched, underground shoots or runners (STOLONS) and thus develop at a little distance from the parent plant. The so-called eyes on the outside of a potato, from which the next year's growth arises, are in reality axillary buds, but the scales which represent their subtending leaves can only be distinguished on very young tubers. The parent plant dies after the formation of the tubers, and the reserve food stored in the tubers nourishes the shoots which afterwards develop from the eyes. Since, in their uncultivated state, the tubers of the Potato plant remain in the ground and give rise to a large number of new plants, it is of great advantage to the new generation that the tubers are produced at the ends of runners, and are thus separated from one another.
The Metamorphosis of Aerial Shoots. — Similar advantages to those obtained by the elongation of the underground shoots in the Potato accrue from surface runners, such as are produced on Straw- berry plants. Surface runners also bear scale-like leaves with axillary buds, while roots are developed from the nodes. The new plantlets, which arise from the axillary buds, ultimately form independent plants by the death of the intervening portions of the runners.
Still more marked is the modification exhibited by shoots which only develop reduced leaves, while the stems become flat and leaf-like and assume the functions of leaves. Such leaf-like shoots are called CLADODES or PHYLLOC LADES, and Goebel proposes to dis- tinguish those flattened shoots which have limited growth and specially leaf-like appearance as phylloclades, and to term other flattened axes cladodes. Instructive examples of such formations are furnished by Ruscus aculeatus (Fig. 27), a small shrub whose stems bear in the axils of their scale-like leaves (/) broad, sharp-pointed cladodes (cl), which have altogether the appearance of leaves. The flowers arise from the upper surface of these cladodes, in the axils of scale leaves.
26
BOTA N Y
In like manner the stems of the Opuntias (Fig. 28) are considerably
Fio. 20. — Part of a growing Potato plant, Solatium tubero.tiiin. The whole plant lias been developed from the dark-coloured tuber in the centre. (From nature, copied from one of UAILLON'S illustrations, J nat. size.)
flattened, while the leaves are reduced to small thorny protuberances.
Fio. 27.— Twig of Jtusciisaculeat'us. f, Leaf; cl, cladode ; bl, flower. (Nat. size.)
I i' . 28.— Opimtta •MMOetMUta, Il,-iw., show-in1,' llower and fruit. (After SCHUMANN. |. nat. size.)
In this case the juicy flat shoots perform not only the functions of
SECT. I
MORPHOLOGY
27
assimilatory organs, but also serve as water-reservoirs in time of drought.
On the other hand a plant may lose its leaves more or less com- pletely without any marked flattening or thickening occurring in the stems, which then take on a green colour ; this, for ex- ample, is the case in the Broom (Spartium scoparium), which-develops only a few quickly falling leaves on its long, naked twigs. As a rule, however, leafless green Phanerogams will be found to have swollen stems, as in the variously shaped species of Euphorbia and Cactus.
Reduction of the Shoot in Parasites. — A great reduction in the leaves, and also in the stems, often occurs in phanerogamic parasites, in consequence of their parasitic mode of life. The leaves of the Dodder (Cusciita, Fig. 202 b) are only represented by very small yellowish scales, and the stem is similarly yellow instead of green. The green colour would, in fact, be superfluous, as the Dodder does not produce its own nourishment, but derives it from its host plant. Cuscuta Trifolii, one of the most fre- quent of these parasites, is often the cause FIG. 2t>. — Ampeiopsis veitckn.
Of the large yellow R> R< Stem-tendrils. (J nat.
f /I 1 sizp-)
areas frequently ob- servable in the midst of clover fields. In certain tropical parasites belonging to the family Rafflesiaceae, the process of reduction has advanced so far that the flowers alone are left to represent the whole plant. Eafflesia .l/'uoldi, a plant growing in Sumatra, is a re- markable example of this ; its flowers, al- though they are a metre wide, the largest flowers in existence, spring directly from the roots of another plant (species of Cisstis).
Tendrillar Shoots. — A peculiar form of metamorphosis is exhibited by some climbing of GietHt- plants through the transformation of certain (4 nat. of their shoots into TENDRILS. Such tendrils assist the parent plant in climbing, either by twining about a support or otherwise holding fast to it. The twining bifurcated tendrils of the Grape-vine, for example, are modified shoots. In some sub-species of the wild vine (Ampelopsis Jtederacea) and in
FIG. 30.— Stem-tin. in schia triawnthos. size.)
28 BOTANY PART i
other species of the genus, e.g. Am^dopsis Veitddi (Fig. 29), the tendrils are able to form adhesive discs at the tips of the branches, and thus to cling to flat supports.
Stem-thorns. — Shoots may undergo a still greater reduction by their modification into THORNS, as a defence against the depredations of animals. Of shoots modified in this manner, the Black Thorn (I'm a us spinosa), the White Thorn (Crataegus), and the Honey Locust (Gleditschia) afford instructive examples. The thorns are simple or branched, hard, pointed bodies. In Gleditschia (Fig. 30) the thorns are developed primarily from the uppermost of several serial buds ; while secondary thorns may develop on older portions of the stem from the lower buds of the series, and thus give rise to clusters of thorns. In Colletia cruciata all the shoots are flattened and spiny. ><> that they perform the duties of the leaves, which are quickly lost, in addition to serving as protective structures. This plant is an American shrub belonging to the Rhamnaceae, and grows in dry sunny situations.
Flowers. — The most marked changes in the form of the shoot, due to the displacement and union of its different members, take place in phanerogamic flowers (Fig. 39). The shoots from which flowers are developed are termed FLORAL SHOOTS, in contrast to the FOLI.MJK SHOOTS, the functions of which are merely vegetative. The axis of the floral shoot remains short, and becomes flattened or even depressed at the tip. The leaves which spring from the floral axis often become united with one another and with the axis itself. In such cases thorough investigation of the development and the comparative morphology of the flower is necessary to reveal the modifications which have taken place during its evolution. In most instances the rule seems to hold that axillary buds are not formed within a flower except in cases of abnormal development.
The Order of Sequence of Shoots. — If the vegetative cone of the primary axis of a plant, after reaching maturity, is capable of repro- duction, a plant with but one axis will result, and the plant is designated UNI AXIAL or HAPLOCAULESCENT. Usually, however, it is not until a plant has acquired axes of a second or third order, when it is said to be DIPLOCAULESCENT or TRIPLOCAULESCENT, or of the nth order, that the capacity for reproduction is attained. A good illus- tration of a plant with a single axis is afforded by the Poppy, in which the first shoot produced from the embryo terminates in a flower, that is, in that organ of Phanerogams which includes the sexual organs. As an example of a triplocaulescent plant may be cited the common Plantain (Plantago major), whose primary axis produces only foliage and scale leaves ; while the secondary axes give rise solely to bracteal leaves, from the axils of which finally spring the axes of the third order, which terminate in the flowers. In the case of trees, only shoots of the nth order can produce flowers. Thus a division of labour commonly occurs in a branched plant, which find-; its expression
MORPHOLOGY 29
in differences of form between the successive shoots. These differ in appearance according to the special function performed by them, whether nutrition, storage, or reproduction. In addition to the essential members in the succession of shoots developed in a deter- mined order, there are non-essential members which repeat forms of shoot already present. These may appear simultaneously with the essential shoots, and serve to increase the size of the plant as in many annuals ; in many perennial plants they arise as yearly innova- tions on the stock. Adventitious shoots, as a rule, repeat members which have already made their appearance. Exceptionally, they form necessary links in the succession of shoots ; this is the case in the Podostemaceae (an aquatic order of Dicotyledons found in the tropics, the plants belonging to which resemble Lichens or Liverworts in external form), where they bear the flowers.
The Habit or General Aspect of Plants is dependent upon the origin, number, mode of growth, and duration of their branches, and on the presence or absence of non-essential shoots. Cormophytes which develop herbaceous aerial shoots, and persist only so long as is requisite for the development and ripening of their fruit, be it one or several vegetative periods, are called HERBS. Herbaceous plants, however, which, although annually dying down to the ground, renew their existence each year by means of new shoots produced from underground shoots, rhizomes, or roots, are further distinguished as PERENNIALS or perennial herbs. SHRUBS or TREES, on the other hand, have woody, persistent shoots, which bear fruit repeatedly. In these the reproductive shoots are shed annually, and, in some, vegetative branches are also cast off, the remaining ones persisting and increasing in thickness. In the Lime the ends of the leafy twigs, in the Scotch Fir the short shoots, and in the Oak, Elm, Willow, and Poplar weak lateral branches are thus lost. The leaves of evergreen trees remain alive for several years, while those of deciduous species only persist for a single vegetative period.
Shrubs retain their lateral shoots, so that their branches are formed near the ground ; trees, on the contrary, soon lose their lower lateral branches, and have a main stem or trunk, which bears a crown of branches and twigs. In many trees, shrubs, and herbs the main shoot is vertical, while the lateral branches assume a horizontal position, or are directed obliquely upwards or downwards. In other cases the main axis is sympodial, a lateral branch continuing the direction of growth of the primary shoot. Sometimes a main axis is indistinguishable among the group of similarly directed branches. The general appearance of the plant is determined by the direction and thickness of its branches and leafy twigs. If these are all directed upwards the shape is pyramidal, while broadly pyramidal, oval, and rounded forms arise when the branches diverge more strongly. The " weeping varieties " of several familiar trees are due
30 BOTANY PART i
to the branches becoming long and pendulous. Herbaceous plants often have stems which creep on the surface of the ground. Other plants, both herbaceous and woody, climb upon various supports by means of hooked hairs, prickles, and modified shoots, by means of roots or tendrils, or by twining movements. Climbing plants are termed LIANES, those which wind round a support being distinguished as TWINING PLANTS. It is the presence of numerous rope-like stems of lianes which renders the tropical forest so impenetrable. On both creeping and climbing shoots the leaves tend to become displaced towards the dorsal surface of the stem, while branches spring from the sides and roots from the ventral surface.
In catalogues and descriptions of plants the duration of the period of growth is usually expressed by special symbols : thus O indicates an annual ; 0 a biennial, and H a perennial herb ; f? is employed to designate both trees and shrubs, and for trees the sign fj is also in use.
Development of the Leaf. — The first appearance of the leaf as a lateral protuberance (Fig. 17 f) on the vegetative cone of the shoot has already been referred to (p. 18). When the apex of a shoot is removed by a transverse section and viewed from above (Fig. 31), the origin of leaves as lateral protuberances is more evident than in a longitudinal section. The embryonic leaf rudiment generally occupies but fa small portion of the periphery of the vegetative cone ; it may, however, completely surround it. In like manner, when the mature leaves are arranged in whorls, the developing protuberances of the rudimentary leaves may form at first a continuous wall-like ring around PIO. si.— Apical view of thfi the growing point and only give rise later to vegetative cone of a shoot the separate leaf rudiments. Leaves take their
of Eiwnvmus iavonicns. • • i e \ e i i
(x 12) origin only from such parts of a plant as have
remained in an embryonic condition. A leaf
never arises directly from the older parts of a plant. In cases where it apparently does so its development has been preceded by the forma- tion of a growing point of a new shoot. The growing point of a shoot has usually 'an UNLIMITED GROWTH, while the growth of a leaf is LIMITED. A leaf usually continues to grow at its apex for a short time only, and then completes its segmentation and development by intercalary growth, which is usually localised near the base. It is true that some leaves, such as those of Ferns, have apical growth and continue to grow for a long time (in some cases, e.g. Lygodium, the growth is unlimited), and bear their pinnae in acropetal order. On the other hand, the leaf-like cladodes, although they are in reality
SECT. I MORPHOLOGY 31
metamorphosed shoots, exhibit a limited apical growth like that of ordinary leaves.
We have already seen that in certain cases the apical cone may give rise to a terminal leaf. The apex of the leaf in many ferns (e.g. Adiantum Edgeworthii) may on the other hand be transformed into a bud from which a shoot will develop (13).
The unsegmented protuberance of the still rudimentary leaf, termed by EICHLER (u) the primordial leaf (Fig. 32 A ft), first projects from the vegetative cone of the shoot (A v). This is usually followed by a separation of the primordial leaf into the LEAF-BASE (g in A and B) and the rudimentary lamina or UPPER LEAF (o in A and B). The leaf-base, or the part of the rudimentary leaf which immediately adjoins the vegetative cone, either Fl0' 3"-Ai)ex of au Klm shoot- Vl"<™ «»»j>erfri«. A,
. ' . , . , Showing the vegetative cone r, with the rudiment!
takes I1O further part in the of a young leaf, 6, still unsegmented, and of the next
Succeeding differentiation of older leaf, exhibiting segmentation into the laminar
thp Ipnf or ir dpvplnns intn rudiment, o, and leaf-base, g; £, showing the older
uie leai, o leveiops into ]|laf viewed obliquely frolll behilld> (x 58 } a LEAF -SHEATH (vagina) or
into STIPULES. The upper leaf, on the other hand, gives rise to the leaf-blade or LAMINA. If the fully developed leaf possesses a LEAF- STALK (petiole), this is later interposed by intercalary growth between the leaf-blade and the leaf-base.
The leaf-tip often develops more rapidly than the rest of the leaf, and GOEBEI. regards this as an arrangement for the protection of the younger parts of the bud (ls). This is seen most strikingly in some tropical plants, especially in climbers. In this case, according to M. RACIBOKSKI (I8), the "fore-runner tips" serve for assimilation before the remainder of the leaf has attained the mature condition.
In leaves the laminae of which do not remain simple, but undergo segmenta- tion as they develop, the lateral segments are as a rule formed in the basipetal direction, i.e. from the tip towards the base (Fig. 33) ; the opposite direction of de- velopment is, however, sometimes found. The segments of the palmate and pinnate leaves of Palms originate by a process of splitting of the leaf blade which is, to begin with, entire. The direction of the splits is determined by the folds of the lamina (17).
Different forms of leaves are most clearly seen in the Phanerogams, in which the various leaf structures found at different regions of the shoot have been distinguished as SCALE LEAVES, FOLIAGE LEAVES, BRACTEAL LEAVES, and FLORAL LEAVES (Fig. 34). These usually succeed one another in definite order. However unlike the leaves may become, their first origin -is similar. Since the scale leaves and bracts can often be shown to be arrested forms of foliage leaves, the latter may be first considered.
32
BOTANY
Foliage Leaves, generally referred to simply as leaves, are the leaf structures on which devolves the task of providing nourish- ment for the plant. Since the exercise of this function is dependent upon the presence of the green pigment, foliage leaves have a green colour. In certain cases, where their form is extremely
FIG. 33. — Acer platanoides. A, external view of a bud, with two young leaves between which the apical cone of the stem is visible ; sp, the leaf-blade, in which five segments are indicated, the uppermost one being developed first ; st, the zone, by the growth of which the leaf-stalk will arise later, li, an older leaf seen from tin- side ; the young vascular bundles, which will later determine the venation, are indicated. C, fully-grown leaf, with the course of the vascular bundles in- dicated diagrammatically. D, a transverse section of the basal portion of a bud showing three vascular bundles in each leaf. ./•,', a similar section at a higher level ; the number of vascular bundles has increased by branching. (After 1 (KIXKOA. from GOEBEL'S Organography. A, li, and K slightly magnified.)
simple, as in the needles of Conifers, the primordial leaf simply increases in length without any further differentiation into parts. In other undivided leaves, however, whether lanceolate, elliptical, ovate, or otherwise shaped, the flat leaf-blade is distinct from the leaf-base, while a leaf-stalk may also be interpolated between them. If no leaf-stalk is developed the leaf is said to be SESSILE, otherwise it is described as STALKED.
SECT. I
MORPHOLOGY
Sessile leaves usually clasp the stem by a broad base. Where, as in the case of the Poppy (Papaver somniferum), the leaf-base surrounds the stem, the leaves are described as AMPLEXICAUL ; if, as in species of Bupleurum, it com- pletely surrounds the stem, the term PEHFOT.I.VTK is used. If the bases of two opposite leaves are united, as in the Honeysuckle (Louicera Capri- folium), they are said to be CON- NATE. Where the blade of the leaf continues downwards along the stem, as in the winged stems of the common Mullein (ferbaseum fhapsiforme), the leaves are dis- tinguished as DECURRENT. The petiole of a leaf merges either directly into the leaf-base, or it swells at its lower end into a LEAF -CUSHION Or PULVINUS, and
is thus articulated with the leaf- base. This is the case, for in- stance, with many of the Legu- minosae. The leaf-blade, in turn, may be either sharply marked oH' from the petiole, or it may be pro- longed so that the petiole appears winged, or again it may expand at its junction with the petiole into ear-like lobes. A leaf is said to be ENTIRE if the margin of the leaf-blade is wholly free from in- dentations, or if the latter are very shallow. When the incisions are deeper, but do not extend half-way to the middle of the leaf-blade, a leaf is distinguished as LOBKD ; when they reach more than half- way, as CLEFT (Fig. 35 sb) ; if the incisions are still deeper the leaf is said to be PARTITE (Fig. 38 I), and if they penetrate to the midrib or base of the leaf- blade it is termed DIVIDED. The divisions of the leaf- blade are said to be PINNATE or PALMATE, according as the incisions run towards the midrib or towards the base of the leaf-blade. Where the divisions of the leaf-blade are distinct and have a separate insertion on the common leaf-stalk or on the midrib, then termed the SPINDLE or RHACHIS, a leaf is spoken of as COMPOUND (Fig. 35 ub) ; in all other cases it is said to be SIMPLE. The single, separate divisions of a com- pound leaf are called leaflets. These leaflets, in turn, may be entire, or may be divided and undergo the same segmentation as single leaves. In this way double and triple compound leaves may be formed. Simply and doubly PINNATE leaves in which the leaflets are attached to the two sides of the rhachis are of common
D
Kio. 34. — Lily of th« Valley (Convallariu mcjalis). ntl, Scale leaves ; Ib, foliage leaves ; hb, bracts ; 6, flower ; i'.-.-.-, rhizome ; aw, adventitious roots. (Somewhat re- duced.)
BOTANY
occurrence. The leaflets are either sessile or stalkeil ; and sometimes also, as in Knit!, tin and Mimosa, their stalks articulate with the spindle by means of swollen
pulvini. The term I-EDATK is ap- plied to leaves in which the seg- ments are further divided on one side only, and the new segments are similarly divided (Fig. 38 I). Variations in the outline of leaves, whether they are entire, serrate, dentate, crenate, incised, etc., as well as peculiarities in their ,-hapc and segmentation, are of use in the determination of plants.
A system of strands known as the VEINS or
Flo. 35.— Ranunculus aquatilis. ub, Submerged leavs ; sb. floating leaves ; l>, flower ; /, fruit. (Reduced.)
NERVES, traverses the flat leaf-blade. They project more or less from the surface, especi- ally on the lower side 'where they often form definite ribs. Frequently the nerve in the median line of the lamina is more strongly developed and is then termed the midrib ; in other cases several equally de- veloped main nerves are present. Lateral nerves spring from the one or more main nerves.
According to their VENATION or NEUVATUUE leaves are described as PAUAU.KI, VEINED or NETTED VEINED. In parallel venation the veins or nerves run either approximately parallel with each other or in curves, converging at tfie base and apex of the leaf (Fig. 36 s) ; in netted veined leaves (Fig. 190) the veins branch olf from one another, and gradually decrease in size until they form a fine anastomosing network. In leaves with parallel venation the parallel main nerves are usually
l-'iu. ::i''. -Part of >tem and leaf <if ;i .urass. //, Haulm: r, leaf- sheath ; k, swelling of the Irut- sheath above the node ; s, part of leaf-blade ; /, ligule, (Nat. sixe.)
SECT, i MORPHOLOGY 35
unitril liy weaker cross veins. Netted or reticulately veined leaves in which tin- side veins run from the median main nerve or MIDRIB are further distinguished as I'iNNATKLY VEINED, or as PALMATELY VEINED when several equally strong ribs separate at the base of the leaf-blade, and give rise in turn to a network of weaker veins. Parallel venation is characteristic, in general, of the Monocotyledons ; reticulate venation, of Dicotyledons. Monocotyledons have usually simple leaves, while the leaves of Dicotyledons are often compound, and are also more frequently provided with stalks.
The nerves or veins give to a leaf its necessary mechanical rigidity and render possible its flattened form. The branches of the veins parallel to the margin of most leaves prevent their tearing : when there are no such marginal nerves in large thin leaves, the lamina is easily torn into strips by the wind and rain. This fre- quently happens to the leaves of the Banana (Musa), which, consequently, when growing under natural conditions in the open air, presents quite a different appear- ance from what it does when grown under glass. The leaves of the Banana, after becoming thus divided, offer less resistance to the wind. In a similar manner the leaves of Palms, although undivided in their bud state, become torn even during the process of their unfolding. A similar protection from injury is afforded to the Aroid (Monstera) by the holes with which its large leaf-blades become perforated. Equally advantageous results are secured by many plants whose leaves are, from their very inception, divided or dissected. The submerged leaves of aquatic plants, on the other hand, are generally finely divided, not only for mechanical purposes, but also to afford a more complete exposure of the leaf surface to the water. Accordingly, in such water-plants as Ranunculus aquatilis (Fig. 35), which possess both floating and submerged leaves, it is generally the latter only that are dissected and filiform in character. The pointed extremities (DRII> TIPS) of the foliage leaves of many land plants, according to STAHL (18), facilitate the removal of water from the leaf surface. Fleshy so-called succulent leaves, like fleshy stems, serve as reservoirs for storing water.
Heterophylly. — Many plants are characterised by the develop- ment of different forms of foliage leaves. Such a condition is known as heterophylly. Thus the earlier leaves of Eucalyptus globulus are sessile and oval, while those subsequently formed are stalked and sickle -shaped. In other cases the heterophyllous character of the leaves may represent an adaptation to the surround- ing environment, as in the Water Crowfoot (Ranunculus aquatilis), in which the floating leaves are lobed, while those entirely submerged are finely divided (Fig. 35). A number of epiphytic Ferns belonging to the genera Polypodium and Platycerium alternately bear stalked, pinnate leaves serving for assimilation, and broad, sessile, nsually cordate NEST-LEAVES which serve to collect humus ; when, as MANTLE- LEAVES, they are closely appressed to the substratum they form humus by their rapid decay. The humus thus collected or produced is penetrated by the roots of the Fern (19). .
The Leaf -base. — In Monocotyledons the leaf-base very often forms a SHEATH about the stem ; in Dicotyledons this happens much less frequently. In the case of the Gramineae, the sheath is open on the side of the stem opposite the leaf-blade (Fig. 36 r), while in the
36
BOTANY
PART I
Cyperaceae it is completely grown together. The sheath of the Grasses is prolonged at the base of the lamina into a scaly out- growth, the ligule (/). Such a sheath, while protecting the lower part of the internodes, which remain soft and in a state of growth, gives them at the same time rigidity. STIPULES are lateral appendages sometimes found at the base of leaves. When present they may be either small and inconspicuous (Fig. 37 nb), or may attain a considerable size. When their function is merely to protect the young growth in the bud, they are usually of a brown or yellow colour, and are not persistent ; whereas, if destined to become assimilatory organs, and to assist in providing nourishment,
FIG. 37.— ttin\Cherry(l'runusAviuin). FIG. 3S.—Helleboru.- fott'idu*. Foliage leaf
Bud-scales (1-3) and the transition (Oiaiul intermediate forms U-tweeii this
forms (4-6) to the foliage leaf (7),; and the bract (h). (Reduced.)
sp, leaf -blade; s, leaf-stalk; nb, stipules. (Reduced slightly.)
they are green, and may assume the structure and form of the leaf-blade, which sometimes becomes modified and adapted to other purposes (Figs. 48, 49). Normally, the stipules are two in number, that is, one on each side of the petiole. In many species of Galium, where*the stipules resemble leaf-blades, the leaf-whorls appear to be composed of six members, but consist actually of but two leaves with their four stipules, which may be easily distinguished by the absence of any buds in their axils. In other species of the same genus (Galium cruciatum and palujtre) there are only four members in the whorls, as each two adjoining stipules become united. In many cases, as in the Rose and the Clover, the stipules have the form of appendages to the enlarged leaf-base. Sometimes both stipules are united into a single one, which then appears to have an axillary
MORPHOLOGY 37
origin ; or the stipules may completely encircle the stem, and thus form a sheath (OCHREA) about the younger undeveloped leaves. This sheath-like fusion of the stipules may be easily observed on the India-rubber tree (Ficus elastica), now so commonly grown as a decorative plant. In this case the stipular sheath is burst by the unfolding of each new leaf and pushed upwards. In the Polygonaceae the stipular covering is similarly torn apart by the developing leaves, but remains on the stem in the form of a membranous sheath.
Scale Leaves possess a simpler form than foliage leaves, and are attached directly to the stem, without a leaf-stalk. They exercise no assimilatory functions, and are more especially of service as organs of protection. Scale leaves exercise their most important function as BUD-SCALES (Fig. 37); they are then hard and thick, and usually of a brown colour. They most frequently take their origin from the enlarged leaf-base ; in that case the upper leaf either does not develop, or exists only in a reduced condition at the apex of the scale. The true morphological value of scale leaves of this nature is very evident in the bud-scales of the winter buds of the Horse-chestnut (Aesculus Hippocastanum) ; for, while the outer scales show no perceptible indications of an upper leaf, small leaf-blades can be distinctly distinguished at the apices of the inner scales. In other cases the scale leaves are modified stipules (Fig. 37), and are then also derived from the leaf-base ; while, in other instances, they correspond to the enlarged, but still undifferentiated, primordial leaves. The bud-scales of the Oak are the stipules of leaves of which the laminae are only represented by minute scales. Scale leaves, usually colourless and in various stages of reduction, are found on rhizomes (Figs. 23, 34), bulbs (Fig. 25), and tubers (Fig. 26). On the aerial stems arising from such subterranean shoots the forma- tion of similar scale leaves generally.,1 precedes the development of the foliage leaves, with which they are connected by a series of transitional forms.
Braeteal Leaves resemble scale leaves in form, and have a similar origin (Fig. 34 hb). They act as subtending leaves for the floral shoots, and are termed BRACTS. They are connected with foliage leaves by intermediate forms (Fig. 38). Though they are not infrequently green they may be otherwise coloured, or even colourless. •
Floral Leaves. — The modified leaves which form the flowers of Phanerogams are termed floral leaves. In the highest development attained by a phanerogamic flower (Fig. 39), the successive floral leaves are distinguished as sepals (&), petals (c), stamens («), and carpels (<?). In form the sepals and petals approach the bracts. In most cases the sepals are green and of a firm structure ; the petals, on the other hand, are more delicate and variously coloured.
BOTANY
-Jc
The stamens are generally filamentous in shape, and produce the pollen in special receptacles. The carpels more closely resemble
scale leaves, and in Angio- sperms by closing together form receptacles within which the ovules are produced. The stamens and carpels of Phanerogams correspond to the spore-bearing leaves of the Vascular Cryptogams. Such spore - bearing leaves are termed SPOROPHYLLS, and even in ,the Vascular Cryptogams exhibit a greater or less departure from the form of other foliage le;i\c>.
Kiu. :!'.i. — Klower of Pneoniii />r/e«/rtrta. /.', Sepals; c,
petals ; „ stamens ; a, carpels. Some of the sepals, k {s cyi(lent fhafc ^ 1(. ^
petals, ami stamens have been removed to show the
pistil, consisting of two serrate carpels. (Half nat. lacteal leaves are to be considered
size.) as rudimentary foliage leaves, not
only from the mode of their de- velopment but also from the possibility of transforming them into foliage leaves. GOEBKL, by removing the growing tip and foliage leaves of a shoot, succeeded in forcing it to develop other foliage leaves from its scale leaves (20). Rhizomes, grown in the light, develop foliage leaves in place of the usual scale leaves, and even on a potato it is possible to induce the formation of small foliage leaves instead of the customary scale leaves.
Leaf -Sears. — After a leaf has fallen, its previous point of insertion on the stem is marked by the cicatrix or scar left by the fallen leaf. In winter, accordingly, when the trees are denuded of their leaves, the axillary buds are seen to be situated above the leaf-scars.
Vernation and ^Estivation. * — A section through a winter bud shows a wonderful adaptation of the young leaves to the narrow space in which they are confined. They may be so disposed that the separate leaves are spread out flat, but more frequently they are folded, rolled (Fig. 40 /), or crumpled. The manner in which each separate leaf is disposed in the bud is termed VERNATION*. On the other hand, the arrangement of the leaves in the bud with respect to one another is designated .-ESTIVATION. In this respect the leaves are distinguished as FREE when they do not touch, or VALVATE when merely touching, or IMBRICATED, in which case some of the leaves are overlapped by others (Fig. 40 /•). If, as frequently occurs in flower-buds, the margins of the floral leaves successively
* [The use of these terms in the following paragraph differs from that customary in England. By VERNATION is understood the arrangement of the leaves in a vegetal i\<- hud as a whole. The folding of each individual leaf in the Imd is termed ITYXIS. The term .ESTIVATION is applied to the arrangement of the parts in a Hower-lmd. — TRANS.]
MORPHOLOGY
overlap each other in one direction, the aestivation is said to be CONTORTED.
The Arrangement of Leaves. — In all erect elongated shoots, and still more so in dwarf shoots, it is apparent that there is a marked regularity in the arrangement of leaves. This regularity may be most easily recognised in cross-sections of buds (Fig. 41), particularly in sections showing the apex of the vegetative cone (Figs. 31, 33), From such sections it is evident that the developing leaf-rudiments stand in the relative position to the pre-existing leaves which best utilises the available space. According to Schwendener (21) the arrangement of the leaves on the axis is determined by purely mechanical causes ; it is dependent on the pressures and tensions
FIG. 40. — Transverse section of a bud of
a lit ni. k, Bml-scales showing imbricated ifstivation [vernation] ; I, foliage leaves with involute vernation fptyxis] ; a, each leaf has two stipules, (x 15.)
Fio. 41.— Transverse section of a leaf-bud of Tsugii <•<< iiiidensis, just below the apex of the shoot, showing a -f3 diver- gence. (After HoKMKISTER.)
induced by the leaves which always stand in contact at their origin. The original arrangement of the young leaves may be modified as growth continues owing to the mutual pressure they exert. ) If the axis does not grow in length, but only in thickness, as the rudimentary leaves increase in size, their points of insertion will be displaced laterally by longitudinal pressure ; if the axis increases in length, and not in thickness, the insertion of the leaves will be displaced by a transverse pressure. The arrangement of the leaves would also be affected by any increase or decrease in the size of the vegetative cone, unaccompanied by a corresponding increase or cessation of the growth of the rudimentary leaves.
In developing flowers sudden changes in the relative position of the parts often occur in which the apical cone undergoes rapid increase in size while the leaf rudiments become smaller. Changes in the usual position of the leaves may also be occasioned by the
40
BOTANY
torsion of their parent stein. Thus the leaves of Pandanus first appear in three straight rows on the vegetative cone, and their subsequent spiral arrangement results from the torsion of the stem. Irregular arrangement of the leaves, such as occurs, for example, on the flower-stalk of the Crown Imperial (Fritillaria imperialis), may result from the unequal size of the leaves at the time of their inception on the vegetative cone. Further observation has revealed an increasing number of cases in which definite leaf arrangements cannot be explained by the spatial relations, and as the effect of contact and pressure. It can only for the present be assumed that the cause of the leaf-arrangement in these cases lies in the inherited organisation of the plant (—).
A frequent mode of arrangement of foliage leaves is the decussate, in which two-leaved whorls alternate with each other (Figs. 31, 33). A whorled arrangement is characteristic of floral leaves. When the number of leaves in each whorl is the same the whorls usually alternate. On the other hand, the number of members in the different whorls of floral leaves will often be found to change, especially on passing from the petals to the stamens, or from the latter to the carpels. In many cases a whorl, the existence of which would be expected from the position of other whorls and from a comparison with allied Fl°: .,; -Dia8ram °r a plants, may be altogether wanting. In this con-
Liliaceous (lower. The r . ' * • * i T • •
main axis is indicated nection a comparison of the flowers of the Liliaceae by a black dot, oppo- an(j Iridaceae will be instructive. The flowers of
site to which is the .1 T •••. /•&• . 0\ j » n
bract the Liliaceae (.tig. 42) are composed of five
regularly alternating, three-leaved whorls or cycles, viz. a calyx and a corolla (each consisting of three leaves, and, on account of their similar appearance, usually referred to conjointly as the PERIANTH), an outer and an inner whorl of stamens, and finally, in the centre of the flower, an ovary of three carpels. In the flowers of the Iridaceae (Fig. 43) the arrangement is exactly similar, except that one whorl, that of the inner cycle of stamens, is lacking, but the three carpels are situated exactly as if the missing whorl of stamens were present. From this similarity of arrangement, despite the absence of the one cycle of stamens, the conclusion has been drawn that, at one time, the inner row of stamens was actually present, but has now disappeared. In constructing a THEORETICAL DIAGRAM of the Iridaceae the missing cycle of stamens is indicated by some special sign (by crosses in Fig. 43) ; a diagram in which theoretical suppositions are not taken into consideration is called an EMPIRICAL DIAGRAM. Diagrams showing the alternate arrangement of leaves, in cases where only a single leaf arises from each node, may be constructed by projecting the
MORPHOLOGY
41
successive nodes of a stem upon a plane by means of a series of concentric circles, on which the position of the leaves may be indicated (Fig. 44). The angle made by the intersection of the median planes of any two successive leaves is called their DIVERGENCE, and is expressed in fractions of the circumference ; for example, when the angular divergence between two successive leaves is 120°, their divergence is expressed by the fraction |. In the adjoining diagram (Fig. 44) a f divergence is shown. Where the lateral distance between two successive leaves is -?- of the circumference of the stem, the sixth leaf is above the first, the seventh above the second, and so on. The leaves form on the axis five vertical rows, which are spoken of as ORTHOSTICHIES. Where the leaves are very much crowded, as in dwarf-shoots, a set of spiral rows called PARASTICHIES, due to the contact of the nearest laterally adjacent mem- bers, becomes much more noticeable than the orthostichies. If the surface of such an axis be regarded as spread outl horizontally, the parastichies become at once distinguishable (Fig. 45), and it will be evident that the sum of the parastichies cut by every cross-section
FIG. 43. — Theoretical diagram of the flower of the Iris. The ab- sent whorl of stamens is indicated by crosses.
FIG. 44. — I )iagram showing j position of leaves. The leaves numbered according to their genetic sequence.
Fio. 45.— The \ position on the outspread surface of the axis, o, Orthostichies ; p, parastichies. The leaves are numbered according to their genetic sequence.
through such an axis must equal the number of the orthostichies. On objects like pine cones, in which the parastichies are easily recognised, they may be used to determine the leaf arrangement. If a line be drawn on the surface of a stem, so as to pass in the
42 BOTANY PART i
shortest way successively through the points of insertion of every leaf, a spiral called the GENETIC SPIRAL will be constructed. That portion of the genetic spiral between any two leaves directly over each other on the same orthostichy is termed a CYCLE. AVhere the divergence is jj, a cycle will accordingly include five leaves, and will in such a case have made two turns about the stem. The most common divergences are the following, -\, .',, V, i;, ,•"'.., .,sn .'. ;>, etc. In this series it will be observed that in each fraction the numerator and denominator are the sum of those of the two preceding fractions. The value of the different fractions varies accordingly between ?, and 7j, while always approaching a divergence angle of 137° 30' 28". The great majority of leaf arrangements can be expressed by the terms of this main series of divergences.
This main series was discovered by CARL SCHIMPEK and ALEXANDER I.KAI v It exhibits a rational relation of the divergences to the circumference of the axis, so that, as the number of leaves increases, definite leaves are situated accurately above one another. As WIKSXKU (2:1) in particular has made clear, it ditl'ns from all other possible series in attaining the most equal distribution of the leaves on the axis bearing them, while requiring the smallest number of leaves. This results in an advantageous utilisation of the available space, a well- distributed loading of the axis and, when the latter is vertical, in the best utilisa- tion of the illumination. The importance of these advantages as determining the leaf-arrangement is seen in those cases in which a plant bears only a few (2-4) leaves. These stand in a whorl at equal distances from one another and thus their weight is equally distributed, and they obtain equal amounts of light. When leaves are arranged alternately on a vertical axis, their size and shape, together with the length of the iuternodes, ensure each obtaining the requisite amount of light. This arrangement is not a convenient one, and as the leaves borne on a vertical axis increase in number their divergence becomes progressively higher. It is otherwise in inclined or horizontal axes ; here the divergence is relatively low, usually £ or an approximation to this, since this corresponds to the most favourable exposure of the leaves to the light. In most instances this advan- tageous result is attained by_twisting of the internodes ; thus when, as is often the case, the leaves are decussate on an erect axis, they form four vertical rows ; but when it is inclined they are brought by twisting to stand in two rows. Similar secondary changes exposing the leaf-blades to the light affect alternately arranged leaves. The position of the foliage leaves is indeed always clearly adapted to the need of illumination. When the leaves form a rosette, the stalks of those lowest on the stem are frequently elongated, so that their blades are not shaded by the more central leaves. This is especially well seen in the floating rosettes of Trapa natans.
While the arrangement of the foliage leaves conforms on the whole to the main series of divergences, this is not usually suitable in the case of the foliar structures of flowers and inflorescences, which have different purposes to serve. Other relations of position also occur in the vegetative region, as has been shown l>y GOEBEL C2*), in particular in the case of dorsiventral shoots. The tips of dorsiventral shoots are frequently coiled ventrally inwards, bearing their leaves either dorsally or on the sides, but, in the latter case, approaching the dorsal surface. The creeping stems of many Ferns or the flower-bearing shoots of
MORPHOLOGY
43
Forget-me-not (ifyvs»tts) are good examples of such dorsiventral shoot.-. Tlie lint- joining successive leaves in such case is, at the best, but a zigzag. On the ground of such observations as these it may be concluded that the actual leaf- arrangements represent adaptations to definite conditions of life, and that with alterations in the latter other arrangements must arise.
The Metamorphosis of Foliage Leaves. — A striking modification of foliage leaves is seen in peltate leaves, in which the petioles are attached to the lower surfaces somewhat within the margin, as in the leaves of the Indian Cress (Tropaeolum majus, Fig. 191). In the process of their development the young leaf -blades, in this case, grow not only in the same direction as the petioles, as a prolonga- tion of them, but also hori- zontally in front of them. The tubular leaves of many insectivorous plants may have commenced their de- velopment in much the same way. The leaves of 'Nepenthes robusta (Fig. 46), for example, in the course of adaptation to the per- formance of their special function, have acquired the form of a pitcher with a lid which is closed in young leaves, but eventu- ally opens. The pitcher, as GOEBEL has shown, arises as a modification of the leaf-blade. At the same time the leaf-base becomes expanded into a leaf-like body, while the petiole between the two parts sometimes fulfils the office of a tendril. By a similar metamorphosis of its leaflets, bladder-like cavities are developed on the submerged leaves of Utricularia (Fig. 47). The entrance to each bladder is fitted with a small valve which permits the ingress but not the egress of small water-animals. While such leaves display a progressive metamorphosis the modification may be of the nature of a reduction, as is the case in many Ferns, which form leaf-runners. Like the modified
I-'K;. 4ii.— Xi-penthes robuxtc. (.', iial. sixr.)
44
BOTANY
shoots of the same name these are elongated and enable the bud produced at the end to develop at a distance from the parent plant. Camptosorus rhizophyllns, an American Fern, is commonly known as the Walking Fern. Such leaf-runners usually lose their pinnae and are reduced to the leaf stalk. A particularly striking appearance is presented in those cases in which the first leaf of the bud in its turn forms a leaf-runner, so that a sympodium of runners results (Asplenium obtusifolium, A. Mannii).
A metamorphosis of the whole leaf lamina, or a part of it, into tendrils (LEAF-TENDRILS) is of comparatively frequent occurrence,
Kic. 17. — l'tricu!<iriii~>-u.l<jari*. A, 1'art of leaf with several bladders (x 2). 11, Single pinnule of leaf with bladder (x 6). C (after GOEBEL), Longitudinal section of a bladder (x 28) ; »•. valve ; ", wall of bladder.
especially among the Papilionaceae. In the leaf of the Pea (Fig. 48), the leaflets of the upper pairs have become transformed into delicate tendrils which have the power of twining about a support. In the case of the yellow Vetchling, Lathyrus Apliaca (Fig. 49), the whole leaf is reduced to a tendril and the function of the leaf -blade is assumed by the stipules (ri). A comparison between these two forms is phylogenetically instructive, as it indicates the steps of the gradu- ally modifying processes which have resulted in the complete reduction of the leaf lamina of Lathyrus. The comparison of the two preced- ing cases with Ampelopsis (Fig. 29) will make the distinction between these tendrils and stem-tendrils clear, and indicate the value of comparative morphological investigation.
In Lathyrus Aphaca the stipules assume the function of the meta-
MORPHOLOGY
45
morphosed leaf laminae ; in other instances, as in the case of the Australian species of Acacia (Fig. 171, 7, 8, 9), it is the leaf petioles which, becoming flattened and leaf-like in appearance, supply the place of the undeveloped leaf-blades. Such a metamorphosed petiole is called
FIG. 48. — Portion of stem and leaf of the common Pua, J'isum sativuin. *, Stem,; n, istipules ; b, leaflets of the compound leaf; r, leaflets modified as tendrils ; «, floral shoot, (i nat. size.)
Fie. i'.'. — l.at/ii/nt-s .I
*, Stem ; n, stipules ; b, leaf-tendril. (£ nat. size.)
a PHYLLODE, and, except that it is expanded perpendicularly, closely
resembles a cladode. From the latter, however, it is morphologically
different, for the one represents a metamorphosed petiole, the other a
metamorphosed shoot. In accordance with
this distinction phyllodes do riot, like cladodes,
spring from the axils of leaves. Just as
stems become modified into thorns (Fig. 30),
by a similar metamorphosis leaves may be
converted into LEAF THORNS. Whole leaves
on the main axis of the Barberry (Berberis
vulgaris) become thus transformed into thorns,
usually three, but in their character of leaves
still subtend axillary shoots provided with
foliage leaves. By a similar metamorphosis,
the two stipules of the leaves of the common
Locust (Eobinia Psewlacacia) become modified
into thorns, while the leaf lamina persists
as a foliage leaf (Fig. 50).
The Root
The third primary member of the corruo- phytic plant in its typical development as
FIG. 50. — Part of stem and com- pound leaf of Sobinin Pseud- ncacin. n, Stipules modified into thorns : ;/, pulvinus. (A nat. sixc.)
an underground root
presents less marked differences in external form than were shown by
46 BOTANY i-.uir i
the other members. This may be put in relation with the uniform conditions to which roots are exposed in the soil. Certain differences are, however, found between the roots of plants adapted to live in different situations (25). The root has as its most important function t la- absorption of water and nutrient substances dissolved in it from tin- soil, and also serves to attach the plant firmly in the soil. Frequently it serves for storage of assimilated substances. The general appearance of an underground root differs as it is more specially adapted to perform one or other of these functions. Those roots which grow in water or mud become more elongated, are little branched, and are often pro- vided with special arrangements for aeration which lead to localised swellings ; they do not, however, undergo any fundamental change of form. Those roots which grow in the air, AERIAL ROOTS, tend t<> be more strikingly modified.
The absence of leaves and the existence of a root-cap protecting the growing point are characteristic of roots, and furnish an easy means of distinguishing them from underground shoots. A ROOT-CAP or CALYPTRA affords the vegetative cone of a root the protection that is provided to the apex of a stem by the leaves of the bud. Although, generally, the existence of a root-cap is only disclosed by a median, longitudinal section through the root-tip, in some roots it is plainly distinguishable as a cap-like covering. The very noticeable cap- mi the water roots of Duckweed (LemtM) are not, in reality, root-caps, as they are not derived from the root, but from a sheath which envelops the rudimentary root at the time of its origin. They are accordingly termed ROOT-POCKETS. As a general rule, however, roots without root-caps are of rare occurrence, and in the case of the Duckweed the root-pockets perform all the functions of a root-cap. The short-lived root of the Dodder (p. 27) affords another example of a root devoid of a root-cap. Characteristic of roots are also the ROOT-HAIRS (Fig. 170, r), which are found at a short distance from their tips. As the older root-hairs die at the same rate that the new ones are developed, only a small portion of a root is provided with root-hairs at the same time. In some few instances roots develop no root-hairs; this is true of the roots of many Conifers.
Branching of the Root. — Just as the shoot may become bifurcated by the division of its growing point (Fig. 18), so a root may become similarly branched. For the most part, this mode of branching takes place only in the roots of Lycopodiaceae, the shoots of which are also dichotomously branched (p. 19). The branches of roots usually occur in acropetal succession, but the lateral roots (Fig. 170 w) make their appearance at a much greater distance from the growing point of the main root than lateral shoots from the apex of their parent stem. By reason of the internal structure of their parent root, lateral roots always develop in longitudinal rows (Fig. 1 70). They are of endo- genous origin, and before reaching the surface must break through the
.MORPHOLOGY
47
surrounding and overlying tissue of the parent root, by the ruptured portions of which they are often invested at the base, as with a collar.
ADVENTITIOUS HOOTS, just as adventitious shoots, may arise from any part of a plant. They are especially numerous on the under side of rhizomes (Fig. 23 w), and also, when the external conditions are at all favourable, they seem to develop very readily from the stem nodes. A young shoot, or a cutting planted in moist soil, quickly forms adventitious roots, and roots may also arise in a similar manner from the bases of leaves, especially from Begonia leaves when planted in soil. The origin of adventitious roots, as of almost all roots, is en- dogenous. Dormant root rudi- ments occur in the same manner as dormant buds of shoots. Willow-twigs afford a special case of the presence of such dor- mant rudiments of adventitious roots, the further development of which is easily induced by darkness and moisture.
When adventitious shoots are borne upon roots they mostly arise at some distance from the apex. In the Adder's Tongue Feru (Ophioglossum), the vegetative reproduction of which is effected by means of buds formed on the root, these arise close to the root-apex. Lastly, just as terminal leaves may arise from the apex of a stem and terminal shoots from the tips of the leaves of Ferns, in a few instances (species of Asplenium and Platycerium) a shoot may arise as the direct continuation of the growing point of a root (2r>).
The Form of Subterranean Roots. — The customary nomenclature for the various root forms is based on their shape, size, and mode of branching. A root which is a prolongation downwards of the main stem is called the main root or TAP-ROOT ; the other roots are termed, with reference to the tap-root, LATERAL ROOTS of different orders, according to the order of their development. A tap-root is present in Gymnosperms and many Dicotyledons, while it is, as a rule, wanting in Monocotyledons. The root system of most Monocotyledons and many Dicotyledons mainly consists of adventitious roots arising from the base of the stem or from underground stems or rhizomes. Roots which serve for storage of reserve materials may enlarge and become turnip-shaped or tuberous (Fig. 51). Such tuberous growths often greatly resemble stem tubers, but may be distinguished from them
Fin. 51. — Root-tubers of iJahlia rariabilis. lower portions of tin- cut stems. (! nat.
The
48
BOTANY
I
by their root-caps, by the absence of any indications of leaf develop- ment, and by their internal structure.
The morphology of the tubers of the Orchidaceae is peculiar. They are, to a great extent, made up of flesh}-, swollen roots, fused together and terminating above in a shoot-bud. At their lower extremity the tubers are either simple or palmately segmented. In the adjoining ligure (Fig. 52) both an old (f) and a young tuber (t") are represented still united together. The older tuber has pro- duced its flowering shoot (b), and has begun to shrivel ; a bud, formed at the base of the shoot, in the axil of a scale leaf (s), has already developed the adventitious roots, which, swollen and fused together, have given rise to the younger tuber. Roots of ordinary form arise from the base of the stem above the tuber.
Metamorphosis of Roots. — The aerial roots of tropical EPI- PHYTES C27) differ considerably in their structure from underground roots. The aerial roots of the Orchidaceae and of many Aroideae are provided with a spongy sheath, the VELAMEN, by means of which they are enabled to absorb moisture from the atmosphere. Aerial roots, in some cases, grow straight downwards, and upon reaching the ground, branch and function as nutritive roots for the absorption of nourishment ; in other instances, they turn from the light, and, remaining comparatively short and un- branched, fasten themselves as CLIMIJINC; ROOTS to any support with which they come in contact. The climbing roots of many Orchids, Aroids, and Ferns branch and form Fio.52.-o,rM*taft/w,-,,. r The lodgment places for humus: and into this
old root-tuber; t", the young , ° , * , , , '. ,.
root-tuber; b, floral shoot; s, absorbent branches of the climbing roots penetrate. Pendent aerial roots generally contain chlorophyll in their cortical tissue. InOrchids belonging to the genera, Angraecuin and Taeniophyllum, the task of nourishing the
plant is left entirely to the aerial roots, which are then devoid of a velamen, and very much flattened. They are distinctly green- coloured, and supply the place of the leaves which lose their green colour and are reduced to scales. The flat, dorsiventral, chlorophyll- containing roots of the tropical Podostemaceae (28), which have been referred to with regard to the sequence of shoots (p. 29), fulfil a similar function. The aerial roots of the epiphytic Bromeliaceae are developed exclusively as climbing roots, while the leaves function not only as assimilating organs, but also assume the whole task of water-absorption. All the aerial roots of epiphytes are, so far as their origin is concerned, adventitious.
The numerous adventitious roots which form a thickly matted covering on the trunks of Tree-ferns become hard after death, and serve as organs of protection.
scale leaf with axillary bud, A-, from which the new tuber has arisen ; r, ordinary adven- titious roots, (i nat. si/.'.)
MORPHOLOGY 49
In some Palms (Acantkarrhiza, Iriartca) the adventitious roots on the lower part of the stem become modified into thorns, ROOT-THORNS. The roots of certain tropical plants, such as Paiidanus and the swamp-inhabiting Mangrove trees, are specially modified. These plants develop on their stems adventitious roots, which grow obliquely downwards into the ground, so that the stems finally appear as if standing on stilts. The Banyan trees of India (Ficus indica) produce wonderful root-supports from the under side of their branches, upon which they rest as upon columns. The lateral roots of certain Mangrove trees become modified as peculiar breathing organs, and for this purpose grow upwards into the air out of the swampy soil or water in which the trees grow, and are provided with special aerating passages. Such KESPIKATORY or AERATING ROOTS surround the Mangrove trees looking like vigorous Asparagus stalks, and enable the roots growing below in the mud to carry on the necessary exchange of gases with the atmosphere. (Cf. Fig. 213.)
Reduction of Roots. — There is "a general relation between the degree of development of the leaf-surface and of the root-system. In saprophytic and parasitic plants, the shoots of which are as a rule extremely reduced, a corresponding reduction of the root-system can be recognised. Special absorbent organs or HAUSTORIA are often developed on the roots of parasites, e.g. on the roots of the green partial parasites Euphrasia, Odontites, Thesium, etc., which become attached to the roots of their host plants by means of disc-shaped or wart-shaped haustoria. In extreme cases no subterranean roots may be developed, and the parasite only bears haustoria which penetrate the host (Cuscuta, Fig. 202 H). The haustoria of the Kafflesiaceae traverse the body of the host -plant as elongated filaments of cells, and are capable of regenerating the parasite. The immense flowers of Rafflesia Arnoldi, which spring directly from the roots of Cissus, originate from similar haustoria. The reduction of the roots may extend to such a degree that, in a number of plants, no roots are formed. It has been already mentioned that in the case of Coralliorrhiza innata (Fig. 24) the rhizome assumes all the functions of the roots, which are entirely absent. Also in many aquatic plants (Scdvinia, Wolffia arrhiza, Utricularia, Ceratophyllum) roots are altogether absent since these plants no longer require them. The beard-like epiphytic Tillandsia usneoides, belonging to the Bromeliaceae, has no roots and obtains the water it requires by means of special scale-like hairs.
Members of Independent Origin
The existence of parts of the plant with the characters of distinct primary members, but to which an independent phylog«netic origin must be ascribed, has been pointed out by GOEBEL (29). The tubers which occur among the Dioscoreaceae, an order of twining plants belonging to the Monocotyledons, are of this nature (Fig. 53). The extreme forms of these tubers are flat, cake-shaped bodies, which in development and structure exhibit a mixture of the characteristics
E
50
BOTANY
of stem and root. The rhizophores of Selaginella (Fig. 54) which have been variously interpreted as leafless shoots, as roots, and as organs sui generis, also find their place here. They resemble leafless shoots, attain a considerable length, and may branch dichotomously and give rise to endogenous roots close to the growing points. As a matter of fact they are in structure and general behaviour inter- mediate between shoot and root, and it is not probable that they have originated from either of these primary members.
FIG. 53. — IKoscorea sinuata. Tuber separated from the shoot ; A, from above : /;. from below. The lower side bears roots. (After K. GOEBEL. A nat. sixc.)
Various outgrowths of the body of the plant to which an inde- pendent origin is to be ascribed are grouped together as EMERGENCES. A phylogenetic significance cannot be attached to the term as here used. It includes structures of the most different origin belonging both to the thallus and the cormus. The rhizoids, which serve to fasten the thallus of many Thallophytes and the cormus of the cormophytic Bryophyta to the substratum, as well as the massive attaching organs of many Brown Sea-weeds (Fucaceae and Lami- nariaceae) are classed here. So also are the structures which contain the asexual and sexual reproductive cells of the cryptogams (sporangia
MORPHOLOGY
51
and sexual organs). In the sense of the term implied here the hairs, prickles, and glands borne on the surface of the highly organised plants must be included as well as the haustoria described above. As an extreme case the attaching organs (hapterae) of the previously mentioned Podostemaceae may be referred to. These hapterse serve
Fio. 54.— Part of the shoot of Sflaginella, Martensii with rhixo]>li< »•>•*. (From GOEBEL, Organography. Nat. size.)
to attach the nutritive roots of these plants firmly to the rocks exposed to rapidly flowing water, upon which they grow They are at first conical outgrowths, but flatten out and become lobed when applied to the surface of the rock. The shoots originate from the flat nutritive roots. There is, indeed, no reason why outgrowths of the vegetative body of the plant should not become adapted to the performance of particular functions (30).
^»-«v
LIBRARY
52 BOTANY
II. INTERNAL MORPHOLOGY
(Histology and Anatomy
A. The Cell 1. STRUCTURE OF THK CKI.L
All plants and animals are composed of elementary organs called cells. In contrast to animal cells, typical vegetable cells are surrounded by firm walls, and are thus sharply marked off from one another. In fact, it was due to the investigation of the cell walls that the cell was first recognised in plants. An English micrographer, ROBERT HOOKE, was thr first to notice vegetable cells. He gave them this name in his Micrographia in the year 1667, because of their resemblance to the cells of a honeycomb, and published an illustration of a piece of bottle-cork having the appearance shown
Km. 66.— Copy of a part of . , , . . . „ /T,. __x -,-,
HOOKES illustration of m the adjoining figure (Fig. 55). ROBERT
bottle-cork, which he dp- HOOKE, however, Was Only desirOUS of exhibit- scribed as "Schematism or • b meang f c]ifferent objects the capabilities
texture of cork. o . L • i i T
of his microscope ; consequently, the Italian, MARCELLO MALPIGHI, and the Englishman, NEHEMIAH GREW, whose works appeared almost simultaneously a few years after HOOKE'S Micrographia, have been regarded as the founders of vegetable histology. The living contents of the cell, the real body or substance, was not recognised in its full significance until the middle of last century. Only then was attention turned more earnestly to this study, which has since been so especi- ally advanced by MEYEN, SCHLEIDEN, HUGO v. MOHL, NAGELI, DE BARY, FER- DINAND COHN, PRINGSHEIM, and MAX
SCHULTZE. KK.. -,i;.-Knibryoiiir c.-ll from th.-
If an examination be made of a thin vegetative <•„!„• of ;, longitudinal section of the apex of a stem
of a phanerogamic plant, with a higher cytoplasm-. -•;,, t-hri)iuatoi>jior-s ;
magnifying power than that used in the '"' cel1 wal1- (s<"l''>w''!" -I'^rain-
°. J .& ,. ,. ,,-,. 1fr. , matic, X circa 1000.)
previous investigation (rig. 17) of the
vegetative cone, it will be seen that it consists of nearly rectangular cells (Fig. 56), which are full of protoplasm and separated from one another by delicate walls. In each of the cells there will be clearly distinguishable a round body^&), which fills up the greater part of
MORPHOLOGY
53
the cell cavity. This body is the cell NUCLEUS. If sections, made in different directions through the vegetative cone, be compared with one another, it will be seen that its component cells are nearly cubical or tabular, while the nuclei are more or less spherical or disc-shaped. The finely granular substance (cy) filling in the space between the nucleus (k) and the cell wall (m) is the CELL PLASM or CYTOPLASM. In the cytoplasm there are to be found, about the nucleus, a number of colourless and highly refractive bodies : these are the pigment-bearers or CHROMATOPHORES (ch). NUCLEUS, CYTO- PLASM, AND CHROMATOPHORES, CONSTITUTE THE ELEMENTS OF THE LIVING BODY OF A TYPICAL VEGETABLE CELL. To designate all
these collectively, it is customary to use the term PROTOPLASM, which is then to be understood as including all the living con- stituents of the cell or PROTOPLAST.
In many animal cells modern investigations have revealed, in addition to the constituents of the protoplasm just mentioned, a small structure situated close to the nucleus, which has been termed the CEN-
TROSOME, CENTRIOLE, or ATTRACTION- SPHERE. Similar structures have been demonstrated in the lower cryptogamic plants (Fig. 57 c), but they appear to be wanting in the cells of the higher Cryptogams and the Phanerogams (31). The nucleus and cytoplasm are the two most essential constituents of the cell, and its vital functions de- pend on the interaction between them. In the lowest plants (Cyanophyceaeand KIU. 57.— A nucleus <>f u c.-u (lf tu.- ymn-
Bacteria) such a division of labour in 1>la",t "' '.'""x """"-;1 B"JU", s":"
' . , weed. '•//, llio nttToandiBg cytoplasm ;
the protoplasm IS not certainly proved, ki the nucleus ; kw, nuclear membrane ;
the existence of a nucleus being still ". nucieoius; .-, ceutrosome; a,, dn-o-
e j- /Q.}\ /~ir matoi>hores. (x 1000.)
a matter of dispute (•"). Cnroma-
tophores" are wanting in the Bacteria and Fungi, as in all animal
cells.
While animal cells usually remain continuously filled with protoplasm, vegetable cells soon form large SAP CAVITIES. It is only the embryonic cells of plants that are entirely filled with protoplasm, as the cells, for example, of an embryo or of a growing point ; they afterwards become larger and contain proportionally less protoplasm. This can be seen in any longitudinal section through a stem apex. At a short distance from the growing point the enlarged cells have already begun to show cavities or VACUOLES (v in A, Fig. 58) in their cytoplasm. These are filled withxa watery fluid, the CELL SAP. The cells continue to increase in si£e, and usually soon attain a condition in which their whole central portion is filled by a single,
BOTANY
large sap cavity (v in £, Fig. 58). The cytoplasm then forms only a thin layer lining the cell wall, while the nucleus takes a parietal position in the peripheral cytoplasmic layer. At other times, however, the sap cavity of a fully-developed cell may be traversed by bands and threads of cytoplasm ; and in that case the nucleus is suspended in the centre of the cell. But whatever position the nucleus may occupy, it is always embedded in cyto- plasm ; and there is always a continuous peri- pheral layer of cytoplasm lining the cell wall. This cytoplasmic peripheral layer is in con- tact with the cell wall at all points, and, so long as the cell remains living, it continues in that condition. In old cells, however, it frequently becomes so thin as to escape direct observation, and is not perceptible until some dehydrating reagent, which causes it to recede from the wall, has been employed. Such a thin cytoplasmic peripheral layer has been described by HUGO v. MOHL under the name of PRIMORDIAL UTRICLE.
Dead cells lose their living protoplasmic contents, and, strictly speaking, should no longer be termed cells, although the name was first applied to them when in that, con- dition. In reality they represent only cell cavities. With their death, however, cells do not lose their importance to a plant. Without such cell cavities a highly-organised plant could not exist, as they perform for it the office of water-carriers, and afford mechanical support and rigidity. The heart wood of a tree consists exclusively of the walls of dead cells.
The Protoplasm. — In order to facilitate . an insight into the real character of proto-
Fio. 08.— Two cells taken at J ,
different distances from the plasm, attention will first be directed to the
growing point of a phanero- SLIME FUNGI (Myxomycetes),a grOUpof Orgail- gamic shoot. k. Nucleus ;. i_-i_ A.-UJUJ it.
!*, cytoplasm; v, vacuoles, 1SmS whlch stand On the b°.rder between the
represented in B by the sap animal and vegetable kingdoms. These cavity. (Somewhat diagram. Myxomvcetes are characterised at one stage
niatic, x circa 500.) , J . . J , , , .1 t t
of their development by the formation of a PLASMODIUM, a large naked mass of protoplasm.
The plasmodium is formed from the protoplasm of the spores. These spores are unicellular bodies (Fig. 59 a, fc), filled with cytoplasm,
MORPHOLOGY
55
in which lies a central nucleus, and are surrounded by resistent cell walls. The spores germinate in water, their contents, breaking through the spore walls, come out (c, d) and round themselves off. A change of form soon takes place ; the protoplasmic mass elongates and assumes somewhat the shape of a pear, with the forward end prolonged into a
FIG. W.—Clwtuli-ioderma di/ornte.. a, Dry, shrivelled spore; l>. swollen spore; c and </, s[>oics showing escaping contents ; «, /, g, swarm-spores ; h, swarm-spore changing to a myxoamoeba ; i, younger, k, older myxoamoeba; I, myxoamoebae about to fuse; m, small plasmodiuiu : H, iwrtion of fully-developed plasmodium. ('<-»», x 540: n, x 90.)
fine whip-like process or flagellum (e, f, g). Thus the contents of the spore have become transformed into a SWARM-SPORE, which now swims away by means of whip-like movements of its flagellum. In addition to the nucleus, which is visible in the anterior end of every swarm-spore, a vesicle may be seen at the other end, which, after gradually increas- ing in size, suddenly vanishes, only to swell again into view. This
56 BOTANY I-AKT i
vesicle is a CONTRACTILE VACUOLE. The presence of such a contractile vacuole in an organism was formerly considered a certain indication of its animal nature. Xow, however, contractile vacuoles have been observed in the swarm-spores of many green Algae, of whose vegetable nature there can be no doubt.
The swarm-spores of the Myxomycetes soon lose this characteristic swarm-movement, draw in their flagella, and pass into the amoeba stage of their development, in which, like animal amoebae, they assume irregular, constantly changing shapes, and are capable of performing only amoeboid creeping movements. In the case of ChondriodermQ tlifforme, a Myxomycete of frequent occurrence in rotting parts of plants (Fig. 59), a number of the amoebae eventually collect together (/) and coalesce. In this way, as is also the case with most other Myxo- mycetes, the amoebae ultimately give rise to a plasmodium (n).
Although each one of the amoebae is so small that it can only be seen with the aid of a microscope, the plasmodium into which they become united may attain a considerable size.
The cytoplasm, both of the single amoeba and of the plasmodium, consists of a clear ground substance, through which granules are dis- tributed. This substance is of the consistence of a tenacious fluid ; its superficial region is denser and free from granules, while these are numerous in the less dense central portion. The granules enable the internal streaming movements of the cytoplasm to be recognised. The currents are constantly changing their direction, moving either towards or away from the margin. The formation and withdrawal of processes of the margin stand in relation to the direction of the currents. When naked masses of protoplasm such as these plasmodia encounter foreign bodies, they can enclose them in vacuoles, and, when of use as food, digest them.
Deprived of its component water the protoplasm becomes hard and tenacious, and, without losing its vitality, ceases to perform any of its vital functions until again awakened into activity by a fresh supply of water. In case of a scarcity of water the plasmodia of the Myxo- mycetes may form SCLEROTIA, that is, masses of resting protoplasm of an almost wax-like consistency. Months and indeed sometimes years afterwards, it is possible for such sclerotia, if water be supplied, to again produce motile plasmodia. Similarly, in seeds kept for a long time, the protoplasm consolidates into a hard mass, which may be easily cut with a knife, while the nuclei will be found to have shrunk and lost their original shape. Nevertheless the protoplasts, after absorbing water, may return again to a condition of activity.
Protoplasm is not a simple substance chemically ; it consists rather of numerous different components, which are subject to continual change. Since albuminous substances are always present, protoplasm always gives a proteid reaction ; when incinerated, fumes of ammonia are given off.
MORPHOLOGY 57
Active protoplasm generally gives an alkaline, and, under certain conditions, a neutral reaction, but never an acid one. The protoplasm of the higher plants coagulates at a temperature not much over 503 C., in the Sehizophyta, however, usually not below 75° C. In the inactive dried condition, as in spores and seeds, it can endure a still higher temperature without coagulating. The spores of many Bacteria can withstand a temperature as high as 105U C. Treated with alcohol or ether, with acids of definite concentrations, with bichromates of the alkali metals, or with corrosive sublimate, protoplasm quickly coagulates, while at the same time insoluble proteid compounds are formed. Coagulating reagents, accordingly, play an important part in microscopic technique ; of especial value are those,' which, while fixing and hardening the protoplasm, change its structure in the least degree. As fixing and hardening reagents for vegetable tissues, alcohol, 1 per cent chromic acid, 1 per cent acetic acid, 0'5 to 1 per cent osmic acid, concentrated picric acid, or corresponding mixtures of these acids, solutions of mercuric chloride and formaldehyde, are used. Iodine stains protoplasm brownish yellow ; nitric acid, followed by caustic potash, yellowish brown (xanthoprotein reaction) ; sulphuric acid, if sugar be present, rose red. Acid nitrate of mercury (MiLLON's reagent) gives to protoplasm a brick-red colour. These reactions occur with all proteid substances though they are not absolutely distinctive of them. Protoplasm is soluble in dilute caustic potash and also in eau de Javelle (potassium- hypochlorite), and accordingly both of these reagents may be recommended for clearing specimens, when the cell contents are not to be investigated. All of the above-mentioned reagents kill protoplasm ; until they have done so, their char- acteristic reactions are not manifested. . A large number of albuminous bodies or albuminates have been named which are said to enter into the composition of living protoplasm. Most of these compounds are still ill-defined ; in nuclei the nucleins are most important, but they are also found in the cytoplasm. They are characterised by containing much phosphorus, and are not attacked by pepsin, and only with difficulty by trypsin. Staining reagents have also become an important help to microscopic investigations for determining the composition of protoplasm. This is due to the fact that the different constituents of protoplasm take up and retain the stain with different degrees of intensity and energy. As a general rule, only coagulated protoplasm can absorb colouring matter, although some few aniline stains can, to a limited extent, permeate living protoplasts. For staining vegetable protoplasts, which have been previously fixed, the various carmines, hcematoxylin, safranin, iodine green, acid fuchsin, eosin, methylene blue, and aniline bhie, gentian-violet and orange, have been found particularly convenient. The different components of the protoplasm absorb the stains with different intensities, and, when reagents are employed to remove the colouring matters, they exhibit differences in their power to retain them. The nucleus generally becomes more intensely coloured than the rest of the protoplasm, especially a part of its substance, which is therefore called OHIIOMATIN. In addition to those substances, which are to be regarded as integral parts of active protoplasm, it always includes derivative products of albuininatcs, particularly amides, such as asparagin, glutamin ; also ferments, such as diastase, pepsin, invertin ; at times alkaloids, and always carbohydrates and fats. The ash left after incineration also shows that protoplasm always contains mineral matter, even if only in small quantities. All such substances which do not enter directly into the composition of protoplasm, but are only included within it, are designated by the term METAPLASM.
The Cytoplasm. — The cytoplasm of vegetable cells, which possess
58 BOTANY PART i
a cell wall, is a more or less tenacious fluid. It partakes of the physical properties of fluids, and on being artificially freed from the cell wall, tends to assume the spherical form. Its cohesion appears to be greater in meristematic cells than in those which are older, while in certain cases a still firmer consistence may be attained as in the cilia borne by swarm-spores.
Both in the case of the Myxomycete and of the vegetable cell enclosed by a wall, the basis of the cytoplasm consists of a hyaline substance termed the HYALOPLASM. When granules are distributed through the cytoplasm it is spoken of as GRANULAR PLASMA or polio- plasm. An extremely thin boundary layer is found at the periphery which is quite free from granules, and a similar layer bounds every vacuole present in the cytoplasm. The wall of the vacuole is characterised by a greater tenacity of life than the rest of the cytoplasm, remaining alive for some time after the latter has been killed by the action of a 10 per cent solution of potassium nitrate. Since the vacuole- wall regulates the pressure exerted by the cell sap contained in the vacuole, HUGO DE VRIES has applied the name TONOPLAST (M) to this layer.
The small granules distributed through the granular plasma consist of various substances, and may be classed together as MICRO- SOMES. Some of them are small cavities filled with dissolved substances, and to these the name PHYSODES has been given.
Even though bounded by a cell wall the cytoplasm frequently exhibits movements comparable to those of the naked amoebae and plasmodia of Myxonwcetes. These movements mostly are found in somewhat old cells, but N. GAIDUKOV (^ has shown by means of the recently invented ultramicroscope that they are of widespread occurrence in vegetable protoplasts. The study of the movements in the Myxomycetes showed that various kinds of movements could be distinguished ; the waving movement of the flagellum of the swarm-spore, the change in external form of amoebae and plasmodia, to which their power of creeping about is due, and finally a streaming movement in the cytoplasm. The cytoplasm, enclosed by a cell- wall, may either exhibit isolated streaming movements, the direction of which may undergo reversals, or a single stream, the direction of which is constant. These two forms of movement are distinguished as CIRCULATION and ROTATION respectively. In rotation, which is found in cells with the cytoplasm reduced to a layer lining the wall, the single continuous current follows the cell wall. In circula- tion, on the other hand, the layer of cytoplasm lining the wall takes no part in the movement, which is found in the strands traversing the vacuole. Circulation is common in cells of land-plants, while rotation is more usual in water-plants. The stimulus caused by wounding the tissues in making the preparation frequently increases the activity of the movement (35).
MORPHOLOGY
59
A particularly favourable object for the study of protoplasm in circulation is afforded by the staminal hairs of Tradescantia virginica. In each cell (Fig. 60) small, fine currents of protoplasm flow in different directions in the peripheral cytoplasmic layer, as well as in the cytoplasmic threads, which penetrate the sap cavity. These cytoplasmic threads gradually change their form and structure, and tli us alter the position of the cell nucleus.
When the protoplasm is in rotation, the cell nucleus and chromatophores are usually carried along by the current, but the chromatophores may remain in the boundary layer, and thus not undergo movement. This is the case with the Stoneworts (Characeae), whose long internodal cells, especially in the genus NiteMa, afford good examples of well-marked rotation.
Properly fixed cytoplasm has a finely reticulate or honeycomb- like structure, small granules being embedded in the network. At particular developmental stages this structure appears to be traversed by special filaments, which can be demonstrated by suitable staining (Fig. 57) (36).
The portion of the cytoplasm which forms the network or honeycomb appears to be specially concerned with the nutritive processes, while the fibrillar plasma influences the process of development ; they are respectively termed trophoplasm and kinoplasm ; the latter has also been called archeplasm. When traced to their origins the limiting layer of the cytoplast is found to belong to the kinoplasm, the walls of vacuoles to the trophoplasm. Albuminous sub- stances, precipitated by the fixing agent, have not infre- quently been mistaken for structural features of the cyto- plasm (s7).
The Cell Nucleus (ys). — The resting nucleus has a reticulate or honeycomb structure forming an anastomosing network (Fig. 56), which, how- ever, in living objects can only be distinguished by the punctated appearance it gives to the nucleus. Streaming movements do not take place within the nucleus. An insight into the nuclear structure is only to be attained with the help of properly fixed and stained preparations. It is then possible to determine that the greater part of this nuclear network is composed of delicate and, for the most part, unstained threads, in which lie deeply stained granules. The substance of the threads 'the'nucieus has been distinguished as LININ (I), that of the by prato- granules as CHROMATIN (c/t). One or more larger bodies, the NUCLEOLI (n), occur at the intersections of some of the linin threads ; these, although deeply
stained, do not take the same tint as the chromatin granules.
The network of the nucleus lies within the nuclear cavity, which is
t:. CO.— Cell from st.-uuinal hair of Tr intia virginicu,
,,ias.,.ic strands, (x
r. i
BOTANY
filled with nuclear sap and surrounded by a membrane (w). The nuclear membrane, strictly speaking, is a part of the surrounding cytoplasm, and is the protoplasmic layer with which the cytoplasm separates itself from the nuclear cavity. The nucleus in young cells with abundant protoplasm is, as a rule, spherical. AVhen situated in the lining layer of cytoplasm of older cells, it is frequently of a
flattened form, while
in elongated cells it
exhibits a correspond-
ing elongation. Ex-
ceptionally in old
cells the nucleus is
forke,., lobed, or of
some other irregular
shape. These changes
in form of the nucleus
are due to slow move-
ments, which cannot,
as a rule, be directly
observed. In em-
bryonic tissues the
nuclei are relatively
large in proportion
to the size of the pro-
toplasts. Glandular
cells are also usually
provided with large
nuclei.
While the cells of the Cormophytes are
almost always uninucleate, in the Thallo-
phytes, on the contrary, multinucleate cells
are by no means infrequent. In the Fungi,
and in the Siphoneae among the Algae, they
are the rule. The whole plant is then
composed either of but one single multi-
extensively
matophores ; p. pvrenoids;
stan-h grains. <XS40.)
Hie. 0-2. — Portions i.r two a«l- .iacfnt cells in a hypha trnrn
t in' stalk lit' ;w/;.
, .Nuclei : »i.. pits. (X o40.)
,,,„, ,
chromic acid and stained with branched (Fig. 295), or it may consist of a .•aniline. n, Nuclei ; ch, ciiro- large number of multinuclear cells, forming
,-> . m, .. , ,
together one organism. Thus, on suitable treatment, several nuclei may be detected in
the peripheral cytoplasm of each of the cells of the common fila-
mentous fresh -water Alga Cladophora ylomerata (Fig. 6, p. 12,
Fig. 61).
The nuclei of the long, multinucleate cells (Fig. 62 n) of fungal
filaments, or HYPHyE, and also of many Siphoneae, are characterised
by their diminutive size.
MORPHOLOGY
til
The Centrioles and Centrosomes. — In a number of cases among the lower Cryptogams (Thallophyta and Bryophyta) structures which correspond to the centrioles and centrosomes of animal cells have been demonstrated. Where centrioles are found, as in some marine Algae (Fucus, Fig. 57), they are dumb-bell shaped. Larger masses of active cytoplasm occur in a number of Fungi, and, though they do not, like them, include centrioles, are comparable with the centro- somes of animal cells. Similar structures occur in the mother-cells of the spermatozoids in Bryophyta, Pteridophyta, and certain Gym- nosperms (39). They are there termed blepharoplasts, and provide the material for the formation of the cilia of the spermatozoids. With the exception of the blepharoplasts no structures comparable .with centrioles or centrosomes have been shown to exist in the cells of the higher plants.
The Chromatophores (4(l). — In the embryonic cells of the embryo and of growing points, where the chromatophores (Fig. 56 ch) are principally located around the nucleus, they first appear as small, colourless, highly refractive bodies. They may retain the same* appearance in older cells (Fig. Ill A, I), but in them they also attain a further development, as CHLOROPLASTS, LEUCOPLASTS, or CHROMOPLASTS. Since these bodies have the same origin they are all included in the one term, CHROMA TOPHORKS.
Chloroplasts. — In parts of plants which are exposed to the light the chromatophores usually develop into chlorophyll bodies or chloroplasts. These are generally green granules of a somewhat flat- tened ellipsoidal shape (Fig. 63), and are scattered, in numbers, in the parietal cytoplasm of the cells. All the chloroplasts in the Cormophytes, and, for the most part also, in the green Thallophytes, have this form. In the lower Algae, however, the chlorophyll bodies may assume a band-like (Fig. 264 e), stellate or tabular shape.
In these cases the chloroplast often includes one or more pyrenoids ; these are spherical protoplasmic bodies con- taining an albuminous crystalloid, and are surrounded by small grains of starch (Fig. 264). The ground sub- stance of the chlorophyll bodies is itself colourless, but contains numerous coloured granules, which are termed OKAXA. These consist of an oleaginous substance, which holds in solution green and yellow pigments. These colouring substances may be extracted by means of alcohol, leaving only the colourless plasmic substance of the chlorophyll body remaining.
The easiest way in which a solution of chlorophyll can be prepared, is to extract the chlorophyll by means of alcohol from green -leaves that have been previously
:. ".'!. — Two cells from ;i leaf of Funaria liygi'n- metrica. cl, Chlompla.st.s; a. nucleus, (x 300.)
62 BOTANY PART i
boiled in water. The green chlorophyll pigment is also soluble in ether, fatty and ethereal oils, paraffin, petroleum, and carbon disulphide. The alcoholic solutions are fluorescent and appear green in transmitted light, blood red in reflected light. When an alcoholic solution of chlorophyll is shaken up with benzole, the latter, as KKAUS first showed, on standing rises to the surface as a green solution, leaving the alcohol yellow.
According to MAHCHLEWSKI and ScHUNCK(41), two green pigments are to be detected in the alcoholic extract from leaves. The one, true chlorophyll, is always present ; the other, which has been termed allochlorophyll, cannot be detected in some plants. The yellow pigments of the chloroplasts are collectively termed xanthophyll. Only chrysophyll, which forms shining red crystals, has been isolated ; the rest of the xanthophyll form amorphous masses. True chlorophyll is characterised by three absorption b;inds in the less refrangible half of the spectrum, and three in the more refrangible portion. As yet only one band, which lies in the red portion of the spectrum, is known for aliochlorophyll. The amount of chlorophyll in a green plant is very small. T.SCHIRCH (4-) has calculated that only O'2-l'O gr. of chlorophyll can be obtained from a square- metre of green foliage leaves.
From the investigations of MARCHLEWSKI and NENCKI it appears that a relationship exists between chlorophyll and haemoglobin (the pigment contained in red blood corpuscles) (43).
The green colour of the chlorophyll in some groups of Algae is more or less masked by other pigments. In addition to the chloro- phyll green, with its accompanying yellow pigments, many of the blue-green Schizophyceae contain a blue colouring matter, phycocyanin, while the red Algae possess a red pigment termed phycoerythrin. These pigments are soluble in water, and are characterised by a beautiful fluorescence. The phycocyanin may often be found as a blue border surrounding a blue-green Fission-Alga which has been dried in a press.
Whether these pigments are mixed with the chlorophyll or are chemically combined with it is as yet undecided (44). Recently HANS MOLISCH (45) has attempted to prove that the brown coloration of the Diatoms, the brown Algae, and especially of a saprophytic Orchid (Neottia nidus a?vis), is not due to a mixture of a brown pigment with chlorophyll. He regards it as due on the other hand to a single pigment, phaeophyll, which is nearly related to chlorophyll and readily undergoes a chemical change into ordinary chlorophyll.
Before the leaves of trees fall in the autumn their cells lose nearly all their cytoplasmic contents, and at the same time the chloroplasts undergo disorganisation. There remains only a watery substance in the cell cavity, in which a few oil globules and crystals, together with a few yellow, strongly refractive bodies, can be seen. Sometimes in presence of abundant sugar this liquid in the cell cavities becomes red, and thus imparts to the foliage its autumnal brilliancy. In the leaves of coniferous trees, which only indicate the approaching winter by assuming a somewhat brownish tint, the cause is different. The
SECT. I
MORPHOLOGY
63
FIG. 64.— Cell from the upper surface of the calyx of Trapaeolum majus, showing chromo- ]. lasts, (x 040.)
chlorophyll-green of their chloroplasts changes to a brownish green pigment, but in the following spring regains its characteristic colour.
In such phanerogamic parasites or humus- plants as are devoid of green colour, the chloro- plasts either do not develop, or they are white, or have only a brownish or reddish colour. No chromatophores are found in the Fungi.
Leueoplasts. — In the interior of plants, where light cannot penetrate, leucoplasts are de- veloped from the rudiments of the chromato- phores instead of chloroplasts. They are of a denser consistency than the chloroplasts, mostly spherical in shape, but often somewhat elongated in consequence of enclosed albuminous crystals. If the leucoplasts become at any time exposed to the light, they may change into chloroplasts. This frequently occurs, for example, in the super- ficial portions of potato tubers.
Chromoplasts. — The chromoplasts of most flowers and fruits arise either directly from the rudiments of colourless chromatophores, or from previously formed chloroplasts. In shape the chromoplasts resemble the chloroplasts, except that they are usually smaller ; in consequence of the crystallisation of their colouring pigment they sometimes assume a triangular, tabular, needle, or fan- shaped form (Figs. 64, 65). The colour of the chromoplasts varies from yellow to red, accord- ing to the predominance of yellow xanthophyll or orange -red carotin. The name carotin has been derived from the Carrot (Daucus Carota), in the roots of which it is particularly abundant (Fig. 65). The frequent crystalline form of the chromoplasts is, in a great part, due to the tendency of carotin to crystallisation, although it may be also occasioned by needle-like crystals of albumin. Xanthophyll, however, is never ,,f present in the chromoplasts except in an amor- phous condition.
Carotin is practically identical with the chrysophyll found in the chloroplasts. Its spectrum only differs from that of chrysophyll in having the absorp- tion bands slightly displaced towards the violet end (46).
Origin and Structure of the Cell Wall (47). — The membrane which encloses the vegetable protoplast is a product of the protoplasm. Many low organisms belonging to the Algae liberate naked protoplasts
KIG. Go. — Cliromoplasts the Carrot, some with included starch grains. (X 540.)
64
BOTANY
PART I
from their cells ; these swarm-spores (Fig. 97 A) serve to multiply the plant vegetatively. They soon settle down, form a thin cell membrane on the surface of the protoplast, and proceed to give rise to a filament. In more highly organised plants the ovum, from which the development starts, has no cell wall until it has been fertilised ; from this stage on, all the cells composing the plant are surrounded by cell walls. At the growing points of plants the cells are separated from one another only by extremely thin membranes or cell walls. As the cells increase in number by repeated division, new cell walls are being continually introduced between the existing ones. The
FIG. t>G.--Htrongly thickened cell from the pith of Cli'indtix rHnllm. , . Middle lamella ; i, intercellular space; t, pit; ie, pitted} cell wall in surface view. (X 300.)
l-'ic. lii.— Part, of asclerenchj- inatous libre IKnii Vln<:« iiiftjm: The striationsof the outer layer* are in<>re appar- ent than those of the inner layers. The thicknessof the wall, as seen in optical sec- tion, is also .shown. (X500.)
rapid growth in length which sets in a short distance from the growing point, as a result of the increase in the size of the cells, must be accompanied by a corresponding GROWTH IN SURFACE of the cell walls. So long as this growth in surface continues, the cell walls remain thin. After the cells have attained their ultimate size, the GROWTH IN THICKNESS of the cell walls then begins. The growth in surface of the cell wall may either involve the introduction of new material, or may take place without this. In the latter case the mem- brane would become thinner if new lamellae were not simultaneously applied to its surface. Growth of the wall by the introduction of new particles between those previously existing is termed GROWTH BY INTUSSUSCEPTION, while that which occurs by the laying down of new lamellae on the surface of the older ones is called GROWTH BY APPOSI-
MORPHOLOGY
. 65
TION. The later growth in thickness of most cell walls takes place by
apposition, and thus the stratification, Avhich such thickened walls
exhibit, is brought about (Fig. 66). Thicker, dense layers alternate
with thinner less dense ones. The denser
layers can be recognised by their high re-
fractive power. In many cases lamella,
deposited by apposition, become further
thickened and otherwise modified by a
process of intussusception. Three distinct
layers can frequently be distinguished in
strongly thickened cell walls, such as those
of the wood, a primary, a secondary, and
a tertiary thickening layer ; these differ in
their optical appearance and their chemical
composition. The secondary thickening
layer is usually the most strongly devel-
oped, and forms the chief part of the cell
wall. The tertiary or inner layer is usually
more highly refractive.
Fl°- 6s. -Surface view of cells from the sensitive side of the tendril of Cucurbita Pepo< showill, ta,.
tile pits, s. (x 540.)
Thicker cell walls or layers of the wall which appear homogeneous frequently exhibit a stratifica- tion when treated with strong acids or alkalies. In many cases the thickening layers exhibit delicate striations in surface view. The striations extend through the whole thickness of the layers, usually running obliquely to the long axis of the cell, and often crossing one another in the different thickening layers (Fig. 67). A similar appearance of crossing of striations may result from the single thickening layers of two adjoining cells being visible at once. In special cases, but only in the formation of repro- ductive cells, an inner thickening layer, completely detached from the others, is produced, as in the formation of pollen grains and the spores of Bryophytes and Pteridophytes, which, enclosed only by this inner membrance, finally become freed from the older thickening layer. This process is often alluded to as REJUVENESCENCE ; in such cases, it should be noted, there are, in reality, no new cells formed.
-phe thickening of the cell wall seldom
Bio. 69. T,a,,sv,rs, section thro^h similar ceils to those in FiK. 08; a
small crystal of calcium oxalate is , , •<• i ,111 <•
present in thetactiiepits. (x4so.) takes place uniformly over the whole surf ace; but some portions are thickened, while, at
other points, the original cell wall remains thin. In this way pores are formed which penetrate the thickening layers. These pores or PITS may be either circular (Figs. 66 w, 71 m), elliptical, or elongated. The pits in adjoining cells coincide, and would form one continuous canal, were it not that the unthickened primary cell wall persists as a PIT MEMBRANE between the two pits. The openings of narrow elliptical pits into adjoining cells usually appear to cross
F
66
BOTANY
one another obliquely. As a result of the continued thickening of
m
6'"'
FIG. 70.— From the wood of the Pine, 7'i/itts sylvextris. A, Hordered pit in surface view ; B, bordered pit in tiingential section; t, torus; '', transverse section of a tracheid ; m, middle lamella, with gusset, M* ; t, inner peripheral layer, (x Mil.)
Ki<;. 71. — Cells from the endo- sperm of Hr/iUhoiial"-' In Hutu in. >[»/. I'its in surface view ; p, closing ineiiilirane ; it, liueleus. (X -40.)
the cell wall, the canals of several pits often unite, and so BRANCH KD PITS are formed. Such branched pits have usually very narrow
^
Fio. 72. — Part of two
sieve - tuU's of the Pine, I'iiniK xi/l'- showint; sieve - pits. (x 540.)
A
KK.. ~X. — A, Part of ;in anniil;ir trnrheid ; ]:. ]>art of a s]>ir:d trai-heiil ; C, l"ir^i- tudinal section through |<iirt of a
retic'iil;ite vessel, showing: the remains of :i partition wall, x. ( • 1MH.)
canals, and occur for the most part only in extremely thick and hard cell walls, as, for instance, in those of the so-called sclerotic cells or
67
sclereides. Simple pits may, on the other hand, expand on approaching the primary cell wall.
Pits widened towards the membrane are found in the external cell walls of many tendrils (4S). These pits, which are filled with cyto- plasm, probably receive the stimulus, and may be termed tactile pits (Figs. 68, 69). The structures known as
BORDERED PITS (Fig. 70)
are a special type of simple pits widened to- wards the pit membrane. The pit may be present on one or both sides of the wall; the former is the case when the water- conducting element abuts on a cell with protoplasmic contents, the latter when the pitted wall separates two water-conducting elements. In bordered pits the closing membrane is thickened at the centre to form a TORUS (Fig. 70, B). By the curving of the closing membrane to one side or the other the torus may so act as to close the pit canal (Fig, 70 P>, f). The bordered pits apparently act as valves. Seen from the surface a
bordered pit appears as two con- centric rings (Fig. 70 A). The smaller, inner ring represents the narrow opening of the pit into the cell cavity ; the larger, outer ring indicates the widest portion of the PIT CHAMBER when it abuts on the primary cell wall.
The pit membrane of specially
Pto. 75.- Epidermal cell from the margin of a ., . , ,, ., , .
r-uiicai leaf of a,,,,,,,,,,,,!,, ,..•,•*;,•//./,•„. The wide P»8 between cells with thin
Km. 74. — Purl of transverse section of a stem of Inipatien.* pai-viflora. e. Epidermis ; c, colleiicliynia ; )>, tliin-walk'<l parenchymatuus ct-Ils ; i, intercellular sjwce. (x 300.)
lens-shaped thickening in this plant silicitied. X 515.)
of the outer wall is
(After HABERLANDT,
walls often shows thicker bands
whjch gjye i(. ft lattjce_work appear-
ance. A similar type of thickening is found in the membranes of the sieve-pits which, on account of their being perforated like a sieve, are termed sieve-plates (Fig. 72).
In cases where the greater part of the cell wall remains un- thickened, its character is determined by its thickened rather than by its unthickened portions ; it is in this sense that the terms annular, spiral, and reticulate are used (Fig. 73). Just as in the case of cells with bordered pits, annular, spiral, and reticulate cell walls
68
BOTANY
I'AUT I
are only acquired by cells that soon lose their contents, and act as water-carriers. Such wall thickenings serve as mechanical supports,
to give rigidity to the cells, and to enable the cell walls to withstand the pressure of the surrounding living cells.
The thickened bands by which thickenings of the cell wall are effected an: attached by narrowed bases to the primary membrane (49). When the mem- Inane separates two water - conducting elements its central portion is thickened like the torus of the bordered pit. The annular and spiral types of thicki-niiii: characterise the water -conducting ele- ments of growing parts of the plant, >ince they allow of extension of the wall iluring growth. The thickening band can often be removed as a continuous spiral from the lamella to which it i* attached.
COLLENCHYMATOUS cells are Fu, 7G.-l>art of transverse section of a leaf of l™g cells, the Walls of which are
Fieus dastica. c, Cystoiitu ; e, «, e, three- thickened principally at the corners
layered epidermis: ;.? palisade ^renchyma : /pjg 74 c\ suc^ ce]Jg occur com. *, spongy parenchyma, (x '-'40.) °, . /, , . , , . ,
monly in the more highly organised
plants and form a special kind of tissue (p. 116). Cells on the surface of plants have usually , only their outer walls thickened (Fig. 74 e).
Unequal thickening of the cell- wall is most noticeable when it is limited to definite small areas. This is the case with those lens-shaped thickenings of the outer walls of the epidermal cells of the upper surface of foliage leaves, which are of frequent occurrence and, according to G. HABERLANPT (M), act as convex lenses to concentrate the light (Fig. 75). By the thicken- ing of cell walls at special points, protuberances projecting into the cell cavity are formed ; in this way the formations known as CYSTOLITHS arise. Certain large cells in the leaves of the India-rubber plant (Ficus elas- tica) contain peculiar clustered bodies, formed by the thickening of the cell wall at a single pointj (Fig. 76). In their formation a stem-like body or stalk first protrudes from the cell wall ; by the addition of freshly deposited layers this
Kn;. 77. -.1, I'nlleii-s.'i-ain nt Cucurbita I1'/-" iii .sin-- face view, and partly in uptiral sertiiin. rendered transparent I'.v treating with oil of lemons (x 240); R, part of transver-r src-tinn of pollen .urain of Cifurliittt re rnif'Sa ( x ">4<i).
SECT, i MORPHOLOGY 69
becomes club-shaped, and, by continued irregular deposits, it finally attains its clustered form.
So far only centripetal wall thickenings have been described. Cells, the walls of which are centrifugally thickened, can naturally only occur where the cell walls have free surfaces. The outer walls of hairs generally show small inequalities and projections. The surface walls of spores and pollen grains (Fig. 77) show a great variety of such centrifugally developed protuberances, in the form of spines, ridges, reticulations, and bands of characteristic structure.
Cell-Wall Substance (51). — The most important constituent of cell Avails is CELLULOSE. It is present in the cell walls of all plants, except in those of the majority of Fungi.
Cellulose is a carbohydrate of which the chemical composition is expressed by the general formula (C6H1005)n. It is insoluble in dilute acids or alkalies. By the action of concentrated sulphuric acid it is converted into dextrose. After treatment with sulphuric or phosphoric acid, iodine will colour it blue ; it shows a similar reaction when exposed to the simultaneous action of a concentrated solution of certain salts, such as zinc chloride or aluminium chloride, and of iodine. Accordingly, chlor-zinc-iodide, on account of the blue colour imparted by it, is one of the most convenient tests for cellulose. GILSON (M) obtained cellulose in a crystalline condition in the form of sphaerites or dendrites.
The cell walls never consist entirely of pure cellulose, but contain a considerable amount of other substances, which are not stained blue by chloroiodide of zinc. In unlignified cell walls PECTIN SUBSTANCES are particularly prominent. They are easily distinguished by the readiness with which they dissolve in alkalies, after being previously acted upon by a dilute acid.
Susceptibility to certain stains — for example, congo red — is a characteristic of cellulose ; while other stains, such as safranin and methylene blue, colour pectin substances more deeply. According to MANGIX (M), the partition wall formed in the higher plants during cell division consists almost wholly of pectin substances ; the next developed laminae, the secondary cell- wall layer, of a mixture of cellulose and pectin substances ; the last formed, or tertiary layer, chiefly of cellulose. If the secondary layer of the cell wall remain unlignified, the amount of the pectin substances contained in it increases with age and helps to strengthen the MIDDLE i, \MKLLA, or primary cell- wall layer.
Among the substances entering into the composition of cell walls, in addition to cellulose and the pectin substances, mention must be made of CALLOSE. It is characterised by its insolubility in cuprammonia and solubility in soda solution, and in a cold 1 per cent solution of caustic potash. It is coloured a red brown by chloroiodide of zinc, with aniline blue it takes an intense blue, and with corallin (rosolic acid) a brilliant red. Its presence in the higher plants is limited to a few special cases ; it envelops the sieve-pits and is always present in calcified cell-wall layers, as, for example, in cystoliths (Fig. 76). Chitin, a proteid substance, according to GILSON (M), takes the same place in the cell walls of the Fungi as cellulose in the cell walls of the higher plants.
Where cell walls become LIGNIFIED or SUBERISED, it is particularly
70 BOTANY I-AKT i
the secondary layer that receives the wood or cork substance, while the tertiary or internal layer retains its cellulose character.
Lignificatiou depends on the introduction into the membrane of certain substances, among which, according to CZAPEK (**), an aromatic aldehyde which he names hadromal is never wanting. According to F. C. vox FAIJEU (•"*) hadromal is not present in all lignified membranes, and on the other hand occurs in many that are not lignified. Associated with hadromal, according to CZAPEK, is coniferin, whieh can be obtained from the youngest xylem. C/AP-EK denies 1 1n-, existence of vanillin in lignified membranes. The proportion in whicli hadromal is found in wood does not exceed 1-2 per cent of its dry weight. To its presence the so-callcil liguin reactions are due, a violet colouration with phloroglucin and hydrochloric acid, and a yellow colouration with acid anilin sulphate. The potassium per- manganate reaction (a red colouration on treatment with a 1 per cent solution of that substance followed by ammonia) is, according to F. C. FABER, a general feature of lignified membranes. With chlor-zinc-iodide lignified membranes stain yellow, not blue.
Corky cell walls contain suberin and take a yellowish-brown colour with chloro- iodide of zinc ; with caustic potash, a yellow. VAN AVissF.r.iNiac (s~) has disputed the presence of cellulose in suberised cell walls, and regards the cork substance or .sruKUix as a fatty body, which is composed of glycerine esters and other com- pound esters, as well as of one or more other substances which are infusible, insoluble in chloroform, and decomposed by a solution of caustic potash.
CUTINISATION, which is similar to but not identical with suberisa- tion, is usually due to the subsequent deposition of cutin in cellulose cell walls.
VAN "\VI.SSKI.INGH has shown that phellonic acid, which is always present in suberiu, is constantly absent in cutin. The behaviour of cutin, as of suberin, varies according to the source from whicli it is derived. Cutin withstands better the action of- caustic potash. In other respects, the reactions given by cutinised cell wralls with chloroiodide of zinc or solutions of caustic potash are almost identical with those of suberised cell walls.
Young cell walls are less elastic, but relatively more extensible than older ones. The power of resisting a stress is increased by lignification. The presence of cutinised and corky membranes at the surface of the plant diminishes the loss of water from it (M
The layers of the cell walls of some cells, particularly the super- ficial cells of certain fruits, as of Sage, and of numerous seeds, such as Flax and Quince seeds, become mucilaginous, ;tnd swell in water to MUCILAGE, which, according to G. KLEBS (59), serves the purpose of attaching the seeds to the soil. Firm cell walls can also be trans formed into GUM, as is so often apparent in Cherry and Acacia trees, portions of the wood of which often succumb to GU.MMUSIS.
The several varieties of gvms and mucilage react differently, according as they are derived from cellulose, callose, pectin substances, or from allied sulistam -e*. According to MANC.IN they may be microchemically distinguished by their ivai-tit>n with ruthenium red, which stains only such as are derived from pectin -nl»t.un .-
SKCT. i MORPHOLOGY 71
or related substances, such as the mucilage of the seeds of the Cruciferae and Quince (Cydonia), that from the mucilage cells of the Malveae, the gums of the Cherry and Acacia, the gum tragacanth from Astragalus gummifer. The mucilage of Orchid tubers, on the other hand, is related to cellulose, and remains uncoloured with the same reagent.
The cell walls of the seeds of many Palms, as also those of Ornithogalum (Fig. 71), have strongly developed thickening layers, which are full of pits. These thickening layers are lustrous white, and, as in the case of the seeds of the Palm, I'/iiff: Icphas macrocarpa, may attain such a degree of hardness as to be technically valuable as vegetable ivory. Such thickening layers may contain other carbo- hydrates in addition to cellulose, and by the action of ferments are dissolved during germination. They are accordingly to be considered as a reserve substance of the seeds.
Cell walls often become coloured by tannin or derivative sub- stances ; in this way, for instance, the dark colour is produced which is often seen in the coats of seeds and in old wood. The colours of the woods of economic value are due to such discoloured cell walls. Inorganic substances are often deposited in large quantities in old cell walls. Among such substances calcium oxalate is often met with, commonly in crystal form ; also, although not so frequently, calcium carbonate. In the cystoliths of Ficus elastica (Fig. 76) so much calcium carbonate is deposited that it effervesces with hydrochloric acid. In many plants, as, for instance, most of the Characeae, the quantity of calcium carbonate in the cell walls is so great as to render them stiff and brittle. Silica is also present in the superficial cell walls of the Gramineae, Equisetaceae, and many other plants, and gives them a very considerable firmness. The lens-shaped thickenings in the outer walls of the epidermal cells of Campanula persicifolia are also silicified.
By withdrawing water from the cells a contraction of the protoplast and its consequent separation from the cell wall is brought about (see Plasmolysis). Such protoplasts are able under certain conditions to surround themselves with a new cell -membrane. • The removal of the cuticle or of the waxy covering from the surface of certain plants (Agave, Aloe, Ricinus, Sedum) is followed by its re generation (60).
Form of the Cell. — As cytoplasm is a viscous fluid, and would tend, if unimpeded, to take a spherical shape, it may be assumed that the natural and primary form for cells is spherical. Such a shape, however, could only be realised by cells which, in their living condition, were completely free and unconfined, or in such as were able to expand freely in all directions. Newly developed cells, in a continuous tissue, are, at first, nearly always polygonal. Through subsequent growth their shape may change. The cubical cells of the growing point either elongate to a prism or remain, owing to repeated division, short and tabular. If the growth is limited to certain regularly arranged points of the surface, they
72
BOTANY
become stellate ; if these points are less uniformly arranged their outline is correspondingly unsymmetrical. In consequence of energetic growth in length, fibre-like, pointed cells are developed. If the walls of such cells become much thickened, they are called SCLERENCHYMA fibres (Fig. 78 A}. These show diagonal markings, due to their elongated pits, which are generally but few in number.
When fully developed, the living contents
A R £L °f such ce^s are smaU i» amount and
frequently they contain only air. In the last case, they merely act as mechanical elements (stereides) and contribute to the rigidity of the plant as a whole. Cells somewhat similar, but shorter and consider- ably wider, not pointed at the ends, and provided with bordered pits, are called TRACHEIDES (Fig. 78 B}. The tracheides, in their fully developed condition, never have any living contents, but serve as water-carriers for the plant. So long as they remain active, they contain only water and isolated air -bubbles; their active functions afterwards cease, and they become filled with air. Tracheides which are speci- ally elongated, and at the same time have only a narrow lumen, and, like the scleren- chymatous fibres, serve merely mechanical purposes, are known as FIBRE TRACHEIDES. Very long tracheides with a wide lumen and thin walls, functioning, like typical tracheids, as water-carriers, are distin- guished as vasiform or VASCULAR TRACHE- IDES (Fig. 78 C). They are characterised by
Fio. 78. — .-I, A sclerenchyinatous ,1 i • • i i' i ± i •
tibre: js.atracheid: c, part of the annular, spiral, or reticulate markings
a spiral tracheid ; D, part of a of their thickening layers, and may also be
latex tube. (A, K, c, x loo ; D, pr0vided with bordered pits. The walls of
tracheides are always lignified, while those
of the sclerenchyma fibres may or may not have undergone this change. Of all the cells in the more highly organised plants, the LATEX CELLS or milk cells, also spoken of as latex tubes, attain the greatest length. In the Euphorbiaceae, Moraceae, Apocynaceae, and Ascle- piadaceae they arise from cells which are already differentiated in the embryo. Growing as the embryo grows, they branch with it and penetrate all its members, and may thus ultimately become many metres long. The latex cells themselves have, for the most part, unthickened, smooth, elastic Avails which give a cellulose reaction. They are provided (01) with a peripheral layer of living cytoplasm
MORPHOLOGY 73
and numerous nuclei. Their sap is a milky, usually white fluid, which contains gum-resins, i.e. mixtures of gums and resins, caoutchouc, fat and wax in emulsion. In addition, they sometimes hold in solution enzymes, leptomin, tannins, often poisonous alkaloids, and salts, especially calcium malate, also in the case of Ficus Carica and Carica Papaya peptonising ferments. Proteid granules often occur in the latex, and in the latex cells of the Euphorbiaceae there are also present peculiar dumb-bell-shaped starch grains. On exposure to the air the milky sap quickly coagulates. In the adjoining figure (Fig. 78 D) is shown a portion of an isolated latex cell dissected out of the stem of an Asclepiadaceous plant, Ceropegia stapelioides.
Special cells, which differ in form, contents, or in their peculiar wall thickenings from their neighbouring cells, are distinguished as IDIOBLASTS. If strongly thickened and lignified, they are called sclerotic cells (stone cells) or sclereides. They often contain ferments; in the Cruciferae and some other orders myrosin is thus present, Avhile Prunus laurocerasus contains emulsin. For the most part they contain excreted substances such as tannins and calcium oxalate. In Fig. 85 an idioblast, containing a bundle of raphides, is represented. Idioblasts, resembling tracheides and functioning as water reservoirs, are found between the chlorophyll-containing cells in the leaves of some of the Orchidaceae and Cactaceae.
Size of Cells. — The corresponding cells of equivalent members of the same plant are usually of nearly the same size, even when the members show a variation in size.
Differentiation of the Protoplasts. — In organisms composed of one or of few cells the separate parts of the same protoplasts may under some circumstances exhibit marked differences. On the other hand, in multicellular organisms whole protoplasts are specialised for functions with the performance of which their definite structure is connected.
Inclusions of the Protoplasm — STARCH. — The chloroplasts in plants exposed to the light almost always contain starch grains. These grains of starch found in the chloroplasts are the first visible products of the assimilation of inorganic matter. They are formed in large numbers, but as they are continually dissolving, always remain small. Large starch grains are found only in the reservoirs of reserve material, where starch is formed from the deposited products of previous assimilation. Such starch is termed RESERVE STARCH, in contrast to the ASSIMILATION STARCH formed in the chloroplasts. All starch used for economic purposes is reserve starch. The starch grains stored as reserve material in potatoes are comparatively large, attaining an average size of 0'09 mm. As shown in the following figure (Fig. 79), they are plainly stratified. The stratification is due to the varying densities of the successive
74
I'.OTAXY
layers; thicker denser layers which appear clear by transmitted light alternate with thinner less dense layers which appear dark. They are excentric in structure, since the organic centre, about which the different layers are laid down, does not correspond with the centre of the grain. The starch grains of the leguminous plants and cereals,
FIG. 70.— Starch grains from a potato. A, simple ; .B, half-compound ; C and D, compound starch grains ; c, organic centre of the starch grains, or nucleus of theii formation, (x MO.)
KKI. So. — Starch grains from the cotyledons <>f Ph<i.--< -. ( x
540.);
on the other hand, are concentric, and the nucleus of their formation is in the centre of the grain. The starch grains of the Kidney Bean, Phaseohis vulfjaris (Fig. 80), have the shape of a flattened sphere or ellipsoid ; they show a distinct stratification, and are crossed by fissures radiating from the centre. The disc-shaped starch grains of wheat are of un- equal size, and only indistinctly stratified. A comparison of the accompanying figures (Figs. 79-81), all equally magnified, will give an idea of the varying size of the starch grains
of different Pknts. The size of starch V*» varies, in fact, from 0'002 mm. to 0*170 mm.
Starch grains 0'170 mm. large, such as those from the rhizome of Canna, may be seen even with the naked eye, as minute bright bodies. In addition to the simple starch grains so far described, half-compound and compound starch grains are often found. Grains of the former kind are made up of two or more individual grains, surrounded by a zone of peripheral layers enveloping them in common. The compound grains consist merely of an aggregate of individual grains unprovided with any common enveloping layers. Both half-compound (Fig. 79 £) and compound
pound grain; B, isolated comjionftnt grains of a com- pound grain, (x ">4o.)
MORPHOLOGY
75
starch grains (Fig. 79 C, D) occur in potatoes, together with simple grains. In oats (Fig. 81) and rice all the starch grains are com- pound. According to NAGELI (62), the compound starch grains of rice consist of from 4 to 100 single grains; those of the oat of about 300, and those of Spinacia glabra sometimes of over 30,000. Starch thus formed from previously assimilated organic substances also requires chromatophores for its production. The grains are formed by means of leucoplasts, which are, in consequence, often termed STARCH- BUILDERS. If the starch grain is uniformly surrounded by the leucoplast during its formation it grows uniformly on all sides and is symmetrical about its centre. If the formation of a starch grain should begin near the periphery of a leuco- plast, the grain would grow more rnpicHy on the side on which the main mass of the leucoplast is present and the starch grain thus becomes excentric (Fig. 82). Should, however, several starch grains commence to form at the same time in one leucoplast, they would become crowded together and form a compound starch grain, which, if ad- ditional starchy layers are laid down, gives rise to a half-compound grain.
It lias been asserted that starch grains are crystalline bodies, so-called sphserites (K!), and Fw. 82.— LeaeoptaiU from an aerial tubwr
are composed of fine, radially arranged, of plu'Jus ;/<"»<''>' <'"<• needle - shaped crystals, which A. MEYER terms trichites. Their stratification, accord- ing to this view, is due to variations in the
form and number of the crystal needles in the successive layers, other hand, H. FISCHER (e4) has explained the stratification as due to zonal splits rich in water, which originate by contraction taking place in the substance of the grain at some distance from its growing surface. HENRY KRAEMER (6o) holds that a crystalloid and a colloid substance are present, but are united in different proportions in the successive lamellae of the starch grain. In a few in- dividual cases, ARTHUR MEYER has succeeded in showing that the stratification of the starch grains corresponds to the alternation of the periods of day and night, i.e. to the interference which is thus caused in the nutritive processes. The growth of starch grains is also, according to him, affected by the solvent action of surrounding substances, whereby the peripheral layers may be partially removed, and then no longer completely envelop the entire grain. Starch grains are composed of a carbohydrate, the formula of which is (C6H1003)n. Most starch-grains consist of amylose and are coloured blue with iodine ; in addition to this they contain, according to L. MAQUEXXK and EUCJ. Roux C66), a mucilaginous substance which they term amylo-pectin. The presence of this substance causes the pasty union of the starch- grains when treated with boiling water or alkalies. The grains swell in water of 60°-703 C., are but little soluble in water at 100° C., but are rapidly dissolved by
Viewed from the side ; /.', viewed from above ; /•.', leucoiilast becoming green and changing to a chloroplast. (x 540.)
On the
BOTANY
super-heated water at 1403-150° C. Starch swells very readily at ordinary temperatures in solutions of potassium, or sodium hydrate. Heated without addition of water, i.e. roasted, starch becomes transformed into dextrin, and is then soluble in water and correspondingly more digestible. The glutinous starch of varieties of Rice and Millet (Oryza saliva, var. ylntinoxx, X»r</hii>/i nil'jurc, ylutinosum) is in the completely unswollen condition coloured brown with iodine ; when swollen by a dilute solution of iodine, it takes a red, wine-red, or purple colour. According to 0. BUTSCHLI (m) it consists of amyloerythrin, a carbo- hydrate resembling starch. In polarised light, starch grains, like inorganic sphferites, show a dark cross. This appearance is usually referred to the doubly refractive nature of the elements of which the starch grain is constructed.
The amount of starch contained in reservoirs of reserve material is often considerable ; in the case of potatoes 25 per cent of their whole weight is reserve starch, and in wheat the proportion of starch is as high as 70 per cent. The starch flour of economic use is derived by washing out the starch from such reservoirs of reserve starch. In the preparation of ordinary flour, on the contrary, the tissues con- taining the starch are retained in the process of milling.
Aleurone. — Aleurone or proteid grains are produced in the seeds of numerous plants, especially in those containing oil. They are formed from vacuoles, the contents of which are rich in albumen, and harden into round grains or, sometimes, into irregularly shaped bodies. The albuminous substances of which they consist are, accord- ing to A. TSCHIKGH and H. KRITZLER (°8), mainly globulins. A portion of the albumen often crystallises, so that frequently one and occasionally several crystals are formed within the aleurone grain. In aleurone grains containing albumen crystals there may often be found globular bodies, termed GLOBOIDS, which, according to PFEFFER (°9), consist of a double phosphate of magnesium and calcium in
combination with some organic substances. Crystals of calcium oxalate are also found enclosed in aleurone grains.
Jt
A
ff The seeds of Ricinus (Fig. 83)
furnish good examples of aleurone grains with enclosed albumen crystals and globoids. The aleurone grains themselves lie embedded in a cyto- plasm that is rich in oil. In the FIG. 83.— A, Cell from the endosperm of Ricinus cereals the aleurone grains which lie wnmttni*, in water; 1'. isolated aleurone grains Q^ JQ tj)e Quter ^ j of the
in olive oil ; I:, albumen crystals ; a, glolioid. . _. ,. 11 i <•
seeds (Fig. 84 at) are small, and free
from all inclusions ; they contain
neither crystals nor globoids. As the outer cells of wheat grains contain only aleurone, and the inner almost exclusively starch, it follows that flour is the richer or poorer in albumen, the more or less completely this outer layer has been removed before the wheat is ground. From the inner layers finer and whiter
MORPHOLOGY
77
flour can be made : while more nourishing Hour is obtained from the outer layers. Reactions for aleurone are the same as those already mentioned for the albuminous substance of protoplasm. Treatment of a cross-section of a grain of wheat (Fig. 84) with a solution of iodine would give the aleurone layer a yellow- brown colour, while the starch layers would be coloured blue.
ALBUMEN CUYSTALS. — Crystals of albumen are of relatively frequent occurrence in vegetable tissues and are often found in aleurone grains (Fig. 83). Their appearance in the seed of Ricinus has been described above, and especially large crystals are found in the endosperm of the Brazil nut (BertlmHrtiti. ov/Vs^ i
if •
Ki<;. S4.— Part of u section M!';I -rain ol' wheat, Tfitii-mn i- nil/are, p, Pericarp ; t, seed coat, internal to which is the endosperm; al, aleurone grains; mn, starch grains ; //, cell nucleus, (x 240.)
KlO. 85.— Cell I'r the cortex
of Drvaienti i-ubrn, tilled with mucilaginous matter and containing a bundle of raphides. /•. (x HiO.)
ing to the Lecythidaceae. They have previously been mentioned as occurring in the chromatophores. In the illustration of the leucoplasts of Phajus grandifolius (Fig. 82), the rod-shaped crystals are represented as light stripes (in B and E). Albumen crystals may also occur directly in the cytoplasm ; as, for instance, in the cells poor in starch, in the peripheral layers of potatoes. Albumen crystals are sometimes found even in the cell nucleus. This is particularly the case in the Toothwoi t (Lathraea squamaria], and in many Scrophulariaceae and Oleaceae (70). Albumen crystals usually belong either to the regular or to the hexagonal crystal system. They differ from other crystals in that, like dead albuminous substances, they may be stained, and also in that they are capable of swelling by imbibition. Subjected to the action of water or a dilute solution of caustic potash, they at first increase in size without losing their crystalline outline.
?8 BOTANY I'AKT i
Crystals of Calcium Oxalate. — Few plants are devoid of such crystals. They are formed in the cytoplasm, within Aracuoles Avhk-h afterwards enlarge and sometimes almost fill the whole cell. In such cases the other components of the cell become greatly reduced; the cell walls at the same time are often converted into cork, and the whole cell becomes merely a repository for the crystal. The crystals may be developed singly in a cell, in which case they are of consiiU-i able size and can be seen to belong either to the tetragonal or monosymmetrical crystal system ; or the crystals are so small and numerous that their form cannot be clearly made out and they appear as a crystalline sand filling the cell. Frequently they form CRYSTAL AGGREGATES, Clusters of crystals radiating in all directions from a common centre. In the Liliaceae, Orchidaceae, and other Mono- cotyledons, compact bundles of needle-shaped crystals of calcium oxalate, the so-called RAPHIDES, are especially frequent (Fig. 85). Such crystal bundles are always enclosed in a large vacuole filled with a mucilaginous substance. The degree of concentration of the mother liquor from which the crystals have separated, deter- mines, according to KNY (71), their form, whether tetragonal or monoclinic.
SILICEOUS BODIES, which are only soluble in hydrofluoric acid, are found in the cytoplasm of many cells, especially of Palms and Orchids, and often completely (ill the whole cell.
TANNIN. — Highly refractive vacuoles filled with a concentrated solution of tannin are of frequent occurrence in the cytoplasm of cortical cells, and may often grow to a considerable size. The dark-blue or green colour reaction obtained on treatment with a solution of ferric chloride or ferric sulphate, and the reddish- brown precipitate formed with an aqueous solution of potassium bichromate, an: usually accepted as tests for the recognition of tannin, although equally applicable for a whole group of similar substances.
FATS and OILS in plants are mixtures of fatty-acid esters. Frequently, a> in most Monocotyledons, a fatty oil appears in the old chlorophyll grains. The occurrence of castor oil in the form of highly refractive drops in the cytoplasm of the aleurone-containing cells in the endosperm of the castor-oil seeds, has already been referred to. Oil usually occurs in this form. But fatty substances may also appear in the cytoplasm as irregularly shaped, more or less soft grains, as for example in the vegetable butters and in the wax of various seeds ; they may even be crystalline, as in the needle-like crystals of Para-nuts (Bertholletid exwlsa) and of Nutmeg (Myristica fragrans). Special cytoplasmic structures in which the drops of oil are embedded, are of frequent occurrence in the epidermal cells of Oreliidac •< -.n- and Liliaceae and go by the name of elaioplasts (72).
CHOI.KSTK.UIN. — Since this substance is a regular constituent especially of givi-n cells, it probably plays an important part in metabolism (7:i).
(ii.vccici.N. — This substance (74), related to starch, and of frequent occurrence in animal tissues, fulfils, according to EKKKKA (75), the same functions in the Fungi as sugar and starch in the higher plants. Cytoplasm containing glycogen is coloured a reddish brown with a solution of iodine. This colour almost wholly disappears if the preparation be warmed, but reappears on cooling.
MORPHOLOGY 79
ETHEREAL OILS AND RESINS. — In many cases the strongly refractive drops found dispersed throughout the cytoplasm are globules of some ethereal oil. It is the presence of such oils in the petals of many flowers that gives to them their agreeable perfume. Under certain conditions the oil globules may become crystal- lised. This occurs, for example, in Rose petals. In 7iio.st cases ethereal oils or resins are formed in special protuberances (76) or strata (77) of the cell wall and only later are stored in the interior of the cell or in special intercellular spaces formed hy separation or destruction of cells. Special cells of this kind, often with corky walls and filled with resin or ethereal oils, are found in the rhizomes of certain plants, as for instance -in those of Acorus Calamus and of Ginger (Zingibcr qfficinalc) : also in the bark, as, for example, of Cinnamon trees (Cinna'niomum) ; iu the leaves, as in the Sweet Bay (Laurus nolnlis) ; in the pericarp and seed of the Pepper (Piper niyrum) ; in the pericarp of Anise (Illicium anisatwn).
Mn ILACJK is often found in the cells of bulbs, as in Alt turn Cepa and L'njiimi Xcilla, in the tubers of Orchids, also in aerial organs, especially in the leaves of succulents, which, living in dry places, are thus enabled to maintain their water supply by means of their mucilaginous cells.
CAOUTCHOUC AND GUTTA-PKRCHA. — These substances are found in a number of plants belonging to different groups, in particular in the Moraceae, Euphorbiaceae, and Sapotaceae. They occur in the latex of special cells in the form of small globules, which, suspended in the watery sap, give it its milky appearance.
FERMENTS. — Bodies of this nature are widely spread in vegetable cells. Their significance in the oxidation processes taking place in the organism has been dealt with by R. CHODAT, A. BACH (78), and others. The name leptomin was given by RACIHOUSKI (~9) to a catalytic enzyme, which he found in the sieve-tubes and laticiferous elements of the higher plants, in the milk of the coco-nut, and in|the tissues of the potato tuber.
SULPHUR. — The presence of sulphur in the form of small refractive grains in the protoplasm of certain Bacteria, the Beggiatoae, is noteworthy. These Bacteria live in water containing much organic matter, and, according to WINOGRAPSKY I80), obtain their sulphur from sulphuretted hydrogen. In fulfilling its function in the Bacteria the sulphur becomes oxidised into sulphuric acid.
The Cell Sap. — Under this term is included especially the fluid which in old cells fills the inner sap cavity. It is generally more watery and clearer than the fluid contained in the smaller vacuoles of the cytoplasm. No sharp distinction can, however, be drawn between the sap cavity and vacuoles, and, moreover, a number of such vacuoles may take the place of the sap cavity itself. The cell sap usually gives an acid reaction, owing to the presence in it of organic acids or their salts. The substances held in solution by the cell sap are very various. The soluble carbohydrates, in particular the sugars (cane sugar, the glucoses, and especially grape sugar), frequently occur in the cell sap. The glucoses may be recognised by their reducing properties.
If preparations containing glucose be placed in a solution of copper sulphate, and, after being washed out, are transferred to a solution of caustic potash and heated to boiling, they will give a brick-red precipitate of cuprous oxide. If cane .sugar or saccharose be present, this same treatment gives only a blue colour to the cell sap.
80 BOTANY r.urr i
Carbohydrates are transported in a plant principally in the form of glucose ; cane sugar, on the contrary, is stored up as a reserve material, as for example, in the sugar-beet, in the stems of sugar- cane, and in other plants from which the sugar of economic use is derived.
INULIN', a carbohydrate in solution in cell sap, behaves in the same way in the ( ''ompositae. Treated with alcohol, inulin is precipitated in the form of small granules, which may be redissolved in hot water. When portions of plants containing much inulin, such as the root tubers of Dahlia variabilis, are placed in alcohol or dilute glycerine, the inulin crystallises out and forms sphserites, spheroidal bodies com- posed of radiating crystal needles arranged in concentric layers. Amides such as GLUTAMIN and ASPARAGIN are also generally present in the cell sap. There are frequently found dissolved in the cell sap TANNINS, ALKALOIDS, and GLUCOSIDES. such as coniferin, hesperidin, amygdalin, solanin, tvsculin, saponin, and also bitter principles related to the glucosides. Organic acids (malic, formic, acetic, and oxalic acids) are also of frequent occurrence in the cell sap ; thus, malic acid is usually present in the leaves of the succulents. For the most part, these organic acids unite with bases, and the salts which are formed often crystallise. Of acid salts, which are less frequent than free acids, the binoxalate of potassium found in Field Sorrel (Rumex) and Wood Sorrel (Oxalis) deserves special mention. Species of Salicornia and Salsola contain sodium oxalate. The cell sap always contains dissolved inorganic salts, especially nitrates, sulphates, and phosphates. The different vacuoles of the same protoplast may have distinct contents ; thus one may contain tannin and another be free from it, or one may have coloured and another colourless sap.
The cell sap is often coloured, principally by the so-called ANTHO- CYANIN, which Hans Molisch, at least in some cases, regards as being a non-nitrogenous glucoside (81). This is blue in an alkaline, and red in an acid reacting cell sap, and, under certain conditions, also dark red, violet, dark blue, and even blackish-blue. Anthocyanin can be ob- tained from the super-saturated cell sap of a number of deeply coloured parts of plants in a crystalline or amorphous form. Blood-coloured leaves, such as those of the Copper Beech, owe their characteristic appearance to the united presence of green chlorophyll and antho- cyanin. The different colours of flowers are due to the varying colour of the cell sap, to the different distribution of the cells containing the coloured cell sap, and also to the different com- binations of dissolved colouring matter with the yellow, orange, or red chromoplasts and the green chloroplasts. There is occa- sionally found in the cell sap a yellow colouring mattter known as xanthein ; it is nearly related to xanthophyll, but soluble in water.
2. ONTOGENY OF THE CELL
The Origin of the Living Elements of Protoplasm. — All the
nuclei in an organism owe their origin to the nuclei of previous
SECT, i MORPHOLOGY 81
generations. The spontaneous formation of a nucleus never takes place. In the same manner, the cytoplasm of every organism is derived from the cytoplasm of the germ cell, and, so far as is yet known, the chromatophores take their origin only from their own kind.
Nuclear Division. — Except in a few limited cases, nuclei reproduce themselves by MITOTIC or INDIRECT DIVISION. This process, often referred to as KARYOKINESIS, is somewhat complicated, but seems necessary in order to effect an equal division of the substance of the mother nucleus between the two new daughter nuclei.
Indirect Nuclear Division (82). — In its principal features the pro- cess is similar in the more highly organised plants and in animals. Its stages are represented in a somewhat diagrammatic manner in the following figure (Fig. 86), as they occur in a vegetative cell such as those which compose the growing point.
The fine network of the resting nucleus (Fig. 86, 1 n) becomes drawn together at definite points and separated into a number of bodies (Fig. 86, 2 ch), the outline of which is at first irregular. Their form soon becomes filamentous, and the filaments become denser and at the same time shorter and thicker (3), and stain more deeply. The stainable substance of the filament, which is called chromatin, becomes arranged in more or less regular transverse discs united by linin (3). The filaments themselves are called CHROMOSOMES (Fig. 86, 3, 4). The chromosomes are moved into the plane of division where they constitute the nuclear or equatorial plate (5 kp, 6, 7). Each chromosome has meanwhile undergone a longitudinal split which continues to become more marked (5, 6, 7). The two halves of each chromosome thus separated move away from one another in opposite directions, and take part in the formation of the daughter nuclei (9 t).
Other changes serve to direct the process thus briefly described. While the nuclear network is separating into the individual chromo- somes, cytoplasmic filaments become applied to the nuclear membrane, surrounding it with a fibrous layer. This layer becomes raised up from the nuclear membrane at two opposite points (3 k) and forms the polar caps. These are filled with a homogeneous substance in which fine filaments appear later. The latter converge at the poles, without, however, coming into contact ; they constitute two pointed bundles, since they diverge from one another as they pass from the polar regions (4 &). At this stage the nucleoli (nl) are dissolved and the nuclear membrane disappears. The fibres proceeding from the polar caps can thus become prolonged into the nuclear cavity (4, 5). Here they either become attached to the chromosomes, or^ filaments from the two poles may come into contact and extend continuously from the one pole to the other. In this way the nuclear spindle is formed (5, 6, 7). The fibres of the spindle attached to the chromosomes may
BOTANY
be termed traction-fibres, those which run from pole to pole supporting- fibres. The nucleoli appear to form a reserve substance which serves to nourish the chromosomes and, later, has the special duty of provid- ing material for the formation of the spindle. Any excess of nucleolar substance passes into the surrounding cytoplasm, where it forms the
FIG. 86. — Successive stages of nuclear- and cell-division in a meristematic cell. «., Nucleus; nl, nucleolus ; tc, nuclear membrane; e, cytoplasm; eh, chromosomes; k, polar cap-.; s, spindle; kj>, nuclear plate ; t, young daughter nuclei ; c, connecting fibres : z, cell-plate ; m, new cell wall. In 1, the resting nucleus ; 2 and 3, separation of the chromosomes ; 4, chromosomes with transverse discs ; 5, the arrangement of the chromosomes to form the cell plate and their longitudinal fission ; 3-5 show the formation of the spindle from the polar caps ; r., the longi- tudinal fission of the chromosomes ; 7, the beginning of their separation to either pole ; 8, the complete separation of the daughter chromosomes; 9, jwssage of the daughter chromosomes to either pole ; 10-12, formation of the daughter nuclei ; in 9-11 the origin of the connecting fibres and of the cell plate is seen, while in 12 the new cell-wall is formed. ( x alnnit 000.)
so-called extranuclear nucleoli. The traction -fibres promote the arrangement of the chromosomes in the equatorial plane (5). After the chromosomes have split longitudinally into the daughter chromo- somes, the latter are separated and drawn towards the two poles by the contraction of the traction-fibres (8, 9). The supporting-fibres afford the necessary resistance in the process. The spindle fibres can often be traced to the limiting layer of the cytoplasm, and their
MORPHOLOGY
83
attachment to this determined. In forming the daughter nuclei, the free ends of the chromosomes first become drawn in (10), and the surrounding cytoplasm separates itself by means of a protoplasmic membrane, the nuclear membrane (11), from the developing nuclei. Within the nuclear cavities which are thus produced, the chromo- somes again assume a reticulate structure and unite with one another
KII:. S7. — Diagrammatic representation of the different positions occupied by the chromosomes iu the spindle and daring their separation, a and b, daughter chromosomes of one mother chromosome ; z, traction-fibres of the spindle.
to form a network within which their individual limits are not dis- tinguishable. We are compelled, however, to assume that the individuality of the chromosomes is not lost (83). The young nuclei enlarge, the extranuclear nucleoli disappear in the surrounding cytoplasm, and one or more nucleoli at length appear in the nuclei ; finally the resting condition is again attained.
The process of nuclear division is described above as it usually takes place iu the young tissues of more highly organised plants. The chromosomes are usually loop-shaped, and the traction-fibres become attached to the middle point of the loop, less commonly toward an end. The traction-fibres from the two poles attach themselves respectively to the two daughter chromosomes of each pair. The orientation of the pairs of daughter chromosomes iu the nuclear spindle, and the way in which they separate from one another, are represented in the accompany-, ing diagram (Fig. 87). In (1) the two shanks of each pair of chromosomes lie nearly in the equatorial plane, and each of the daughter chromosomes on their separation (2) assumes the form of the letter U, remaining attached by the shanks. More commonly, while one of the shanks of the paired chromosome lies in the equatorial plane, the other is directed towards one of the two poles (3). In this case a condition of things results, when the daughter chromosomes separate, which is represented in 4 or 5. The former shows the resulting appearance when the daughter chromosomes remain for a time attached to one another at both ends ; the latter when separation soon follows at the end directed towards the nuclear pole. In all cases the separation proceeds from the point of attachment of the traction-fibres. When a paired chromosome is attached to the spindle near one of its ends, the separation of the daughter chromosomes naturally commences near this end ; when the attachment is by the middle of the chromosome the daughter chromosomes remain longer attached by their ends. In Fig. 86 the behaviour of
84 BOTANY PAKT i
the chromosomes is represented as in the diagrams 3 and 4 (Fig. 87). As a rule it does not appear so clearly, but more or less combined with the other type.
The changes occurring in a mother nucleus preparatory to division are termed the PROPHASES of the karyokinesis. These changes extend to the formation of the nuclear plate, and include also the process of the longitudinal division of the chromo- somes. The stage of the nuclear plate is the METAPHASK. The separation of the daughter chromosomes is accomplished in the ANAPHA.SE, and the formation of the daughter nuclei in the TELOPHASE of the division. The real purpose of the whole process is attained in the quantitative and qualitative division of the chromosomes, resulting from their longitudinal splitting (Fig. 86, 5, 6, 7 ; Fig. 87). The anaphases and telophases of the karyokinesis are but a reverse repetition of the prophases.
The number of chromosomes occurring in any nucleus is a definite one, and \vla-n a deviation from the usual number is met with, it is due to some of the chromo- somes having remained united end to end. The smallest number of chromosome which has yet been found in the nuclei of vegetative cells of the more highly organised plants has been eight ; as a rule the number is larger, amounting often to several times this number.
A special type of nuclear division, to which the name of REDUCTION DIVISION is given,, is met with in those reproductive cells which start a new generation, such as the spore-mother-cells of the higher Cryptogams and Phanerogams. In the prophase of this division the chromosomes become united in pairs (Fig. 88, 1, 2), and there then occurs a mai'ked contraction of the nuclear contents, which is characteristic of this process of division and is called SYNAPSIS (3). After this the double chromosomes become again loosened out as a delicate double thread (4), which soon unites to a correspondingly stout thread, forming a loose skein (5). The doubled nature of this thread soon becomes recognisable again (6). The skein consisting of the as yet unbroken double thread now falls into segments (7), each of which corresponds to one paired chromosome. The number of these segments is half as great as the number of chromosomes in the tissue cells of the same plant, since two chromosomes are represented by each segment. The paired chromosomes become shorter and thicker and are distributed around the periphery of the nucleus ; this is the con- dition that has been termed diakinesis (8). At this stage kinoplasmic filaments are becoming applied to the nuclear membrane (8) ; the latter disappears and the nuclear spindle, which is at first multipolar (9), but ultimately becomes bipolar (10), originates from the kino- plasmic fibres. The paired chromosomes become attached to the fibres of the spindle and arranged in an equatorial nuclear plate (10). Shortly afterwards the separation of the chromosomes, until now united in pairs, takes place (11). In this process, in which the essential of the reduction division is effected, it is not longitudinal halves of chromosomes but entire chromosomes which separate from one another. The result of this is that each daughter nucleus receives only half as many chromosomes as were found in the tissue cells of the
MORPHOLOGY
85
IS
;. S8.— Dividing polleii-iiiother-cells of a Lily, somewhat diagrammatic. 1, Mother-cell with resting nucleus ; 2, the separation of the chromosomes ; 3, synapsis ; 4, double filament in process of fusion ; 5, spirem consisting of an apparently single filament derived from the fused double filament ; 6, reappearance of the longitudinal split, the spirem still unsegmented ; 7, spirem transversely segmented, into paired chromosomes ; 8, diakinesis ; 9, multipolar spindle ; 10, spindle of the mother-nucleus, the nuclear plate composed of paired chromosomes ; 11, re- duction division, the separating chromosomes showing partial separation of their longitudinal halves ; 12, young daughter nuclei ; 13, the longitudinal halves of the chromosomes (daughter chromosomes) are arranged in pairs on the nuclear spindles ; 14, the spindles of the daughter nuclei ; 15, separation of the daughter chromosomes ; 16, young stage of the grand-daughter nuclei, (x about SOO.)
86
BOTAXY
same plant. During their passage towards the poles of the spindle a longitudinal split can be detected in each chromosome. This split was indeed already complete in the prophase before the nuclear plate was formed, but was not followed as in an ordinary division by a separation of the halves. The two halves of each chromosome remain on the other hand in relation to one another and pass to the ? same daughter nucleus. The formation of the daughter nuclei is completed (12) as in an ordinary division, but following promptly on the first reduction division, which is also known as the HETEROTYPE division, comes a second or HOMOTYPE division (84). In this no new longitudinal splitting of the chromosomes takes place, but the two halves of each chromosome, which existed in the daughter nuclei, become separated from one another, and become the chromosomes of the grand-daughter nuclei.
The steps of this homotype division agree in other respects with those of an ordinary nuclear division, and will be clear from Fig. 88, 13-16. In 13 an early 'stage and in 14 the completed condition of the spindles of the dividing daughter nuclei are seen; 15 shows the division of the nuclear plate, and in 16 the young grand-daughter nuclei are completed. One of the characteristic features of the whole process is that the two divisions succeed one another immediately or very quickly. The heterotype and homotype nuclear divisions, which may together be termed the ALLOTYPIC division, may be con- trasted with the ordinary or typical nuclear division. At a particular stage of development corresponding phenomena to those of the allo- typic division are met with in animals as well as plants.
The smallest reduced number of chromosomes known for the nuclei of the
more highly organised plants is four, i.e. the half of the smallest number met with in the tissue cells.
In those lower Cryptogams, the nuclei of which possess an individualised centriole, the latter undergoes division into two at the be- ginning of karyokinesis. The two halves from one another (Fig. 89 c) and ultimately reach the jioints which will become the poles of the spindle. Round such cen- ~' ^LyT/'-v ;• trioles a definite portion of protoplasm form-
?~\ ing the centrosome is usually marked off,
and around this kinoplasmic radiations (kp) FIG. MI. — A nucleus of a vouii" plant of ,. v IITI. j.i j_- i
form an astrosphere. When the centnoles the Brown Seaweed, Fitcus serratus,
preparing to divide. The two centrioles have reached the poles the nuclear membrane (c), which have arisen l.y the division disappears, and spindle fibres appear in the of a sin-le one, have already separated nuclear cavity itself. These clearly proceed from one another;/.-?, radiations of the from the centrosomes and become atta.-h.-<i nbnllar plasma; s, chromosomes ; » _, 1.1.1
nucleolus (x 1000) to ^e chromosomes. The complete nuclear
spindle (Fig. 90) has a centrosome with kino- lasmatic radiations (kp] at each pole, but in other respects agrees with the spindles
,"•> v«. j j; » — - — —
* .- «' , -.'•>," , ginning
^.y: '..-.; * «p»»te
X-^ltfePRSr^? « ultimate!
^rV^ESag - the pole,
MORPHOLOGY
87
of the higher plants, from which centrioles are absent. The main features of
the division and the formation of the daughter nuclei are also similar in the two
cases. The centrioles persist in the daughter nuclei
and .divide into two on each subsequent nuclear
division ; the kinoplasmatic radiation around the
centrosome is, however, only present during the
karyokinetic process (M).
-kj, -C
Direct Nuclear Division. — In addition to the mitotic or indirect nuclear division there is also a DIRECT or AMITOTIC division, sometimes called FRAGMENTATION (Fig. 91). This may have been the original mode of nuclear division, and among the lowest organisms transitions between it and in- direct division are found. In the higher plants direct division of the nucleus is a reduced, or indeed a senile process which usually occurs in old cells, or in cells in which the cell contents become disorganised* shortly after the nuclear division.
Instructive examples of direct nuclear division are afforded by the long internodal cells of the Stoneworts (Characeae), and also
FIG. !K). — A nuclear spindle of a cell of a young plant of the Brown Seaweed Fm-us serrd.tvs with split chromosomes in tin- nuclear plate, c, Centrioles /, /i, radiations of fibrillar plasma astrospheres; sp, spindle fibres; 8, longitudinally divided chromosomes funning the nuclear plate, (x 1000.)
by the old internodal cells of Trades- cantia (Fig. 91). The direct nuclear divi- sion is chiefly a process of constriction which, however, need not result in new nuclei of equal size. In the case of the Stoneworts, after a remarkable increase in the size of the nucleus, several successive rapid divisions take place, so that a continuous row of bead-like nuclei results. The old internodal cells of Tradescantia (Fig. 91) very frequently show half-constricted nuclei of irregular shape. While in uninuclear cells indirect nuclear division is, as a rule, followed by cell division, this is not the case after direct nuclear division.
Cell Division. — In the uninuclear cells of the higher plants cell division FKI. 9i. -old cells from the stem of and nuclear division are, generally, closely virginica, showing associated. The supporting fibres of
pole persist as CONNECTING FIBRES between the developing daughter nuclei (Fig. 86, 90; Fig. 88, 12, 16),
nuclennpro, ,
BOTANY
Fio. 92. — Three stages in the division of a living cell of Epipactis paliistrls. (After TREUB, x 365.)
and their number is increased by the interposition of others (Fig. 86, 10). In consequence of this a barrel-shaped figure is formed, which eithe/ separates entirely from the developing daughter nuclei, or remains in connection with them by means of a peripheral sheath, the CONNECTING UTRICLE. The first is the case in cells rich in cyto- plasm, the latter when the cells are more abundantly supplied with
cell sap. At the same time the con- necting fibres become thickened (Fig. 86, 10) at the equatorial plane, and the short rod-shaped thickenings form what is known as the CELL PLATE. In the case of cells rich in proto- plasm or small in diameter, the con- necting fibres become more and more extended, and touch the cell wall at all points of the equatorial plane (Fig. 86, 11). The elements of the cell plate unite and form a cytoplasmic limiting layer, which then splits into two. In the plane of separation the new partition wall is formed of cell-wall substance, and thus SIMULTANEOUSLY divides the mother cell into two daughter cells (Fig. 86, 12). If, however, the mother cell has a large sap cavity, the connecting utricle cannot at once become so extended, and the partition wall is then formed SUCCESSIVELY (Fig. 92). In that case, the partition wall first commences to form at the point where the utricle is in contact with the side walls of the mother cell (Fig. 92 A}. The protoplasm then detaches itself from the part of the new wall in contact with the wall of the mother cell, and moves gradually across until the septum is completed (Fig. 92 B and 0) ; the new wall is thus built up by successive additions from the protoplasm. *
The connecting fibres of the spindle consist of kinoplasm. The origin of the cell plate from swellings of these fibres indicates its kinoplasmatic nature also. By the splitting of the cell plate the limiting layers of the two sister cells are com- pleted across the place of separation. The separation of the complex of connecting fibres into two halves effects an equal division of the kinoplasm between the two new cells.
In the Thallophytes, even in the case of uninuclear cells, the partition wall is not formed within connecting fibres, but arises either simultaneously from a previously formed cytoplasmic plate, or successively, by gradual projection inwards from the wall of the mother cell. It was a division process of this kind (Figs. 93, 94), first investigated in fresh-water Algae, that gave rise to the conception of cell division, which for a long time prevailed in both animal and vegetable histology. In this form of cell division the new wall com-
SECT. I
89
mences as a ring-like projection from the inside of the wall of the mother cell, and gradually pushing farther into the cell, finally extends
FIG. 93.— Cell of Spirogyra in division. FIG. 94. — Portion of a dividing cell of Clado-
n, One of the daughter nuclei ; w, de- phora fracta. w, Newly-forming partition
veloping partition wall ; ch, chloroplast wall ; eh, dividing chromatophore ; k,
pushed inward by the newly-forming nuclei, (x 600.) wall. (X 230.)
completely across it (Figs. 93, 94). In a division of this sort, in uninuclear cells, nuclear division precedes cell division, and the new wall is formed midway be- tween the daughter nuclei (Fig. 93). In the multinuclear cells of the Thallophytes, on the other hand, although the nuclear division does not differ from that of uninuclear cells, cell division (Fig. 94) is al- together independent of nuc- lear division. And in multi- nuclear, unicellular Thallo- phytes, nuclear division is not followed by a cell division.
The interdependence of nuclear and cell division in uninuclear cells is necessary to ensure a nucleus to each •>
daughter cell. In multi- -SffillS nuclear cells it is not essen- tial that cell division should always be accompanied by
nuclear division, as in any FIG. 95.— Portion of the peripheral protoplasm of the
Case a Sufficient number of embryo-sac of Keseda odorata, showing the commence-
T . .,, , , ,. ment of multicellular formation. The process pro-
nuclei will be left to each gresses from above downwards. (x 240.) daughter cell.
Free Nuclear Division and Multicellular Formation. — The nuclear division in
90
BOTANY
TAUT I
the multinuclear cells of the Thallophytes may serve as an example of free nuclear division, that is, of nuclear division unaccompanied by cell division. In plants with typical uninuclear cells, examples'of free nuclear division also occur ; although. in that case, the nuclear division is customarily followed by cell division. While the nuclei increase in number by repeated division, this process is not accompanied by a corresponding cell division. When, however, the number of nuclei is com- pleted, the cytoplasm between the nuclei divides simultaneously into as many portions as there are nuclei. In this process we have an example of multicellular formation. This method of development is especially instructive in the embryo- sac of Phanerogams, a cell, often of remarkable H/C and rapid growth, in which the future embryo is de- veloped. The nucleus of the rapidly growing em- bryo-sac divides, the two daughter nuclei again divide, their successors repeat the process, and so on, until at last thousands of nuclei are often formed. Xo cell division accom- panies these repeated nuc- lear divisions, but the nuclei lie scattered throughout the peripheral, cytoplasmic lining of the embryo-sac. When the embryo-sac ceases to en- large, the nuclei surround themselves with connect- ing strands, which then radiate from them in all
round the mass of cytoplasm ; in C this process is com- directions (Fig- ^5). Cell
plete, but the limiting layer produced by the fusion of radia- plates make their appear-
tions of the fibrillar plasma is still connected with the polar ance in these connecting
side of the nucleus ; in D this kinoplasniatie connection strands and from them between nucleus and limiting layer has disappeared ; *, n „ 11 ; T~ <-\ •
,'r. cell walls arise. In tins nuclear network ; n, nucleolus. (After HARPER, x 1500.)
manner the peripheral
protoplasm of the embryo-sac divides, simultaneously, into as many cells as there are nuclei. Various intermediate stages between simultaneous, multibellular forma- tion and successive cell division can often be observed in an embryo-sac. Win-re the embryo-sac is small and of slow growth, successive cell division takes place, so that multicellular formation may be regarded as but a shortened process of successive cell division, induced by an extremely, rapid increase in the size of the cell.
Free Cell Formation. — Cells produced by this process differ conspicuously from those formed by the usual mode of cell division, in that the free nuclear division is followed by the formation of cells, which have no contact with each other, and in the formation of which the whole of the cytoplasm of the mother cell is not used up. This process can be seen in the developing embryo of the Gymnosperms, in
Fio. 96.— Successive stages of the delimitation of a spore in the ascus of Erysiphe eommunis. A, Before delimitation has begun ; the tibrillar plasma (kp) radiates into the cytoplasm around; in B the fibrillar plasma has commenced to grow
SECT, i MORPHOLOGY 91
Ephedra, for example, and also in the formation of the spores of the Ascomycetes. A single nucleus is present to begin with in each ascus of the Ascomycetes. By successive divisions eight nuclei lying free in the cytoplasm are derived from this. A definite portion of cytoplasm around each of these nuclei becomes limited by a peripheral layer which then forms a cell wall. Thus eight separate spores arise (cf. Fig. 327). As the researches of Harper (86) have shown, the formation of the peri- pheral layer proceeds from a centrosome-like mass of kinoplasm which formed a pole of the spindle in the preceding nuclear division. The nucleus is drawn out towards this mass of kinoplasm. From the latter kinoplasmic radiations proceed which surround the spore as it becomes delimited, and finally fuse to form its peripheral layer (Fig. 96).
Cell - Budding. — This is simply a special variety of ordinary cell division, in which the cell is not divided in the middle, but, instead, pushes out a protuberance which, by constriction, becomes separated from the mother cell. This mode of cell multiplication is characteristic of the Yeast plant (Fig. 2, p. 11) ; and the spores, known .as conidia, which are produced by numerous Fungi, have a similar origin (Fig. 353).
Conjugation (87). — A sexual cell is, with few exceptions, only able to continue its development after fusion with another sexual cell. The two cells so uniting are either alike, and in that case are called GAMETES, or unlike, and are then dis- tinguished as EGG and SPERMATOZOID. The spermatozoid is the male, the egg the female sexual cell. The gametes may be motile (Fig. 97 B) or non-motile. The motile gametes frequently resemble the swarm-spores (Fig. 97 A) generated by the same parent for the purpose of asexual reproduction. As a rule, however, they are smaller than the swarm-spores, and have usually only half as many cilia. In the more highly specialised sexual cells the egg usually retains the structure of an embryonic cell, but the spermatozoid undergoes profound modifications. A cytoplasmic cell body, a nucleus, and the rudiments of chromatophores are always present in the egg. The spermatozoid (Fig. 98), on the other hand, becomes transformed, in the more extreme cases, into a spirally twisted body, provided with cilia, and exhibiting an apparently homogeneous structure. Only a knowledge of the history of its development, and the greatest care in hardening and staining, have rendered it possible to recognise the homology of the structure of such a spermatozoid with that of an embryonic cell. It has been shown that the hinder part of its spiral body corresponds to the cell nucleus (k), the anterior, together with the cilia, to the cytoplasm, especially the kinoplasm (c), and the vesicle (I), at the other extremity, to the sap cavity of a cell (8tJ).
Motile male cells provided with cilia occur only in the Cryptogams and, as has been recently demonstrated (89), in some Gymnosperms (Cycadaceae, Ginkgo). In the Cryptogams the spermatozoids are set free from the sexual organs and require water for their dispersal. They reach the egg-cell, which usually remains in its place of origin, by swimming. In the Gymnosperms, which form motile spermato- zoids, the latter are brought near to the ovum by means of the pollen tube developed from the pollen grain. In a similar way the non-motile male cells of the other Gymnosperms and the Angiosperms are conducted to the egg through the pollen tube (Fig. 99). In the union of the two sexual cells in the act of fertilisation, the egg nucleus (ek) and the sperm nucleus (sk) fuse and form the nucleus of the fertilised egg-cell. The cytoplasm of the male cell also commingles with that of the female cell, but the chromatophores of the embryo are derived from the egg-cell alone. When the spermatozoid, as in animals and in Thallophytes, is provided with a centrosome, this does not fuse with the centrosome belonging to the ovum. The
92
BOTANY
centrosome of the fertilised egg- cell appears to be derived from that of the .s[>ermatozoid only.
FIG. 97. — A, An asexual swarm-spore of Ulothrix zonata ; B, \, a gamete ; 2 and 3, conjugating gametes ; 4, zygote, formed by the fusion of two gametes, (x 500.)
FIG. 98. — A, Spermatozoid of Cham fruijilis ; B, Spennatozoid of the Fern, Onodeastruthiopteris. (After SHAW.) The nucleus (k) is more deeply shaded ; c, the cytoplasmic portion ; c7, cilia which in B arise from the tinner edge of the spiral band ; 6, vesicle. (A x 540, JB X 850.)
Kio. 00.— Fertilisation of a phanerogamic Angio- sperm, somewhat diagrammatic. .-1, End of pollen tube ; in it the generative cells gz, each of which contains a sperm nucleus : /•/., til-- vegetative nucleus in process of dissolution. B-D, Egg in successive stages of fertilisation, — B, showing the generative cell with its sperm nucleus, nk, penetrating the egg; syn, the degenerating synergidae; C, the union of sperm nucleus, sk, and egg nucleus, ek ; D, the germ nucleus, kk, resulting from the fusion of the sperm and egg nuclei; cJi, rudiments of chromatophorcs. ( x about 500.)
The nuclei of the sexual cells always possess only the reduced number of chromosomes. By fertilisation the number becomes doubled, and after a shorter or longer period is again halved in the reduction division.
MORPHOLOGY 93
Although it is by fertilisation that the further development of the sexual products is made possible, there are exceptional cases in which a gamete or an egg forms an embryo without being fertilised. If development follows in this way from a gamete or egg with the reduced number of chromosomes the phenomenon is called PARTHENOGENESIS. On the other hand the reduction of the number of chromosomes may be suppressed in the development of the egg so that the full number necessary for further development is already present in it. In this case its development is not true parthenogenesis but a vegetative production of an embryo, which has come about through loss of sexual- ity and falls under the conception of APOGAMY (go). The cases of production of embryos from unfertilised eggs among the higher plants belong to the latter category : this is known in Antennaria alpina (91), Alchemilla (92), Thalictrum purpurascens (93), Taraxncum (w), Hieracia (95). In lower plants, in some Fungi, e.g. Saprolegnieae, and in one of the Characeae (Chara crinita) (M) true partheno- genesis is found. In Marsilia (97), one of the Hydro- FJG 10o.— Chlorophyll pterideae, the segmentation and further development of grains from the leaf of unfertilised eggs has also been observed. Funaria hygrmnetriva,
resting, and in process
Multiplication of the Chromatophores. — This of division, smaii m- is accomplished by a direct division, as a result eluded stan* grains are
r » . '. present in the grains.
of which, by a process of constriction, a chro- (X 540.) matophore becomes divided into nearly equal halves. The stages of this division may best be observed in the chloroplasts (Fig. 100).
B. Cell Fusions
The connection of the living protoplasts with one another is less complete in plants than in the animal body. This is evident from the existence of the cell wall which surrounds the vegetable protoplast. Recent researches have, however, shown (9S) that the protoplasts of the plant are united together by extremely fine cytoplasmic filaments, which proceed from the boundary layer of the cytoplasm. Such filaments are mostly confined to the pit-membrane (Fig. 101), but may also penetrate the whole thickness of the cell wall (Fig. 102). The existence of these connecting filaments of living substance between the protoplasts confers an organic unity on the whole body of the plant.
The members which make up the sieve-vessels, or, as they are commonly called, the sieve-tubes, are united by thicker strands of cytoplasm, which facilitate the transfer of substances through the tube. The transverse walls traversed by these strands of cytoplasm have been referred to above (p. 67) as sieve-plates. The pores attain their greatest diameter in some Angiosperms (Fig. 103). It is worthy of special note that, despite the fact that the nuclei of the sieve-tube segments disintegrate, the cytoplasm, which lines the
94
BOTANY
I'AllT 1
wall, remains alive. The walls of sieve-tubes are always unliguitied. Their sap-cavities contain a watery, and more or less dilute, solution of albuminous substances, and small starch grains are also present in most cases. As a rule, the sieve-tubes remain functional only through one or a few vegetative periods. Before their activity ceases the sieve-plates become covered (Fig. 103 C) with highly refractive callus (p. 69).
A more complete fusion of protoplasts is found in the formation of laticiferous vessels. These have the same structure and contents
-
~"
FIG. 101. — A cell from the cortex of the Mistletoe (Viseum album) ; the protoplast has been properly fixed and stained and the wall (m) swollen. The pit-membranes (x) are traversed by connecting threads ; ck, chloroplasts ; n, nucleus, (x 1000.)
, -• .
i pi
'
.»
B
Flo. 10:.'.— A, A swollen portion of cell wall from tin- endosperm ol tlie Vegetable Ivory Palm (1'hyteli ill/a* //m.-,v,r,./y><i). At t, s, simple pits tilled with cytoplasm ; in the intervening pit-membrane are fine connect- ing threads ; pi, other threads traversing the whole thickness ol the wall, (x 375.) li, The contents of two opposed pits and the connecting threads of the pit -membrane. (X 1500). C, The opening of a pit ami tin- connecting threads ot the pit-membrane viewed from the surface, (x 1500.)
as latex cells (p. 72). Their occurrence, like that of latex cells, is limited to a few distinct plant families, such as the Papaveraceae, of which the Poppy (Paparer) or Celandine (Chelidonium), with its characteristic orange-coloured "sap," are familiar examples, or the Composited, of which in particular the Lettuce (Lactuca) may be cited. Latex vessels are distinguished from latex cells only by the method of their development, which has resulted from the fusion of rows of elongated cells, the separating transverse walls of which have become more or less completely absorbed. Such vessels as a rule possess lateral branches, which, on meeting with one another, fuse by the absorption of their end walls (Fig. 104). The mucilage tubes (61) which are found in many Monocotyledons present points of resemblance to the laticiferous vessels. Their
MORPHOLOGY
95
mucilaginous sap contains albumen, starch, glucose, and tannins in addition to inorganic compounds.
A cell fusion also takes place in the formation of VESSELS or TRACHEA, but it should not be considered as a union between living cell bodies, but merely as one between cell cavities. The mature vessels are dead tubes serving for water conduction. They are formed from rows of cells, the lateral walls of which are peculiarly marked by spiral or reticulate thickenings, or, as is more frequently the case, by bordered pits, while the transverse walls become more or less completely absorbed. In cases where the transverse walls are
FIG. 103.— Parts of sieve-tubes of Cucurbita Ptpo, hardened in alcohol. A, Surface view of a sieve- plate ; JJ, C, longitudinal sections, showing segments of sieve-tubes ; D, contents of two sieve- tube segments, after treatment with sulphuric acid ; s, companion cells ; it, albuminous con- tents : pr, peripheral cytoplasm ; <:, callus plate ; c*, small, lateral sieve-pit, with callus plate ( X 540.)
at right angles to the side walls, they usually become perforated by a single large round opening while the remains of the wall forms a thickening ring (Fig. 73 C). When the transverse walls are oblique, they are perforated by several openings, between which portions of the wall remain, like rungs of a ladder (Fig. 105 q). According to the mode of their wall thickening, vessels are distinguished as SPIRAL, RETICULATE, or PITTED. When the transversely elongated pits of a vessel are arranged in more or less parallel rows (Fig. 105), it is called a SCALARIFORM VESSEL. The thickening of the vessel walls is always lignified. The living contents of the cells, after the perforation of the transverse walls, become completely absorbed, and the fully formed vessels or tracheae contain only water and a limited amount of air.
96
BOTANY
There is no difference between vasit'orm tracheides and vessels other than that the former are single elongated cells, and the latter fused cell rows. Generally speaking, tracheides are formed in parts of plants still in process of elongation, vessels in parts where growth in length has already ceased. True vessels make their first appearance in some of the Ferns, for instance, in the common Bracken (Pteris aquilina). In the main, despite the name Vascular Cryptogams, Ferns have only vasiform tracheids. Even in the Gymuosperms the Gnetaceae are the only family regularly provided with vessels. It is in the Angiosperms that vessels first become of frequent occurrence. Vessels are not of an unlimited length. A few plants, however, such as the Oak, and especially climbing woody plants, or
Km. 104. — Tangential section through the periphery of the stem of Scorzonera hispanica, showing re- ticulately united latex vessels. (x 240.)
Fi<;. 105.— Lower third of a scalari- form vessel from the rhizome of the common Bracken KITH. J'ttrif miiiiii mi. t, Transversely elongated pits in the lateral walls ; q, scalari form perforations of the terminal wall. (After DK BABY, x >.>'•.)
Lianes, have vessels several metres long ; but, as a rule, their length is not more than a metre, and in plants the woody portion of which conducts \\ater only by vessels, the vessels have an average length of only ten centimetres. The length of an individual vessel is denned by the presence of transverse walls, which are not perforated but bear bordered pits.
A similar fusion to that seen above to occur in the formation of laticiferous vessels is also met with in fungal hyphse ; by an absorp- tion of a part of the wall where two branches come into contact, their protoplasmic contents unite. A still more complete fusion is exhibited by the naked amoebae of a Myxomycete in forming the plasmodium
MORPHOLOGY
97
(p. 56). The fusion of the sexual cells in the process of fertilisa- tion possesses special characteristics which place the process in a distinct category.
Tissues (")
Origin and General Properties of Tissues. — A continuous aggregation of cells in intimate union is called a tissue. The origin of vegetable tissues is, in general, attribut- able to cell division. It is only in the Fungi and Siphoneae that a tissue arises through the interweaving of tubular cells or cell filaments (Fig. 106). In such cases, where the filaments are so closely inter- woven as to form a compact mass of cells, the tissue thus formed has the same appearance as the tissues of higher plants (Fig. 107).
The mutual interdependence of the cells of a tissue is manifested both by the con- Jfi junction of their pits (Figs. 66, 70, 71), " and by the general similarity of their wall thickenings.
The cells in a tissue may either fit no. 100.— Longitudinal section of closely together (Fig. 70), leaving no open- the stalk of the fructification of ings or spaces, or so-called INTERCELLULAR
SPACES may be left between the individual cells. Where cell fila- ments are interwoven into a tissue, the intercellular spaces are repre- sented by the openings left between the loosely-woven filaments (Fig. 106). In tissues resulting from cell division the intercellular spaces arise subsequently, as the partition wall between two cells formed by cell division is originally a simple membrane.
Such a partition wall may ultimately split and so give rise to intercellular spaces, but this only occurs after it has been thickened. The cause of such splitting is to be found in the hydrostatic pressure existing within the cells, and their con- sequent tendency to assume a spherical shape. The formation of intercellular spaces commences, therefore, at the cell corners, where the primary wall, consisting of pectinose material, becomes swollen.
The simplest and at the same time most frequent intercellular spaces are triangular or quadrangular in outline, as seen in cross- section (Figs. 66 i, '74 i). In cases where special portions of adjoining cells are in extremely energetic growth, intercellular chambers and passages, of more or less irregular shape, may be formed between them. If the growth of adjoining cells is very unequal, it may lead to a complete separation of their cell walls ; or the cells, or even a whole system of tissues, may be stretched and
H
98 BOTANY PART i
torn apart. It is by such a process that hollow stems are formed.
Intercellular spaces arising from a splitting of adjoining cell walls are termed SCHIZOGENIC ; those formed by tearing or dissolution of the cells themselves are called LYSIGENIC INTERCELLULAR spaces. In many cases intercellular spaces that begin as schizogenic are increased in size by de- struction of surrounding cells. Most inter- cellular spaces contain only air, although in special instances they may contain water or excreted products, such as gum, mucilage, resin, or ethereal oils, and in other still
FIG. io7.-Transverse section of rarer cases latex. Schizogenic intercellu-
the sclerotium of Claviceps •, 11 .en j -ii. • i
purpurea. (x 300.) lar spaces are usually filled with air, and
form the ventilating system of the plants,
while the lysigenic spaces often contain either water or secretion products.
Of the schizogenic intercellular spaces, those filled with ethereal oils or resin, on account of their frequency, should be particularly noticed. Short cavities and longer passages, or ducts, containing ethereal oils, are to he found in the stems, roots, and leaves of numerous plant families. The Umbelliferae are especially rich in these, and the oil-ducts form the characteristic markings (vitte) on their fruits. The Conifers are especially characterised by resin-ducts (Fig. 141 A, /t), which, even during their formation by the separation of the cell walls, seem to lill with the excretion from the cells. The enlargement of such intercellular spaces is accompanied by a division of the surrounding cells, the number of which is thus correspondingly increased. The cells themselves remain thin-walled, and in close contact laterally, but bulge out somewhat into the ducts. Lysigenic intercellular spaces, acting as receptacles for secretions, have the appearance of irregular cavities in the tissue. Where they contain oil or resin, they develop from a group of cells in which these substances appear in the form of drops. The cell group then becomes disorganised by the gradual absorption of the cell walls. In this way are formed the receptacles filled with ethereal oils, in the Orange and Lemon. The formation of the so-called resin-galls, in the case of coniferous trees, is preceded by the formation of abnormal tissues, which afterwards become converted into resin. Such was also the origin of amber, which is the fossil resin of the Amber-fir (Picea succinifera). The formation of gum in lysigenic gum cavities is due to the modification of the cell walls, and either normal tissues participate in this process, as in the case of the gum-arabic of the Acacia, or abnormal tissues are first developed and then transformed into gum, as, for example, the gum on Cherry trees. Latex does not occur in lysigenic intercellular spaces.
The separating walls resulting from cell division are simple lamellaj. In tissues which have arisen by cell division these lamellae are common to the cells they separate. That part of the partition wall between two cells which stands out so distinctly in a cross-section does not. consist of the original primary cell wall alone. It is made up of both the primary wall and the primary thickening layers. The former is called the MIDDLE LAMELLA (Figs. 66 m, 70 m). In soft tissues the middle lamella, according to DEVAUX (10°), is composed for the mo>t part of pectic substances ; in woody and corky tissues it is also lignified. By
MORPHOLOGY 99
boiling soft tissues in water, the cells may often be easily isolated through the consequent swelling and dissolutipn of the middle lamella. In ripe fruit, an isolation of the cells frequently takes place spontaneously, through the dissolution of the middle lamella. A lignified middle lamella, on the other hand, seems able to withstand more effectually the action of oxidising agents. Consequently, it is possible, by subjecting a section of pine -wood to the action of SCHULZE'S MACEKATING MIXTURE (potassium chlorate and nitric acid), and subsequently treating with concentrated sulphuric acid, to remove all secondary and tertiary thickening layers, so that only the middle lamellae remain as a delicate network. If the macerating process be continued for a longer time, without the subsequent treatment with sulphuric acid, the middle lamellae become finally dissolved. The thickening layer will then be left free from all lignification. SCHULZE'S macerating method may accordingly be employed to isolate the elements of lignified tissues. The peculiar relation of the middle lamella towards chemical reagents gave rise at one time to the presumption of a special intercellular substance which, like a glue, bound together the cells of a vegetable tissue. The supplementary deposition of pectic compounds in the middle lamellae (p. 69) frequently gives rise to the formation of rod -like protuberances and excrescences, which project into the intercellular spaces, or these spaces may be filled up by the formation of gussets (Fig. 70 C, m*).
Kinds of Tissue. — The mature tissues of the plant may be divided into two groups, PARENCHYMA and PROSENCHYMA, between which, how- ever, no sharp distinction can be made. A typically developed paren- chymatous tissue is one in which the thin-walled cells are equally expanded in all directions, and possess a layer of protoplasm lining the wall, and other cell contents. Typical prosenchymatous tissue, on the other hand, consists of thick-walled, elongated cells, either in the form of fibres or spindle-shaped cells, with interlocking, pointed ends, and with little or no protoplasmic contents. A parenchymatous tissue, in which the cells are thick-walled and elongated, resembles prosenchyma, but may be distinguished from it by the absence of pointed cell terminations, and especially by the greater abundance of protoplasm. Thin- walled prosenchyma is not, on the other hand, necessarily lacking in protoplasm, but is characterised by its pointed and interlocking cells.
An undifferentiated tissue, the cells of which are still capable of division, is termed embryonic tissue, or MERISTEM. The meristem of embryonic rudiments and of the growing point is called PROMERISTEM, and all meristematic tissue which can be shown to have been derived directly from such promeristem is termed PRIMARY. A primary meristem may still retain its meristematic character in the midst of a mature tissue. Fully differentiated tissue is designated PERMANENT tissue, or somatic tissue, in contrast to meristematic tissue. At times, permanent tissue may again become capable of division, and thus give rise to a SECONDARY MERISTEM.
A mass of tissue so united in the body of a plant as to form a higher histological unit constitutes a tissue system. In the more
100 BOTANY PART i
highly organised plants three such systems may be distinguished — the
TEGUMENTARY SYSTEM, the VASCULAR BUNDLE SYSTEM, and the FUNDAMENTAL TISSUE SYSTEM.
The tissues which make up the different tissue systems are distinguished as PRIMARY and SECONDARY, according to their origin. The primary are those which are derived from the pro- meristem and primary meristems before growth in length has been completed ; the secondary are those which owe their origin to the primary or secondary meristems after growth in length is finished.
The primary constituents of the tissue systems will be considered first.
A. The Primary Tissues
The Tegumentary System. — In the Pteridophytes and Phanero- gams the plant body is covered by a distinct outer layer of cells or EPIDERMIS; this is wanting in plants of the lower groups. A typically developed epidermis consists of one layer of cells (Fig. 74 e), the outer walls of which on the free surfaces of the plant are more strongly thickened. This is especially the case in all aerial parts of plants adapted for a long life, but on the more perishable parts of a plant, such as the floral leaves, or on subterranean parts such as the roots, the cells of the epidermal layer are generally thin-walled or only slightly thickened. For such reasons the epidermis of the root was formerly termed the epiblem. Even when the external walls of the epidermal cells are considerably thickened, the side walls, at least in part, remain unthickened. The external walls are also more or less cuticularised, while their outermost layer, which is more decidedly cuticularised and capable of withstanding even the action of concentrated sulphuric acid, extends as a CUTICLE continuously over the surface of the epidermis. This cuticle appears to be always wanting from the epidermis of roots (101). The cuticle has its origin in the primary walls of the younger epidermal cells, which, during the increase in size of the plant, grow in area, and at the same time are strengthened by the deposition of cutin. The cuticle of leaves in the tropics is often specially smooth and shining. It reflects a portion of the sunlight, giving rise to the sparkling appearance characteristic of tropical foliage, and serves to protect the plant from too strong insolation. The cuticle frequently becomes folded, and so assumes a striated appearance (Fig. 114). By the thickening and the cutinisation of their outer walls the mechanical rigidity of the epi- dermal cells is increased, and the loss of water by transpiration is lessened. Plants in dry climates, or so situated that, for any reason, transpiration from their outer surfaces must be diminished, are char- acterised by the greatly thickened and cuticularised walls of their epi- dermal cells. In some of the Gramineae, Equisetaceae, and many other
MORPHOLOGY
101
plants, the cell walls of the epidermis are silicified. In the Equisetaceae the impregnation with silica is so considerable that these plants are used for polishing. Heating, even to redness, does not destroy the structure of such silicified epidermal cells.
Deposits of wax are also present in the cutinised layers of the epidermis, and consequently water will flow off the epidermis without wetting it. The wax is sometimes spread over the surface of the cuticle as a wax covering. This is the case in most fruits, where, as is so noticeable on plums, it forms the so-called bloom.
Fio. 108.— Transverse section of a node of the sugar-cane, Saccharum oflleinaru,m, showing wax- incrustation in the form of small rods, (x 540.)
The wax coverings may consist of grains, small rods (Fig. 108), or crusts.
The wax deposits attain their greatest thickness on the leaves of some of the Palms ; on the Peruvian Wax Palm, Geroxylon andicola, the wax covering is more than 5 mm. thick. This wax, as well as that obtained from the fruit of Myrica cerifcra, is known as vegetable wax, and possesses an economic value. The wax incrustations may be melted by heat ; they are soluble in ether and in hot alcohol. In many cases, in place of the wax coverings, small grains and scales of a fatty substance, which is soluble even in cold alcohol, are excreted. The dusty cover- ings thus formed appear either mealy white or golden yellow, and are the cause of the striking appearance of the Gold and Silver Ferns, especially in species of Gymnogramme.
In many cases, slimy or sticky excretions are produced between the thickening layers of the epidermis and the cuticle ; these press up the latter and finally burst it. Such excreting surfaces often occur on bud-scales. Sticky zones are frequently formed on stems, as in the case of Lychnis viscaria and other Sileneae, as a means of protection to the flowers higher on the stem from undesirable visitors. Small creeping insects, which would otherwise rob the flowers of their honey,
102
BOTANY
Fid. 109. — Surface view of the epidermis from the upper side of a leaf of Mercuri- alis perennis. (x 300.)
seem as little able to pass beyond such a sticky zone, as other larger animals to surmount the rings of tar often placed around the trunks of fruit trees for a similar protective purpose. Excreting epidermal surfaces form also the nectaries of flowers, which by means of their
sweet secretions attract such animals, generally insects, as are instrumental in their pollination.
The cells of the epidermis are in uninterrupted contact with each other, and as a rule have undulating side walls (Fig. 109). The protoplasm of epidermal cells generally appears to be reduced to a thin, peripheral layer, and the sap cavities filled with a colourless or coloured sap. Around their nuclei cluster the colourless rudi- ments of the chromatophores, showing that, although exposed to the light, their further development into chloroplasts may cease in cells not destined to take part in the assimilatory processes. Such epidermal cells with undeveloped chromatophores, besides acting as an external protection, serve as water-reservoirs; their side walls, by means of folds in theunthickened parts, can expand and collapse as a bellows, according to the varia- tions in their supply of water. In plants which grow in shade chlorophyll is usually pre- sent in the epidermal cells. The external walls of the epidermal cells of this upper surface of foliage leaves not uncommonly project or exhibit a lens- shaped thickening in the middle (Fig. 75) ; this ar- rangement serves to con- centrate the rays of light. The refractivity of the lens-shaped thickening is rendered greater by pec- tinisation, ctitinisation, silicification, or waxy secretions. In the cases where the optical apparatus is formed by a bulging of the uniformly thickened wall, the cell-sap containing much tannin serves as the refractive medium (1C2). The formation of stomata (103) in the epidermis is characteristic of all parts of the more highly-developed plants which are exposed to the air. Each stoma consists of an intercellular passage perforating the epidermis and bounded by two elliptical epidermal cells, termed
FIG. 110. — Epidermis with stomata from the lower surface of the leaf of Helleborvs niger. (x 120.)
MORPHOLOGY
103
GUARD-CELLS (Figs. 110, 111 A). The guard-cells always contain chloroplasts, and are also characterised by their peculiarly thickened walls. These, as is best seen in transverse sections, form ridge-like protuberances projecting above and below from the sides of the
Fid. 111. — Epidermis from the under side of a leaf of Tradescantia virginiai. A, In surface view ; B, in transverse section ; I, colourless rudiments of chromatophores surrounding the nucleus. (X 240.)
guard-cells adjoining the air-passage (Fig. Ill B). Midway between the projecting ridges, on the other hand, the walls of the guard-cells remain unthickened (Fig. 112).
The unthickened parts of the walls of guard-cells jut out into the pore (Figs. Ill £, 112), and thus facilitate its closing. In addition, the external thickened
FIG. 112. — Transverse section of the epidermis of Aloe nigricans. i, Inner, uncutinised thickening
layer, (x 240.)
walls of the two adjacent epidermal cells become, in some cases, suddenly thin on approaching the guard-cells. By this means a hinge-like connection is formed which renders the guard-cells more or less independent of the other epidermal cells. At other times the same result is accomplished by raising the stomata above the epidermis, or, more frequently, by sinking them below the less thickened epidermal
104 BOTANY PAHT i
walls. The elevation serves to increase the evaporation, and is on this account met with in Ferns which grow in damp situations. The depression diminishes the transpiration by creating a still atmosphere above the guard-cells. On this account it is met with in plants of dry regions (xerophytes). Frequently the epidermal cells adjoining the guard-cells are less thickened or lower than the other cells of the epidermis (Fig. 111). Such special epidermal cells are called SUBSIDIARY CELLS, and have the same use as the hinge joint mentioned above.
The stomata are formed by the division of a young epidermal cell into two cells of unequal size, one of which, the smaller and more abundantly supplied with protoplasm, becomes the stoma mother-cell ; while the larger, containing loss protoplasm, usually forms an ordinary epidermal cell. The stoma mother-cell becomes elliptical in outline and divides again, by a vertical wall, into the two guard-cells, between which, by a splitting of the wall, the intercellular passage (pore) is formed. Before the formation of the definitive stoma mother -cell, succes- sive divisions of the young epidermal cell often occur ; in such cases the finally developed stoma is generally surrounded by subsidiary cells.
Stomata are chiefly developed on the green parts of plants, but are sometimes found even on the coloured floral leaves. They are naturally found in greatest numbers on the leaves, as it is there that they are most needed to facilitate the interchange of gases in the processes of assimilation. In dorsiventral leaves the stomata occur, for the most part, if not exclusively, on the under surface, and average about 100 to the square millimetre, although in some plants their number may reach 700. Leaves which are alike on both sides have their stomata equally distributed on the two surfaces. Floating leaves of aquatic plants have stomata only on the side exposed to the air. In some cases, as in the Oleander (Nerium Oleander), numerous stomata are situated together in pit-like depressions of the under surfaces of the leaves. In the tissue directly under each stoma there is always a large intercellular space, termed the RESPIRATORY CAVITY (Fig. Ill B), which is in direct communication with the other intercellular spaces extending throughout the leaf tissue. In plants grown in abundance of moisture, these intercellular spaces are usually larger than in the case of plants growing in drier situations.
In contrast to the stomata, which as air-pores serve for the interchange of gases, a few plants also possess WATER-STOMATA or WATER-PORES, situated at the ends of the veins or nerves of the leaves. These pores serve as organs for the discharge of water or watery solutions. Calcium carbonate, in solution, is frequently excreted in this way, and in many species of Sdxifraga it forms white scales on the margins of the leaves. Although water-pores may often be found at the apices and tips of the marginal teeth of young leaves, they seem to dry up as the leaves become more mature. The guard-cells of water-stomata usually lose their living contents early, and thus the passage between them remains continually open. The water-stomata (Fig. 113) are always larger than the air-stomata.
MORPHOLOGY
105
Fid. 118.— Water-pore from the margin of a leaf of Tropaeolum majus, with surrounding epidermal cells, (x 240.)
Although submerged leaves of aquatic plants are devoid of air-
stomata, water-stomata often occur on them.
Hairs or TRICHOMES and EMERGENCES are characteristic of the
tegumentary system. The cells of the hairs may retain their living
contents, or die and become
filled with air ; in the latter
case the hairs appear white.
The simplest form of hairs are
the PAPILLAE, which are merely
epidermal cells, the external
walls of which have protruded
in a conical form. Papillae are
often developed on the petals
of flowers, and are the cause
of their velvety appearance
(Fig. 1 14). Longer hairs, such
as the root hairs (Fig. 170 r),
are also prolongations of single
epidermal cells ; these are
characteristic of definite
regions of the root, and only
a limited portion of the outer
wall of the epidermal cell
protrudes as a hair. The woolly hairs found in young buds are
generally similarly prolonged . epidermal cells which, as a protec-
tive covering, surround the young growing tissues and some-
times remain on fully developed parts of plants to protect them
against too rapid evaporation and direct insolation. The hairs developed from some of the epidermal cells of the seed coats of various species of Gossy- pium attain an unusual length, and supply the cotton of com- merce (Fig. 115). These cotton
hairs are Sometimes 6 Cm. long, an(j in ^^ fu|iy developed State . • . i • n n
contain only air ; their cell walls are thicker than those of ordinary hairs, and covered with a delicate cuticle. They are usually some- what flattened and at the same time twisted ; and are wider in the middle than at either end. Only a certain number of the epidermal cells of the seed grow out as cotton hairs (Fig. 115 BJ. BRISTLES are short, pointed hairs, in the thickened cell walls of which calcium carbonate or silica has been deposited (Fig. 116, below, to the right). The STINGING HAIRS (Fig. 116), such as those of Nettles (Urtica)
FIG. 114.— Surface of the upper epidermis of a petal of Viola tricolor, showing ridge-like projections from the lateral walls, and protruding papillae.
(x 250 )
106
BOTANY
and of the Loasaceae, are special forms of bristles, and arise as prolongations of single epidermal cells. These, however, swell in the course of their development, and becoming surrounded by adjoining epidermal cells present the appearance of being set in sockets ; while, at the same time, by the multiplication of the cells
FIG. 115. — Seed-hairs of the cotton, Gossypium her- baceum. A, Part of seed-coat with hairs (x 8) BI insertion and lower part, B% middle part, and •83 upper part, of a hair, (x 300.)
Fio. 116.— Stinging hair of Urtica dioica, with a portion of the epi- dermis, ami, to the right, a small bristle, (x (X).)
in the tissue at their base, the whole hair becomes elevated on a column-like protuberance. The hair tapers towards the apex and terminates, somewhat obliquely, in a small head, just below which the wall of the hair remains unthickened. As the wall of the hair is silicified at the end and calcified for the rest of its length, the whole hair is therefore- extremely stiff. Such hairs furnish a means of defence against animals. The heads break off at the slightest touch, and the hairs piercing the skin pour out their poisonous contents,
MORPHOLOGY
107
FIG. 117. — Glandular hair from the petiole of Primula sinensis. (After DE BARY, x 142.)
which, especially in the case of some tropical nettles, may cause
severe inflammation.
The cells surrounding the base of a hair are often arranged in
a ring or in radiating lines, or are otherwise different from the
surrounding epidermal cells. Such cells may
be called subsidiary cells to the hairs.
UNICELLULAR HAIRS, such as we have
so far considered, may terminate in well- defined heads resulting from the swelling of
their tips, or their side walls may develop
irregular excrescences ; on the other hand,
they may remain short and expanded like a
balloon, or remain close to the surface of
the epidermis as spindle-shaped or stellate
hairs. MuLTlCELLULAR HAIRS may be
merely simple rows of similar cells, as the
hairs on the stamens of Tradescantia (Fig.
60) ; or their terminal cells may become
swollen into globular heads (Fig. 117), like
those on the Chinese Primrose (Primula
sinensis) ; or an epidermis may be covered
with disc-, star-, or bowl-shaped hairs (Fig. 118). Sometimes the
hairs become variously branched, lose their living contents, and form
a silky or woolly protective covering similar to that formed by uni- cellular hairs. In special cases, as in the scale hairs of Ferns, they may even have the shape of a small leaf.
EMERGENCES, unlike hairs, are not formed solely by epidermal cells, but a number of cells, lying more or less deeply in the sub-epidermal tissues, also take part in their formation. Thus, for example, while only a few rows of sub-epidermal cells enter into the formation of the emergences
FIG. 118,-Gianduiar scale from the (Fig. 119) on the margins of the stipules of female inflorescence of the Hop, the Pansy (Viola tricolor}, much deeper- lying
Humvlus Lupulus, in vertical tisgue participates in the development of
section. A, before, B, after the
cuticle has become distended by the emergences which, as PRICKLES, serve the excretion, in B the ex- jn tne case of Roses as a means of protection,
cretion has been removed by j ,1 , • r . ,
alcohol. (After DE BARY, x H2) and at the same time are of assistance in climbing. The thick emergences, which
spring from the roots of the Podostemaceae, and serve to attach them to rocks, are parenchymatous throughout, but vascular bundles may be included within emergences, as is well shown in the club- shaped digestive glands or tentacles (Fig. 120) on the leaves of the Sundew (Drosera). Some emergences resemble in structure certain
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of the metamorphosed members of the plant body described in the preceding chapter ; the resemblance between prickles and thorns, and between haptera and lateral roots will serve as examples. They are not, however, to be traced back in origin to such members (p. 50). Both hairs and emergences frequently act as secreting organs, and are then termed GLANDS. In many cases they are concerned with the active exudation, and at times also the absorption of water. They then belong to the class of organs designated HYDA- THODES (104) by HABERLANDT. Other glandular hairs excrete 'a resinous substance. The hairs of Primula sinensis (Fig. 117) are in reality such glands, in which the cuticle of the terminal globular head is pressed away from the cell wall by the resinous matter excreted from the hair, until finally the
Fir.. 110.— Glandular colleter bulging CUticle IS ruptured from a stipule of Viola trl- an(j the resinOUS Secretion color, showing also a uni- . .
cellular hair, (x 240.) exudes. 1 he hairs of this and other species of Pri- mula (especially P. obconica) are capable of excit- ing inflammation in the skin of those handling them (10C). Only some persons are susceptible to the effect. The similar but more complicated glandular hairs of Hops (Fig. 118) produce a secretion called LUPULIN, to which beer owes its bitter taste and distinctive aroma. The secretion is set free by the bursting of the cuticle, the latter having been previously raised up from the underlying cell wall as a continuous membrane (Fig. 118 B}. Hairs and emergences with abund- ant protoplasmic contents occur on irritable stamens, perianth leaves, and pulvini, and, as HABERLANDT (105) showed, act as tactile papillae, hairs, or bristles in the reception of stimuli. In other cases they are not themselves irritable, but FIG. 120.— Digestive gland serve to conduct a stimulus mechanically towards from Drosera rotu"fli- the irritable tissue.
folia, (x 60.)
The mucilaginous matter produced in young buds by the mucilage papillae or COLLETERS results from the partial dissolution of the cell wall under the cuticle. After the mucilaginous secretion has been discharged by the ultimate rupture of
MORPHOLOGY 109
the cuticle, another new cuticle forms over the cell wall, and the process is again repeated. The colleters are special forms of hairy structures, and are often de- veloped in buds to protect the young organs from drying, by means of the mucilaginous modification of their cell walls. Where the dissolution of the cell wall is accompanied by secretions from the underlying cells, the colleters assume rather the character of glandular hairs. Such GLANDULAR COLLETERS are common in the winter buds of trees ; in the Horse-chestnut (Aesculus ffippocastanum), for example, the bud-scales of the winter buds are stuck together by a mixture of gum and resin, which has exuded from colleters of this nature. The glandular hairs of the Pansy (Fig. 119) act in a similar manner. The emergences on the leaves of the Sundew (Drosera), described as digestive glands (Fig. 120), discharge glistening drops of mucilaginous matter, not under the cuticle, but from the free surface of the glands at the ends of the tentacles. Small animals are caught by means of these sticky excretions, and are afterwards digested by the plant. The nectaries also often excrete sugary solutions directly from their surfaces. In flowers these serve to attract insects, which effect pollination, while on other parts of the plants they are known in certain cases to attract ants, which protect the plant. The osmotically active substances in the nectar are in the first instance derived by transformation of the outer cell walls, or are secreted by the cells. The presence of these substances on the surface of the nectary attracts water from the tissue beneath, and thus leads to the continued formation of the nectar.
In some of the Piperaceae and Begoniaceae, and in some species of Ficus, the epidermis is composed of several layers ; but this is a comparatively rare occurrence. Such a many-layered epidermis results from a division of the young epidermal cells parallel to their external surface. The epidermis of Ficus elastica (Fig. 76) has three layers, and serves as a reservoir for accumulating water. The cystoliths of Ficus elastica, already referred to (p. 63), occur in single swollen epidermal cells. A many-layered root epidermis is also met with, as in species of Asparagus, Crinum and Lycoris (107). The many - layered epidermis of the aerial roots of many Orchids, and of various Aroids, undergoes a peculiar modification and forms the so-called VELAMEN RADICUM (p. 48), a parchment-like sheath surrounding the roots, and often attaining a considerable thickness. The cells of this enveloping sheath are generally provided with spiral or reticulate thickenings, and lose their living contents. They- then become filled with either water or air, depending upon the amount of moisture contained in the surrounding atmosphere. These root- envelopes absorb water like blotting-paper ; when the velamen is filled with water, the underlying tissues impart a greenish tint to the root ; but if it contains only air the root appears white. The epidermis of fruits, and particularly of seeds, exhibits a considerable variety of modifications in its mode of thickening, and in the relations the thickening layers bear to one another. The purpose of these modifications in the epidermis becomes at once evident, when it is taken into consideration that, in the case of fruits and seeds, in addition to protecting and enclosing the internal parts, the epidermis has often to provide for their dissemination and permanent lodgment.
The Vascular Bundle System. — The PRIMARY VASCULAR BUNDLES extend in the form of strands throughout the body of the higher plants. In more transparent stems, such as those of Impatiens parvi- flora, the bundles may be clearly distinguished and their course followed. The arrangement of the bundles of leaves is apparent from
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the venation. In many parallel-veined leaves the bundles are easily isolated. This is often done accidentally, as when, for example, in picking a leaf of Plantain (Planfayo media) a pull is given at the same time.
Special strands of tissue serving for the transport of substances through the plant are found in the more highly differentiated Thallo-
Fio. 121.— Transverse section of a vascular bundle from the internode of a stern of Zea Mais, a, King of an annular tracheid ; sp, spiral tracheid ; n and m', vessels with bordered pits ; v, sieve- tubes ; s, companion cells ; cpr, compressed protophloem ; I, intercellular passage ; vg, sheath ; /, cell of fundamental tissue, (x 180.)
phyta ; examples are afforded by some of the lied and Brown Seaweeds (Rhodophyceae and Phaeophyceae). In the Laminariaceae these con- ducting tracts contain elements which closely resemble sieve-tubes (108). The thallus of some Liverworts is traversed by a strand which resembles the nerve of a leaf. Bundles sharply limited from the surrounding tissues first appear in the Mosses ; they occur commonly in the leaves, less often in the stems. A fairly simple example of this kind of conducting bundle is that of the stem of Mnium which is represented in tranverse section in Fig. 161.
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Such bundles reach their highest differentiation in the Polytrichaceae. In them the stem has a central cylinder composed of elongated cells with scanty contents, of elements resembling sieve-tubes, and of elongated cells ; the three kinds of element serve respectively to transport water, albuminous substances, and carbohydrates (109). The arrangement of the tissue of these complicated strands is often similar to that found in the vascular bundles of more highly organised plants. This is a good example of a striking resemblance between
*p f /*
I
FIG. 1-2-2. — Longitudinal section of a vascular bundle from the stem of Zea Mais, a, and a', Rings of an annular tracheid ; v, sieve-tubes; s, companion cells; cp, protophloem ; I, intercellular passage ; vg, sheath ; sp, spiral tracheides. (x 180.)
structures which are analogous but have arisen independently in the course of evolution.
It is, however, in the Cormophytes, which possess roots, that a high degree of differentiation of the vascular bundles is first attained. Since the absorption of water is limited to the roots, the arrange- ments in the conducting tracts require to be more perfect. Two distinct components can be distinguished in these vascular bundles,
the TRACHEAL Or XYLEM PORTION, and the SIEVE-TUBE or PHLOEM
PORTION. While these may form independent strands, they are gener- ally united in one VASCULAR BUNDLE. Other terms often used to designate the vascular bundles are FIBRO-VASCULAR BUNDLES and MESTOME. The vascular portion is also termed the XYLEM or
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HADROME, and the sieve-tube portion the PHLOEM or LEPTOME (no). The distinction of the two components of the vascular bundle is most evident from transverse sections (Figs. 121, 123), with which the longitudinal section (Fig. 122) should be compared. The vascular portion contains TRACHE/E and TRACHEIDES as most essential for the fulfilment of its function of water conduction (a, sp, m, Figs.
Fio. 123. — Transverse section of a vascular bundle from a stolon of Ranunculus repens. s, Spiral tracheides ; m, vessel with bordered pits ; c, cambium ; v, sieve-tubes ; vg, sheath, (x 180.)
121, 122), or tracheides alone, and, in addition, living, elongated parenchymatous cells that may be designated XYLEM or WOOD PARENCHYMA. In the phloem portion the most essential elements are the SIEVE-TUBES (#), which serve for the conveyance of albuminous matter. They are always accompanied by other living cells ; either by the so-called COMPANION CELLS (s), or in addition by elongated parenchymatous cells, or by the parenchyma alone. Companion cells only occur in the phloem of Angiosperms. They are sister
MORPHOLOGY 113
cells of the sieve-tubes, having arisen by longitudinal division from the same mother cell. The companion cells are not so large as the sieve-tubes, and may be distinguished from them by their more abundant protoplasmic contents, and especially by the fact that they retain their nuclei, while the nuclei of the sieve-tubes soon disappear. In Monocotyledons (Figs. 121, 122), and in the Ranunculaceae among the Dicotyledons (Fig. 123), the phloem consists solely of sieve-tubes and companion cells ; in the other Dicotyledons parenchymatous elements are also present, and these are accordingly distinguished as PHLOEM PARENCHYMA ; no companion cells are found in Gymno- sperms and Pteridophytes,-and in addition to sieve-tubes the phloem contains only phloem parenchyma.
The bundles of the Phanerogams (Gymnosperms and Angiosperms) are generally COLLATERAL in structure, that is, the xylem and phloem are in contact on one side only. In stems the most usual arrange- ment of the two portions of a collateral bundle is that in which the xylem lies nearest the centre ; in leaves the xylem portion lies nearer the upper, and the phloem portion nearer the lower surface. Closely allied to the collateral type is the bicollateral type of bundle. In this the xylem is accompanied by phloem both on the outside and inside. Such bicollateral bundles are characteristic of the Cucur- bitaceae (1U). The xylem and phloem of roots generally form separate strands (Fig. 124 s, v), and the xylem strands are differently oriented ; while in stems the narrow vessels are nearer the centre and the wider nearer the circumference, in roots this order is exactly reversed.
The "CONCENTRIC" vascular bundles of the Pteridophyta (Fig. 125) contain tracheides (sp), and only in exceptional cases tracheae (sc). The latter are as a rule wanting in Pteridophyta, although this group goes by the name of Vascular Cryptogams. The water-con- ducting elements exhibit scalariform thickenings, only the narrowest having spiral markings (sp) ; they are surrounded by xylem paren- chyma (Ip). Outside this comes a zone of tissue consisting of sieve-tubes (v) and phloem parenchyma (s).
A number of similar vascular bundles are present in the stem of most Ferns and species of Sclaginella. In Lycopodium they are fused into a single central cylinder. In the stem of Equisetaceae vascular bundles of collateral structure appear.
The vascular bundles are developed from strands of meristematic tissue which are called PROCAMBIUM STRANDS. Within each strand a zone of tissue commences to divide tangentially, and behaving as a primary meristem produces to both the inside and the outside new cells in radial order. If the whole meristematic tissue of a procambium strand is exhausted in this process, the vascular bundles are said to be CLOSED ; but if any of the meristematic tissue remains in an undifferentiated condition between the xylem and phloem
I
114
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portions, the bundles are spoken of as OPEN (Fig. 123 c). The Pteridophytes have, almost without exception, closed bundles ; in
Monocotyledonsalso the bundles are always closed (Fig. 121); Gymnospermsand Dicotyledons (Fig. 123), on the contrary, have open bundles.
In those portions of plants which are still actively growing in length, the procambium strands remain un- differentiated, except at definite points, where single rows of cells lose their meristematic condition and form narrow, annular, and spiral vessels and sieve-tubes, or sieve-tubes and companion cells ; the structure of all of these is of such a nature as to render their elongation possible. Such prim- ary vascular elements are termed while the correspond-
Fio. lL'4. — Transverse section of central portion of . . , ...
ing sieve elements are in like manner the root of Aeenu Calamus, m, M'-dnlla ; .--, 6
xylem; r, phloem ; p, pericycle ; c, endodermis ; designated PROTOFHLOKM. The pro- c, cortex, (x 90.) . toxylem occuj>ies the innermost, the
protophlocm the outermost side of a
procambium strand, from which a collateral bundle is eventually formed. After the growth in length of any part of a plant ceases, the differentiation of the pro- cambium strand into a collateral vascular bundle is continued from the inner and outer sides of the strand toward the centre.
In fully developed vascular bundles the protoxylem and protophloem cease to perform their functions. The protoxylem elements become compressed and ruptured by the tension resulting from the continued vertical growth (a and a', Fig. 122), so that in their stead a lysigenic intercellular space is often formed (Figs. 121, 122). The protophloem elements (cf. Figs. 121, 122) at the same time become disorganised, and their sieve-plates closed by a covering of callus.
In accordance with the inverted orientation of the xylem, the protoxylem of roots is found on the outer, not on the inner side of the vascular strands (Fig. 124).
The Terminations of the Vascular Bundles. — In leaves, particularly in the foliage leaves of Angiosperms, the vascular bundles become much branched until finally they are reduced to extremely fine strands. In the leaves of Gymnosperms this branching of the bundles does not usually take place, but instead, a single vascular bundle frequently runs throughout the whole length of the leaf. The vascular bundles of the reticulately-veined leaves of Dicotyledons illustrate the most extreme form of branching.
The minute distribution of the bundles in the leaf-lamina facilitates the regular conduction of water to all parts of the leaf-tissue, and at the same time renders easier the removal of the assimilated products. In the same degree as the ramifica-
MORPHOLOGY
115
tions of the vascular bundles are continued, the bundles themselves become attenuated and simpler in structure (Fig. 126). The vessels first disappear, and only spirally and reticulately thickened tracheides remain to provide for the water conduction. The phloem elements undergo a similar reduction. In Angiosperms, in which the sieve-tubes are accompanied by companion cells, the sieve-tubes become narrower, whilst the companion cells retain their original dimensions. Finally,
FIG. 125. — Transverse section of a concentric bundle from the petiole of 1'teris aquilina. 'sc, Scalariform vessels ; sp, protoxylem (spiral tracheides) ; sc*, part of a transverse wall showing .scalariform perforations ; Ip, xylem parenchyma ; v, sieve-tubes ; j>r, protophloem ; pp, starch layer; e, endodermis ; s, phloem parenchyma, (x 240.)
in the cells forming the continuation of the sieve-tubes, the longitudinal division into sieve-tubes and companion cells does not take place, and TRANSITION CELLS are formed (112). With these the phloem terminates, although the vascular portion of the bundles still continues to be represented by short spiral tracheides. The ultimate branches of the bundles either terminate blindly or anastomose with other vascular bundles.
The Fundamental Tissue System usually forms the main bulk of the primary tissues of the body of a plant. The whole tissue of the lower plants, as it shows no internal differentiation, may, in a certain sense, be considered fundamental tissue. The other tissues
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have gradually arisen from the fundamental tissue in the course of phylogenetic development. The fundamental tissue in the higher plants is enclosed by tegumentary tissue, and traversed 1>v the vascular bundle system. While the tegumentary tissue protects the plant externally, and the vascular bundle system performs the office of conduction, and also of mechanically strengthening the plant, the duty of providing for the nutrition of the plant and of storing reserve food material falls chiefly to the fundamental tissue. The fundamental tissue consists, therefore, for the most part of
parenchymatous cells containing chloro- phyll, at least to such depth as the light penetrates; internally a colourless parenchyma is found. The fundamental tissue system also takes part in pro- viding for the mechanical rigidity of plants, and in connection with this function it possesses collenchyma (Fig. 74 c) and sclerenchyma as its special mechanical tissues. -The COLLENCHYMA (p. 68) is unlignified and very elastic, and thus fitted for stretching ; it is the form of mechanical tissue suitable for those parts of plants still undergoing growth in length. The sclerenchymatous fibres, which are often unlignified but have greatly thickened walls, on the other hand, are formed after growth in length has ceased, and sclereides (p. 73) arise even later. The elongated cells of the fundamental tissue also perform a certain share of the work of conduction, and serve for the transport of carbohydrates. Secondary or waste pro- ducts, resulting from chemical changes, are also deposited in special cells of the fundamental tissue. Consequently idioblasts (p. 73), containing crystals or rows of crystal-containing cells, are often met with in the fundamental tissues, together with cells, tubes, cavities, or canals containing tannin, gum, resin, ethereal oils, latex, or alkaloids. Such waste products are for the most part deposited near the surface of a plant, in order to serve as a defence against destructive animals, or that they may afterwards be thrown off along with the superficial tissue. Cells containing these waste products, particularly crystal cells and latex tubes, are often found, accompanying the phloem portion of the vascular bundles.
The Aroideae, Nymphaeaceae, and several other plant families possess a peculiar form of idioblast, in the so-called internal hairs, which project into the intercellular spaces of the fundamental tissue. In the wide intercellular passage*
FIG. 126.— Termination of a vascular bundle in a ,leaf of Imputiens po,rvi flora, (x 240.)
MORPHOLOGY 117
'of the petioles and flower stems of the Water-Lily these idioblasts are stellate in form. Their walls are strongly thickened, and provided with short protuberances in which small crystals of calcium oxlate are deposited.
The Distribution of the Primary Tissues (m)
In the body of multicellular plants a distinction between an outer small-celled and firm tissue and an inner large-celled looser tissue soon becomes apparent. The outer tissues are best adapted for pro- tection, the inner for conduction and storage. The cells of the inner tissues accordingly become elongated for the purpose of conduction. The outer tissues in plants, which must provide independently for their own nourishment, contain chromatophores fitted for assimi- lation, and are correspondingly coloured, while the inner tissues remain colourless. The outer portion of the fundamental tissue thus differentiated is called the CORTEX, the inner the MEDULLA or PITH. An epidermis, distin- guishable from the cortex, is found in some of the Mosses, but a sharp distinction between these tissues is first found in the more highly organised plants.
In the Stem of a Phanerogamic plant there is an outer skin or epidermis (Fig. 128 B, e) on the FK-. I-JT. Tr:msv.-i-s>- sirti.m ui .m mt«Miio<ie of external surface ; then follows the th" sti:"' 'f * • ^ p,r,ilniiry c?rte,x :
' pc, pencycle ; co vascular bundles ; go, fuuda-
PRIMARY CORTEX (Figs. 127, 128 mental time of the eentnl eyliwter. (x 2.) A, pr), and internal to this the
so-called CENTRAL CYLINDER, for which VAN TIEGHEM has proposed the name STELE (column) (114). The innermost layer of the primary cortex, which may be designated by the term PHLOEOTERMA, is for the most part not distinctly differentiated, but can be recognised in the aerial stems of land-plants as a starch-sheath ; while in the rhizomes of land-plants and in the stems of water-plants it forms the ENDODERMIS. Differentiated as a starch-sheath (Fig. 128 A, B, xt), the phloeoterma is rendered conspicuous by the quantity of movable starch contained in its cells. A starch -sheath is often present in the young shoots, while it disappears or becomes limited to certain parts of the older shoots (llr<). When developed as an endodermis, portions of the lateral walls of its cells become suberised (uo). In a cross-section these suberised portions of the cell walls of
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the endodermis appear as dark spots (Fig. 130), but in a tangential section as a wavy band. The CENTRAL CYLINDER of the stem contains vascular bundles (cv), which, in the Equisetaceae, the Gymnosperms and Dicotyledons (Fig. 128 A\ are arranged in a circle, whereas in Monocotyledons (Fig. 127) they are irregularly distributed. In all these cases the xylem portion of the vascular bundle is directed towards the centre, and the phloem portion away from the centre of the stem. That part of the peripheral tissue of the central cylinder lying outside of the bundles is called the PERICYCLE (pc). If the
B
FIG. 128. — A, Part of a transverse section of a young stein of Arittnliu-hiu xifiho. e, Epidermis ; jv, primary cortex ; st, starch-sheath ; c, central cylinder ; pc, pericycle, in this case with a ring'of sclerenchyma fibres ; ctf, phloem, and cv", xylem portions of the vascular bundle ; cb, cambium ring; m, medulla; ins, primary medullary ray. (x 48.) B, Small portion of the periphery of a similar section of a still younger stem, e, Epidermis ; pr, primary cortex ; st, starch- sheath with easily movable starch grains ; pc, outer layers of the pericycle. (x 350.)
bundles are arranged in a circle (Fig. 128 A\ that part of the central cylinder enclosed by them is the PITH or MEDULLA (m), and the tissue between the different bundles the PRIMARY MEDULLARY RAYS. In the case of scattered bundles (Fig. 127), a distinction between medulla and medullary rays is no longer possible. Wherever there is no sharp distinction between primary cortex and central cylinder, comparative investigation alone can determine whether a tissue belongs to the primary cortex or to the central cylinder.
Although the fundamental tissue of the primary cortex is mainly a chlorophyll-containing tissue, portions bordering on the epidermis frequently become converted, for mechanical purposes, into strands of collenchyma or sclerenchyma. Such a mechanical tissue, which serves to strengthen the epidermis, is known as a HYPODKKMA. Of the tissues composing the central cylinder, the pericycle, the
MORPHOLOGY
119
primary medullary rays, and medulla consist of fundamental tissue, and are chiefly composed of colourless paren- chyma. A part, however, of the tissue of the pericycle may become sclerenchymatous (Fig. 128 A, pc) ; sclerenchy- matous elements also often surround individual bundles as sheaths, or accompany the phloem portion in the form of strands (Figs. 121, 123). When- ever such a sheath of scleren- chyma is developed about a bundle, it is interrupted on both sides of the bundle, at the junction of the xylern and phloem portions, by paren- chymatous cells, or by cells which are only slightly thick- ened and lignitied. The exist- ence of these unthickeiied places facilitates the exchange of water and food material between the vascular bundles and the fundamental tissue. When
FIG. 129. — Transverse section of an adventitious root of Allium Cepa. ep, Remains of the epidermis ; ex, exo- dermis ; c, primary cortex ; e, endodermis ; <x, central cylinder, (x 45.)
FIG. 130.— Transverse section of an adventitious root of AlHitm Cepn. r, Primary cortex ; c, endo- dermis ; p, pericycle; a, annular tracheides ; sji, spiral tracheides; sc and sc* ; scalarifonn vessels; v, phloem, (x 240.)
a common starch-sheath is not present in a stem, starch-sheaths are sometimes
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BOTANY
found surrounding the individual vascular bundles, or particular rows of cells provided with movable starch grains are present. The central cylinder of Phanerogams is simple, and occupies a more or less central position. In a few cases, as in the stems of Auricula and Gunnera, the central cylinder is broken vip into several partial cylinders.
In Roots, the division between primary cortex and central cylinder is sharply marked by the endodermis, into which the innermost layer of the primary cortex is usually transformed (Figs. 124, 129, 130 e). The central cylinder becomes completely shut oft' from the primary
cortex by the suberisation of the lateral walls of the endodermal cells, and by their close and uninterrupted contact. While, by this means, the passage of gases from the intercellular spaces of the cortex into the central cylinder, with the consequent obstruc- tion of the water -channels, is pre- vented, the passage of water from the cortex to the central cylinder can, at the same time, go on unhindered through the unsuberised inner and outer walls of the endodermal cells. In this manner it is possible for the water, absorbed from the soil by the root- hairs or by the surface of the roots, to Fir,. i3i.-i>art of a transverse section of \>Q transferred to the tissues of the
a root of [rin Jlorentina. e, Endodermis, , i- j T ^v. u
showing ceil walls thickened on one central cylinder. In the older parts
side.; /, transfusion cell; p, pericycle ; of the TOOtS, which no longer absorb
/; phloem ; s, vessel of xylem ; c, cortex. (X 240.)
water from the soil, the cells of the endodermis become greatly thickened,
but generally on one side only ; they may also be cutinised.
Should thickening occur at an early stage, special endodermal
cells, directly external to the xylem strands, remain unthickened
and serve as TRANSFUSION CELLS (Fig. 131 /).
While the root-hairs are as a rule developed from the cells of a definite region of the epidermis, they may, in case the epidermis is thrown off at an early >t;igc, arise from the outermost cortical layer, which then assumes the functions of an epidermis. In any case the epidermis soon disappears, and the outermost cortical layer becomes cuticularised and, as an EXODKKMIS, takes its place. Frequently some of the cells of such an exodermis remain uucutinised and serve as transfusion cells. They may be characterised by their smaller size, and be regularly distributed between the cutinised cells. In aerial roots the epidermis usually forms a many-layered root-sheath or velamen (cf. p. 109), within which comes the exodermis.
The primary cortex of the root is composed of colourless tissue, which is usually parenchymatous. In the outer layers the cells are
MORPHOLOGY 121
in close contact with one another, but intercellular spaces are present more internally. These intercellular spaces often widen into air- cavities or passages. In many roots a hypoderma giving mechanical support to the epidermis or exodermis is present. The outermost layer of cells of the central cylinder (Figs. 124, 131 p) forms the pericycle, which is also called the pericarnbium ; this is usually a single layer, and in rare cases is wanting. The xylem and phloem portions form separate strands (p. 113), radially disposed and alternating with each other (Figs. 124, 130). It has already been shown that the narrowest elements of the vascular strand are outermost. Roots are described as diarch, triarch, polyarch, according to the number of the vascular strands. For example, the roots of Acorns Calamus (Fig. 124) are octarch, those of Allium Cepa (Fig. 130) hexarch. The vascular strands may eitHer meet in the centre (Fig. 130), or they may surround a central pith (Fig. 124).
The Leaves are composed of fundamental tissue (which is here termed mesophyll), bounded by an epidermis and traversed by vascular bundles. Sheaths are present around the bundles, extend- ing to their fine terminations. The cells composing these mesophyll sheaths are as a rule elongated and not separated by intercellular spaces. Besides limiting the vascular bundles from the mesophyll, the sheaths perform the important function of conducting soluble carbohydrates from the leaf to the stem. The larger vascular bundles are usually accompanied by strands of sclerenchyma ; these disappear from the finer branches. Other strands of sclerenchyma not connected with the vascular bundles may also occur in the mesophyll and contribute to the rigidity of the leaf.
The mesophyll passes into the primary cortex of the stem, while the vascular bundles are continuous with the central cylinder. Thus in the leaf, tissues corre- sponding to the cortex and central cylinder of the stem remain distinct from one another.
The mesophyll of the coloured FLORAL LEAVES of the Angiospei'ms usually consists of a somewhat loose tissue, containing intercellular spaces and traversed by vascular bundles. The laminae of many assimilating FOLIAGE LEAVES, especially of shade-loving plants, may have a similar uniform structure ; but they are usually more com- plicated, and exhibit a difference in the structure of their upper and lower sides (Fig. 132). In such dorsiventral structures the upper epidermis is succeeded by one or more layers of cylindrical parenchymatous elements elongated at right angles to the surface, and known as the PALISADE CELLS. These are especially rich in chlorophyll, and are often separated laterally from one another, so that the con- duction of substances is limited to the direction of their longer axis (Fig. 132). Adjoining the palisade parenchyma, and extending to the epidermis (ep") on the under surface of the leaf, is a loose
122
tissii£ called the SPONGY PARENCHYMA. In contrast to the palisade cells, the cells of the spongy parenchyma are less abundantly supplied with chlorophyll ; they are also much more irregular in shape, and have large intercellular air-spaces between them. The palisade cells are elongated in the direction in which the rays of light penetrate the leaf-lamina, and by this means are particularly adapted to their special function of assimilation. The spongy paren- chyma, on the other hand, is arranged to facilitate the free passage of gases, and to that end develops large intercellular spaces in direct communication with the stomata of the lower epidermis. HABER- LANDT (m) has estimated that to every square millimetre of surface in a leaf of Ricinus communis there are, in the palisade cells, 403,200
FIG. 132.— Transverse section of a leaf of Vagus sylvaticc. ep, Epidermis of upper surface ; ep", epidermis of under surface ; ep"', elongated epidermal cell above a vascular bundle ; pi, palisade parenchyma ; s, collecting cells ; sp, spongy parenchyma ; k, idioblasts with crystals, in k' with crystal aggregate; st, stoma. (x 300.)
chlorophyll granules ; in the cells of the spongy parenchyma only 92,000 ; that is, 82 per cent of all the chlorophyll granules belong to the upper surface of the leaf, and only 18 per cent to the under side. The palisade cells are often arranged in groups, in which the lower ends of the cells of each group converge (Fig. 132). In this way several palisade cells come into direct contact with a single expanded cell of the spongy parenchyma, which thus functions, apparently, as a collecting cell for a group of palisade cells. The products of assimilation are passed on from the collecting cell through the spongy parenchyma, to be finally carried to the mesophyll sheath surrounding the vascular bundles. The sheaths serve as a conducting tissue towards the stem.
At the base of the lamina the tissues close together and pass into the leaf-stalk, where one is present. The dorsiventral structure becomes less marked in the petiole. The cells are mostly elongated
SECT, i MORPHOLOGY 123
in the length of the petiole, a, modification which facilitates the con- duction of food material. They are often thickened and so arranged as to meet the altered requirements for mechanical rigidity. The vascular bundles pass from the leaf-stalk into the stem and there either arrange themselves among the bundles of the central cylinder or at once fuse with some of them. In the leaf-stalks of Angiosperms the bundles usually appear arranged in a curve open above, but may form other figures. In the petioles of Ferns, the partial cylinders are accompanied, as in the stem, by sclerenchy- matous fibres forming strands or plates. It is the peculiar arrange- ment of those brown-walled sclerenchymatous masses which forms the double eagle apparent on cross-sections of the petiole of Pteris aquilina, from which the plant derives its specific name.
In certain families of the Dicotyledons, particularly iu the Crassulaceae, the mesophyll of the leaf-lamina forms peculiar masses of tissue called the EPITHEMA between the swollen terminations of the bundles and the epidermis. The cells of the epithema are small and, for the most part, devoid of chlorophyll ; they are full of water, and joined closely together, leaving only very small interspaces, which are filled with water. They are internal hydathodes (cf. p. 108) and serve for the active excretion of water. Water pores are usually situated above such epithemata. The leaf-tips of a number of aquatic Monocotyledons show a depression into which the terminations of tracheides project. These depressions arise by the destruction of water-pores or of these together with the epidermis. They may be closed by the persisting cuticle. These apical openings serve in the same way for the excretion of water (118).
The Course of the Vascular Bundles (119). — The bundles exhibit a definite course and arrangement within the body of a plant. It is sometimes possible, by maceration, to obtain preparations in which the course taken by the bundles may be followed. By allowing a leaf, stem, or flower to lie in water until it has become softened and disintegrated, a skeleton formed by the more imperishable vascular system may be obtained.
Vascular bundles which pass from a leaf into a stem form within the latter what are known as LEAF-TRACES. The leaf-traces may be composed of one or more vascular bundles, and are accordingly dis- tinguished as one-strand or many-strand leaf -traces. When, as is usually the case, the vascular system of the stem is entirely composed of leaf-traces, each vascular bundle of the trace after passing down- wards for some distance unites with another entering from a lower leaf. The arrangement of the bundles in a stem varies according to the distance and direction traversed before the coalescence of the bundles takes place. A relatively simple case is afforded by the young twigs of the Dwarf Juniper (Juniperus nana) (Fig. 133). The leaves are in whorls of three, the leaves of successive whorls alter- nating with one another. From each leaf a leaf-trace consisting of a single vascular bundle enters the stem. This divides into two about
124
BOTANY
the middle of tbe internode below, and the divisions diverge and unite with the leaf-traces of the whorl below. The arrangement of the bundles may be shown diagrammatically by representing the bundles as if on the surface of an unrolled cylinder, so that they all appear in one plane. This is done in Fig. 133, which also shows the origin of the vascular bundles of the axillary shoots (k).
The arrangement of the bundles iu the Yew (Taxus laccata), although its leaf-traces have only one bundle, is much more compli- cated (Fig. 134), for the bundles maintain a distinct course throughout twelve internode 5 before coalescing. Each bundle at first descends in a straight direction through four internodes ; it then curves to the
side to give place to a newly-entering leaf-trace, with which it finally coalesces at the twelfth internode. The position of a leaf necessarily determines the point of entrance of its leaf-trace into the stem, and accordingly a diagram (Fig. 134) of the bundles of Taxus will exhibit a divergence of the leaf-traces corresponding to the yV divergence of the leaves. The course taken by the leaf -traces in the stem, however, is independent of the leaf position, and
FIG. 133.— Diagram of the course of tin- .r . , , , . ,.£
vascular bundles in a young branch of varies considerably m different stems, jnniperuf nnnn shown on the unrolled although the divergence of their leaves
surface of the cylinder At I* the be the game The gtem Qf Ckmaiis
vascular bundles passing to the axillary /
shoots are seen. (After GEYI.KR.) vtiicella afford s an example of leaf traces
consisting of three vascular bundles.
The leaves are in whorls of two, the successive whorls alternating with one another. The median strand of each leaf-trace (Fig. 135 ad, uk, nq, te) has a free course through one internode, and at the node below divides into two arms which coalesce with the adjacent lateral strands of the leaves inserted at this node. The two lateral strands of each leaf-trace (Fig. 135 be, ef, hi, Im, op, rs) also are free throughout the internode, but at the node below curve inwards and become attached to the same lateral strands as the arms of the median bundle of the trace.
The traces of the axillary buds of most Gymnosperms and Dicotyledons unite to form two strands which enter the mother shoot and are inserted upon its leaf-traces (Fig. 133 k).
As a general rule, the leaf-trace bundles in Gymnosperms and Dicotyledons arrange themselves in a circle in the stem. There are, however, Dicotyledons in which the vascular bundles form two (Cucurbita, Phytolacca, Piper) or more circles (Amarantus, Papaver, Thalidrum). In such cases the inner circle is usually more or less irregular.
SECT. I
MORPHOLOGY
125
In the stems of Monocotyledons (Fig. 127) the vascular bundles are scattered, and without any apparent regular order. Their scattered arrangement is due to the varying distances to which the bundles of the leaf-traces penetrate into the central cylinder of the stem. This results from the prolonged growth in thickness of the growing point after the procambial strands have been laid down (12°). A common arrangement of the bundles in monocotyledonous stems is that of the so-called Palm type, in which each leaf-trace consists of
FIG. 134. — Diagram showing the course of the vascular bundles in a shoot of Taius Imccvtv.
the numerous bundles which pass singly into the stem from the broad leaf-base. The median bundle penetrates to the middle of the stem. The depth to which the lateral bundles penetrate varies with their remoteness from the median bundle. In their descending course the bundles gradually curve outwards, and finally join other bundles near the periphery of the stem. The number of internodes, therefore, through which a bundle passes before coalescence, is variable ; the median bundle, however, continues distinct for the longest distance. The deeper penetration and greater length of the median bundle become apparent in a median longitudinal section of such a stem
126
BOTANY
PART 1
(Fig. 136). The numerous bundles entering the stem from axillary shoots pursue a similarly curved course to those entering from leaves.
In addition to the leaf- trace bundles or COMMON BUNDLES, which are common to both leaf and stem, there are others, called CAULINE BUNDLES, which belong solely to the stem, and again others,
FOLIAR BUNDLES, which, Oil
entering the stern from the leaf, at once coalesce with other bundles and have no independent existence in the stem. The bundles of the Pteridophytes are continued as cauline bundles in the stem,
Fio. 135. — Cieiiuitis viticella. End of a branch which has been made transparent by the removal of the superficial tissues and treatment with caustic potash. The emerging strands have been slightly displaced by gentle pressure. The two uppermost pairs of young leaves (bli, W2) are still without leaf-traces, r, Apical cone. (After NACF.LI.)
and those from the leaves join on to the bundles of the stem.
3/tl
nn
FIG. 136.— Diagram showing the course of the vascular bundles of Monocotyledons of the Palm type, with alti-rriatiiix, two- ranked amplexicaul leaves. The numbers indicate the sequence of the leaves ; m, median bundle. (After DE BARY.)
The stems of many Dicotyledons (Begonia, Aralia) possess cauline bundles in addition to leaf- traces. The cauline bundles are situated in the
pith within the ring of leaf-trace bundles in the internode, and at the nodes are connected by branches with the leaf-trace bundles.
MORPHOLOGY 127
In the central cylinder of roots, the radially arranged strands of xylem and phloem pursue a straight course parallel to one another. The xylem and phloem strands of lateral roots are inserted upon the corresponding strands of the parent root. Those of adventitious roots are attached to the corresponding tissues of the stem or root from which the adventitious root arises. The strands of xylem and phloem of the main root of a phanerogamic seedling are continuous with the leaf-trace bundles in the hypocotyl (12°).
Increase in Thickness by Continued Enlargement of the Primary Meristem. — The growing points of those Monocotyledons which have large stems (Palms, Pandanaceae, some Liliiflorae) exhibit this feature. The growing point thus attains a considerable thickness, and the stem has from an early period its definite circumference. The increase in number of cells takes place in a zone lying at the periphery of the growing point (m).
A Special Form of Growth in Thickness of the Stem by means of the Continued Enlargement of the Fundamental Tissue. — This is often exhibited by Palms, and leads in them to the formation of stems which may be more than 1 metre in diameter. EICHLER (122) has shown that the growth in thickness is solely due to the continued expansion of the already existing cells of the fundamental' tissue of the central cylinder. In this process, by the expansion of the cell lumen and increased thickening of the walls, the strands of sclerenchy- matous fibres accompanying the vascular bundles on their phloem sides also become greatly enlarged. In this form of growth in thickness no new elements are formed.
Enlargement due to Continued Cell Divisions in the Fundamental Tissue. — This is seen most strikingly in the growth of the fruits of many Angiosperms. In their development from the small rudiment in the flower there may be a great increase in size, although no new meristematic tissue has been formed. The growth is due to repeated divisions in the epidermal cells and in those of the fundamental tissue. The tissue thus produced may when mature be thin-walled, or its cells may undergo various changes in form and in the thickness of their walls. From small structures such large fruits as those of the Gourds rnay develop in this way.
The Secondary Tissues (123)
After growth in length is finished through the activity of a cambial tissue, functioning either as a primary or secondary meristem (p. 99), secondary tissues are added to the previously existing primary tissues, or even substituted for them. Although, phylo- genetically considered, secondary tissues seem to have been developed first in the Pteridophytes in forms now only known in a fossil
128
BOTANY
condition (Caltimarieae, Sigillarieae, Lepidodendreae) they first became of general occurrence in the Phanerogams. In them the formation of secondary tissues is almost exclusively confined to the roots und stems ; secondary growth is met with in some foliar structures but only in a slight form.
Cambium (124). — The primary and secondary meristems, the activity of which gives rise to secondary growth, are given the name of CAMBIUM. These tissues persist in a meristematic condition and undergo successive divisions by walls parallel to one another, so that the cells produced from them are arranged in radial rows. A similar mode
of division characterises the prim- ary meristem in the procambial strands, which give rise to col- lateral vascular bundles (cf. p. 113), and this might also come under the term cambium. It is advisable, however, to restrict this term to meristems giving rise to secondary growth. As a rule the cambium forms new tissue-elements on both sides, but cases are not wanting in which this takes place to one side only.
As a rule a single persisting initial layer is present in the cambium from which on one or both sides the rows of cells take their origin. In many cases in which the cambial activity is confined to one side, according to J. C. SCHOUTE, the original initial cells are used up in the tissue forma- tion, and new initial cells arise from adjoining cells of the ground-tissue.
Growth in Thickness of the Stem in Gymnosperms and Dicoty- ledons.— The cambium of the open vascular bundles of Gymnosperms and Dicotyledons, which exhibit a growth in thickness, commences its activity almost directly after the formation of the primary tissue. The primary meristem remaining between the xylem and phloem of the bundle continues its active growth as the cambium. Its cells are full of protoplasm and continue to divide by means of tangential and occasionally radial walls. The new cells thus continuously given oft' from the initial cells toward the xylem and phloem sides of the bundles experience another tangential division before attaining their definite form as elements of the xylem or phloem portions. The vascular bundles of Gymnosperms and Dicotyledons which undergo secondary growth are usually arranged in a circle. After the cambium in the bundles begins its activity, a zone of tangentially dividing tissue,
Fio. 137. — Transverse section of a stein of Aristc lochia Sipho 5 mm. in thickness, m, medulla fv, vascular bundle ; el, xylem ; cb, phloem ; /c fascicular cambium ; \fc, interfascicnlar cam bium ; p, phloem parenchyma ; pc, pericycle sk, ring of sclerenchyma ; e, starch -sheath c, primary cortex ; cl, collencliyma in primary cortex, (x 9.)
MORPHOLOGY
1-29
called the INTERFASCJCULAK CAMBIUM, develops in the primary medullary rays between the original bundles, and, uniting with the cambium in the bundles, forms a complete cambium ring. This cambium ring is thus composed of two distinct forms of meristematic tissue : for while the cambium of the bundles or the FASCICULAR
-.1C
rrp
FIG. 138. — Transverse section of a stem of Aristolochia Siplw in tin; Hist year of its growth, showing a vascular bundle with cambium in active division, p (Vascular .parenchyma • i'lp proto- xylein ; TO' and TO", vessels with bordered pits; ic, interfascicular cambium in continuation with the fascicular cambium ; v, sieve-tubes ; cftp, protophloem pc, pericycle ; sk, inner part of ring of sclerenchymatous fibres, (x 130.)
CAMBIUM consists of primary meristem (p. 99), the connecting zone of interfascicular cambium is of later development, and js consequently a secondary meristem (p. 99). A cross-section of a young stem of Aristolochia Sipho, with the cambium ring in process of formation, is represented in Fig. 137; in Fig. 138 a single bundle of the same cross-section, more highly magnified, shows the fascicular cambium with the interfascicular cambium to either side in a condition of
130
BOTANY
active division. \\7ithin the bundle may be seen two large vessels (m"), in a still incomplete state ; while in the adjoining primary medullary rays the cells which give rise to the interfascicular cambium may still be plainly distinguished. All the tissue arising from the inner side of the cambium ring goes to form the secondary WOOD, while that produced on the outside is termed secondary BAST. The vascular portions of the wood form the WOOD STRANDS, the sieve portions within the bast the BAST STRANDS. By the activity of the interfascicular cambium, the primary medullary
s
Fir,. 139. — Portion of a four-year-ol<l stem ot tin- 1'inc, 1'innxlsi/lcexti-i*, cut in winter. •/, Trans- verse view ; /, radial view ; t, tangential view ; /, spring wood ; s, autumn wood ; m, medulla ; p, protoxylem ; 1, 2, 3, 4, the four successive annual rings of the wood ; i, junction of the wood of successive years ; ma, ins', ?«.<"', medullary rays in transverse, radial, and tangential view ; ins", radial view of medullary rays in the bast ; c, cambium ring ; b, bust ; //. n-sin canals ; In; bark external to the first jwriderm layer, and formed from the primary cortex, (x H.)
rays are continued throughout both the wood and bast. As the wood and bast strands enlarge, SECONDARY MEDULLARY RAYS are developed from the fascicular cambium. In one direction the secondary medullary rays terminate blindly in the wood, and in the other in the bast ; the later they develop, the less deeply they penetrate the tissues on either side of the cambium.
The primary medullary rays are therefore often distinguished as long, the secondary as short medullary rays. The expression transvcrx- parenchyma is also sometimes used to designate the medullary rays, winch arc in fact composed almost exclusively of parenchymatous tissue. The cells given oil' by the initial layer of the cambium for the formation of medullary ray- do not undergo a further tangential division, as in other cases, but assume at once the character of medullary ray cells.
MORPHOLOGY
131
The cambium cells have, for the most part, the shape of right-angled prisms, of which the radial diameter is smaller than the tangential. The ends of these prisms are usually one-sided, tapering to a point, alternately on the right and left sides. The length of the cambium cells varies in different plants, but those from which mi'ilullary rays are formed are the shorter. The primary vascular por- tions of the bundles projecting into the medulla constitute what is known
as the MEDULLARY SHEATH.
Owing to climatic varia- tions, the cambial tissue of woody plants exhibits a period- ical activity which is expressed by the formation of ANNUAL KINGS of growth (Figs. 139, 141 A, 146). In spring, when new shoots are being formed, wider tracheal elements are developed than in the follow- ing seasons (Figs. 141 A, 148). For this reason a difference is perceptible between the EARLY WOOD (spring wood), which is composed of large elements especially active in the convey- ance of water (Fig. 141 /), and the LATE WOOD (autumn wood), consisting of narrow elements which impart to a stem its necessary rigidity (Fig. 141 s}. Throughout the greater part of the temperate zone, the formation of wood ceases in the latter part of
AugUSt Until the following ,,l(i uo._1Magnml to illuslral(. the secondary growth
-spring, when the larger ele- inthickneMCrftheitemaadvQofcofaGyBmoapenn
mentS of the Spring WOod are or Dicotyledon. «, Cambium, indicated by; 11 dotted
•, ! j f* . line ; m, pith ; 1, 2, 3, 4, successive animal rings
again developed. Owing to ,jf W01K, . ,, h.lst . S(,% limit ,„,, UW)I Imiin stem _,,„,
the contrast in the struc- tap-r.».t.
ture of the spring and the
autumn wood, the limits (Fig. 139 i) between successive annual
rings of growth become so sharply defined as to be visible even
to the naked eye, and so serve as a means of computing the age
of a plant.
Under certain conditions tin- number of annual rings may exceed the number of years of growth, MS. for instance, when MIDSUMMKII cii'wni occurs, such as
132
BOTANY
PAKT I
commonly happens in the Oak, when, after the destruction of leaves by caterpillars, a second formation of spring wood is occasioned by the new outgrowths thus induced. In the wood of tropical plants the annual rings'may be entirely absent. This occurs, for example, in the tropical Conifers of the genus Araucaria, which, in this respect, show a marked contrast to the Conifers of the northern zone. Any interruption of growth, such as would occur during a drought, followed by a period of renewed activity, may occasion the formation of annual rings even in tropical plants.
Although a cessation in the formation of wood takes place so
Kio. 141. — A, Transverse section of the wood of a Pineat'the junction of two annual linus. ./. Earl\ wood ; s, late wood ; t, bordered pit ; u, interposition of a new row of tracheides resulting from the radial division of a cambium cell ; A, resin canals; m, medullary vays.; </, limit of lat>- wood, (x 240.) If, Part of a transverse section of the stem of a Pine, s, Late wood; c, cambium ; v, sieve-tubes ; p, bast parenchyma ; A-, cell.of bast parenchyma containing crystal : cv, sieve-tubes, compressed and functionless ; HI, medullary ray. (x 240.)
early, the cambium tissue continues to form bast so long as climatic conditions permit. As a rule, however, far'fewer elements are added to the bast than to the wood. Owing to the continued growth in length, the higher a cross-section is made of a gymnospermous or dicotyledonous stem the fewer will be the annual rings. As the diagram in Fig. 140 shows, the older rings disappear first on passing towards the apex, and in the same way the older layers of the bast are unrepresented nearer the apex. Up to a certain period, in the age of woody plants, the elements of both wood and bast exhibit a progressive increase in size.
The living elements may remain in a state of greater or less
MORPHOLOGY
133
activity throughout the whole of the wood, extending even to the pith ; such wood is called splint wood : the Beech (Fagus sylvatica) may be quoted as an example. In other trees which form heart-wood, the living elements die after a certain time, so that only dead tissues ure found at a certain distance from the cambium. Before the death of the living cells, they usually produce certain substances, such as tannin and gums, which permeate the cell walls of the surrounding elements, and also partially close their cavities. The tannins impart to the dead wood a distinct colour, often very characteristic, especially when it has been transformed into wood dyes, or so-called XYLOCHROME. The tannin in the woody walls acts as a preservative against decay, while the gums close the functionless water-courses of the dead wood. The dead portion of the wood of a stem is called
the HEART-WOOD Or DURAMEN, in
contrast to the living SAP-WOOD or ALBURNUM. Usually the splint or sap wood is at once distinguish- able from the heart-wood by its lighter colour. In some stems, however, the heart-wood does not change its colour. In that case, as the protecting materials are generally absent, it is liable to decay, and then, as so often occurs in the Willow, the stem becomes hollow.
The sap-wood is limited, ac- cording to the kind of wood, to a larger or smaller number of the
younger annual rings, and tO it FIG. 142.— Transverse section of a vessel fron
falls the task of water conduction.
the heart- wood of Robinia Pscwlt'cttcia, closed by tyloses]; at a, a is shown the connection between the tyloses and the cells from which they have been formed, (x 300.)
'• The distinction between sap- and heart- wood is sharpest where the latter is dark-coloured, as in the Oak, with its brown heart- wood, and in species of Diospyms, whose black heart- wood furnishes. ebony. The darker the heart- wood, the harder and more durable it usually is. The following may be mentioned as examples of woods which yield dyes and colouring principles — Haematoxylon campcchianum, L. (Campeachy wood, logwood), with a red heart-wood from which H^MATOXYLIN is extracted ; Pterocarpus santalinus, L. fil. (red sandal-wood), from the heart-wood of which SANTALIN is obtained ; Caesalpinia brasiliensis, L., and G. ecltinata, Lam. (Brazil wood, Pernambuco wood), with a red heart-wood which supplies BRASILIA ; and the Alsage Orange, Madura aurantiaca, Nutt. (yellow Brazil wood), which has a yellow heart- wood from which MORIN is derived. Inorganic substances may also be deposited in the duramen ; thus calcium carbonate is found especially in the vessels of the Elm and the 'Beech, while silicic acid occurs in those of the Teak ( Tectona grandis).
TYLOSES (Fig. 142) are also instrumental in closing the water-courses of the
134 BOTANY I-AHT i
1 1 cart -wood. These are intrusive growths from living cells, which i>cnctr;itc tin- cavities of the adjoining tracheal elements during the transition of the sap-wood into heart-wood. In the formation of tyloses the closing membranes of the pits of pitted vessels form bulging ingrowths into the cavities of the vessel. Such ingrowths increase in size until several meet, and so more or less completely close the cavities of the vessels into which they have intruded. The closing membrane of the bordered pits in the heart- wood is pushed to one side, so that the torus presses against the opening of the pit and completely closes it. According to H. MA.YH (12B), resin does not penetrate the walls of wood cells under normal con- ditions ; the wood of Conifers only becomes resinous through the impregnation of the cell walls with resin, after they have become dried up through wounds or other causes. The resin-ducts of Conifers may also be closed by the formation <>t tyloses.
The elements of secondary growth differ in Gymnosperms and Dicotyledons. The vascular strands of Gymnosperms are composed almost exclusively of tracheides (Fig. 141 A}. These are provided with bordered pits which are situated, for the most part, in their radial walls. The tracheides of the spring-wood (/) have larger cavities than those formed later (s). Parenchyma is also present in the wood, though in relatively small amount ; in some Abietineae resin-passages occur in it (Fig. 141 h).
Except in the Gnetaceae, true vessels are not found in the secondary growth, nor in the primary vascular portions,, of the bundles of Gymnosperms. The wood produced by the cambium consists of radial rows of tracheides, the number of which is occasionally doubled by the radial division of a cambium cell (Fig. 141 A, a). The tracheides are often over a millimetre long, much longer than the cambium cells from which they are developed. They attain this length by a subsequent growth, during which their growing ends become pushed in between one another. In addition to the tracheides, small amounts of wood parenchyma are also produced in Gymnosperms by a transverse division of the cambium cells. It is in the parenchymatous cell rows of the wood of Pines, Spruce-Firs, and Larches that the schixogenous resin-ducts are produced (Fig. 141 A, h). In other Conifers the wood parenchyma consists of simple rows of cells, which afterwards become filled with resin.
Besides tracheides (t) and wood parenchyma (hp), other element > take part in the composition of the secondary wood of a Dicotyledon ; these are the vessels (tracheae, g), and the wood fibres (h) (Fig. 145 A, B}. The cells of the wood parenchyma are short and have abundant contents, the woods fibres are thick- walled, long cells with pointed ends. The elements with wider lumens, especially the vessels, are abundant in the spring-wood, in which water conduction is important. The autumn -wood, on the other hand, consists of narrow elements, among which the wood fibres, which contribute to the rigidity of the plant, are numerous. On account of these differences between spring and autumn wood the annual rings are well marked (Fig. 148).
SECT. I
MORPHOLOGY
135
All the elements entering into the formation of the wood of Dicotyledons can be derived from the two classes of tissue already met with in the Gymnosperms, the tracheal tissue and the parenchymatous tissue of the wood. The tracheides and vessels belong to the former class, while under the parenchymatous tissue are included the wood parenchyma, fibrous cells of greater length but with similar contents (Fig. 145 ef), and the wood fibres.
The tracheal tissue consists of elements which lose their living contents at an early stage, and in their fully developed condition are in reality only dead cell cavities. In this class are included TRACHEIDES having relatively wide lumina and large bordered pits, and ultimately also spirally thickened walls, which serve as water-
sni
FIG. 143.— Radial section of a Pine stem, at the junction of the wood and bast, s, Late tracheides ; t, bordered pits ; c, cambium ; v, sieve-tubes ; at, sieve-pits ; tin, tracheidal medullary raj- cells ; sm, medullary ray cells in the wood, containing starch ; sm', the same, in the, bast ;' em, medullary ray cells, with albuminous contents, (x 240.)
carriers (Fig. 145 A, t) ; VASCULAR TRACHEIDES (gt), with similar functions, but with the structure and thickening of vessels ; FIBRE TRACHKIDES (ft), with small lumina and pointed ends, having only small, obliquely elongated bordered pits, and, in ex- treme cases, exercising merely mechanical functions ; and finally TRACHEA (</), formed by cell fusion, and provided with all the different forms of thickenings by which they are distinguished as annular, spiral, reticulate, or pitted vessels. All vessels function as water-carriers. If they have small lumina and resemble tracheides, they may be distinguished as TRACHEIDAL VESSELS (tg) ; if, as is generally the case, they, have bordered pits on their lateral walls, they are usually provided with tertiary thickening layers in the. form of thin, spiral bands (Fig. 149 m). In the parenchymatous tissue of the wood the cells (Fig. 145 B) generally retain their living contents, and never develop the true bordered pits with a torus in the closing membrane, which are so characteristic
136
BOTANY
PART I
\-t
,v
of the water-conducting elements. All tissues of this class may be best derived from wood parenchyma. The wood parenchyma is produced by transverse divisions of the cambium cells, and accordingly consists of rows of cells (hp] with transverse division walls, and others obliquely disposed, which correspond to the alternately differently pointed ends of the cambium mother cells. The cells of the wood parenchyma are provided with simple round or elliptical pits, varying in size in different kinds of wood ; they generally contain starch, and some of them also take
up bye-products, resulting from metabolism, or from the chemical changes taking. place within A plant in the processes of its nutrition and growth. The cells having the ••closest resem- blance to those of typical wood parenchyma are the so-called FIBROUS CELLS (ef). In their contents, as well as in their wall thickenings, they are similar to the cells of wood parenchyma, but each [is formed directly from one entire cambium cell. In' their formation, the cells of the cambium tissue become more or less elong- ated and fibrous. The LIBRIFORM FIBRES or WOOD FIBRES (h) have a similar origin, but are even more elongated and have thicker walls, and, at the same time, narrow, obliquely elong- ated, simple pits. Although the wood fibres may continue living, in the more extremely developed forms (h) they lose their living contents. They are then filled with air, and their function is -merely mechanical. Under certain conditions, by later transverse divisions, the libriform fibres may become transformed into SEPTATE WOOD FIBRES (gh). The trans- verse septa thus formed remain thin, and form a striking contrast to the more strongly thick- ened lateral walls. \ While the tracheal tissues are engaged in providing for the conduction of water, the duty of conducting and storing the
pit; tm, tracheidal medullary ray products of assimilation, in particular the cells; am, medullary ray cells con- carbohydrates, is performed by the parenchy- ' matous tissues of the wood. Both forms of
tissue, however, aid in maintaining the rigidity of the plant body, and, in their most extreme development, furnish such elements as the fibre tracheides on the one hand, and on the other the empty wood fibres, which are only capable of performing mechanical functions.
The wood of Dicotyledons is made up of the elements of these two classes of tissue, the tracheal and the parenchymatous, but all the different elements are not necessarily represented in any one kind of wood.
Drimys, a genus belonging to the Magn»liaceae and two genera of the related order Trochodeiidraceae, are the only Dicotyledons the wood of which is formed of tracheides only. These Dicotyledons closely resemble the Conifers in structure. In numerous Leguminosae, Willows, Poplars, and species of Ficus, on the other hand, the tracheal tissues are only represented by vessels, which perform the task
FIG. 144. — Tangential section <>t the autunm-wood of a Pine, t, Bordered
on one side; i, intercellular SJKII-I- in the medullary ray. (X 240.)
MORPHOLOGY
137
of water conduction. In the wood strands of these plants there are also present wood parenchyma and a large proportion of wood fibres. The vessels in climbing plants (lianas) are especially wide.
The distribution of the living elements in the wood strands always bears a dis- tinct relation to the water-courses which they accompany, enclosing them in a more or less complete sheath. The living cells adjoining the tracheal elements are in communication with them by means of one-sided bordered pits. When such pits
ft
Fio. 145.— .4, Elements of the tracheal tissue of the wood ; diagrammatic. /.', Elements of the parenchymatous tissue of the wood ; diagrammatic. For description see text.
occur between living cells and tracheal (elements the pit cavities are absent on the side of the living cell, but present in the tracheal elements ; they differ from the true bordered pits in the absence of a torus on the pit membrane.
The Elements of the Secondary Phloem in Gymnosperms and woody Dicotyledons are sieve-tubes, or these together with companion cells, bast parenchyma with abundant cell contents, and long narrow bast fibres with strongly thickened walls. The sieve-tubes serve to conduct proteid materials ; the companion cells, or in their absence special rows of the bast parenchyma, take up substances from the
138
BOTANY
I'AUT I
sieve-tubes ; storage and conduction of carbohydrates take place in the parenchyma in which bye-products of metabolism, such as tannins and calcium oxalate, also accumulate.
As in the case of the wood, the elements of the bast may be referred to two forms of tissue, the sieve-tube and the parenchymatous. The former is represented by the sieve-tubes or by these together with companion cells, the parenchymatous portion by the phloem parenchyma and the bast fibres, between which there are intermediate forms of element.
In the bast strands of Gymnosperms, the phloem elements pro- duced by the cambium (Fig. 141 ft, c) consist solely of sieve-tubes,
••-.".
;'// tm
fZ¥fe.~-*J
; .T - -
-pm
FIG. 146.— Transverse section of a stem of Tilin
in the fourth year of its growth. pr, Primary cortex ; c, cambium ring ; cr, bast ; pm, primary medullary ray; pm', expanded ex- tremity of a primary medullary ray ; am, second- ary medullary ray ; g, limit of third year's wood. (x 6.)
KM.. 147.- A radial section of the wool of Tiliii ulmijuliii, showing a small medul- lary ray. g, Vessel ; I, wood fibres ; ////. medullary ray cells in communication with the water -courses by means of pits ; snt, conducting cells of the medul- lary ray. (x '240.)
the parenchymatous cells of the bast parenchyma (p and /,•), and, in certain cases, of bast fibres. These elements of the bast generally form alternating bands.
The Araucarieae, Taxineae, and Cupressineae have definite, vertical rows of bast parenchyma cells which take the place of the companion cells wanting in these plants. At a certain distance from the cambium the sieve-plates become overlaid by callus. During the vegetative period following their development, the sieve-tubes become empty and compressed together (Fig. 141 B, cv). The rows of bast parenchyma cells containing albuminous substances, which are found in some Conifers, undergo disorganisation at the same time as the adjacent sieve-tubes ; the bast parenchyma cells which contain starch, on the other hand, continue living for years, and even increase in size, while the sieve-tubes become compressed.
The elements of the phloem tissue included in the bast strands
SECT. I
MORPHOLOGY
139
of woody Dicotyledons (Fig. 150) are represented by SIEVE-TUBES (v) and COMPANION CELLS (c). To the parenchymatous tissues of the bast belong BAST PARENCHYMA (p), BAST FIBRES (/) and transitional forms between them.
The bast fibres,' like the fibres of the wood, may occur in an unthickened' form as FIBROUS CELLS, either with or without living contents, or they may be filled with starch, or finally may become septate.
The elements of the bast of Dicotyledons fre- quently exhibit a great regularity in their ar- rangement. Thus in the Linn- (Fig. 150) there is a regular alternation of sieve -tubes (v) and companion cells (c), bast parenchyma containing starch (p) and crystals (Jc), tangentially flat- tened bast parenchyma (p), and then another zone of sieve-tubes. The sievo-tubes of Dicotyle- dons as of Gymnosperms only remain functional for a short time, after which they become empty and crushed. The companion cells, which are sister cells of the
sieve-tubes, undergo the yiff ^ f
same fate. The starch- containing parenchyma, on the other hand, may remain for years un- altered. The differences
in the appearance of the bast of dicotyledonous trees are due to the greater or less diameter of' the sieve-tubes, the presence or absence, of bast' fibres, and the arrange- ment of the various elements.
The Medullary Rays of the Gymnosperms (Fig. 139 ms) and woody Dicotyledons (Fig. 146 pm, sm) form radial bands, composed wholly or in part of parenchymatous elements. Their function is to supply the cambium and wood with the products formed in the leaves and conveyed away by the bast ; they also conduct water outwards from the xylem. The medullary rays in this way link together by radial bands of living cells the protoplasm-containing elements of the bast and wood, thus uniting all the separate living tissues of the stem. The medullary rays are in turn accompanied or, if many-layered,
!•'[<;. 148.— Portion of ;i transverse section of the wood of 'Cilia idmifolia. TO, Large pitted vessel ; t, tracheides ; I, wood fibre ; p, v.ood parenchyma ; r, medullary ray. ( x 540.)
140
BOTAXY
PART I
traversed by intercellular spaces filled with air. These, beginning in the periphery of the stem, penetrate the cambium and communicate with all the intercellular spaces throughout the living elements of the wood and bast. All the living elements are kept in communication
with the atmosphere by means of the intercellular spaces of the medullary rays, and the necessary interchange of gases is thus rendered possible.
The substances contained in the parts of the medullary rays within the wood, chiefly consisting of starch, tannins, resin, and crystals, are essentially the same as those in the woo. 1 parenchyma. In the medullary rays of certain Gymnosperms, particularly in the Pine, single rows of cells, without living contents and situ- ated usually at the margin of the medullary bands, become tracheidal in structure (Figs. 143, 144 (m), and united with one another and with the tracheides by means of bordered pits. Their purpose is to facilitate the transfer of water radially between the tracheides. In other Conifers, where such tracheidal elements are not found in the medullary rays, bordered pits are developed in the tangential walls of the tracheides of the autumn- wood, and by means of them the transfer of water in a radial direction is effected. The living cells of the medullary rays of the wood bear the same relation to the water-carriers as does the wood parenchyma, and like them are connected with the water- conducting elements by means of bordered pits on one side of the cell wall. They take up water from them and give it out again, as it may be FIG. 149.— Tangential section of the wood needed, toother living cells ; on the other hand, of Till,, ..tmifoli,,. m. Pitted vessel ; in the wring, at the beginning of the season t, spiral traoheides ; j>, wood paren- .
,-hyma; I, wood llJ.n-s ; r, m.Nlullary of gH"**, theJr press m*0 *he water-cour.ses rays, (x 160.) the products of assimilation, in particular
glucose and small quantities of albuminates,
in order that these substances may be transferred in the quickest way to the points of consumption. Accordingly, during the winter and in the beginning of spring, sugar and albumen may be detected in the tracheal elements (}26), and may thm be obtained from the watery sap of "bleeding " trees, or from artificial borings or incisions, particularly in such trees as the Maple, Birch, and Hornbeam. In the wood of Dicotyledons it is usually only special rows of the medullary ray cells which stand in such close relation with the tracheal tissues. In these special rows, generally on the margins of the medullary rays, the cells- are elongated vertically, and on that account have been distinguished as VERTICAL MEMUI.LAUY KAY CELLS. Theiother cells, or those of the middle layers of the medullary bauds, on the other hand, are called HORIZONTAL MEDULLARY KAY CELLS ; they are narrower and more elongated radially (I27). These have no especial connection with the tracheal
SECT. I
MORPHOLOGY
141
elements, but are designed for conducting and storing assimilated substances. Within the bast xone the medullary rays of Dicotyledons have a simpler structure than in the wood. It is evident, not only from the pits between the cells of the medullary rays and the bast parenchyma, but also from the similar relations in Dicotyledons between the medullary ray cells and the companion cells of the sieve-tubes, that the function of the cells of the medullary rays in the phloem is to take up substances passing down the bast strands.
In the Pine and other Abietineae, whose bast parenchyma is devoid of cells functioning as conductors of albuminous matter, their place is taken in this respect by rows of medullary ray cells (Fig. 143 em}. These maintain an intimate connec- tion with the sieve-tubes by means of sieve-pits. They lose their contents in the
Fio. 150. — Portion of a transverse section of the bast of Tilin ulmifnli". v. Sieve-tubes ; v*, sieve- plate ; c, companion cells; k, cells of bast parenchyma containing crystals ; ;>, bast pui-en- chyma ; f. bast fibres ; /•, medullary ray. (x :>W.)
same manner as the sieve-tubes, 'and, like them, become compressed and dis- organised. On the other hand, the cells of the medullary rays, which contain starch, like the similar cells of the bast parenchyma, increase in size, and continue living for years.
The width and height of the medullary rays may be more easily determined from tangential than from radial sections. In such tangential sections the medul- lary rays appear spindle-shaped (Figs. 144, 119). With few exceptions, as in the Oak and Beech, the medullary rays are of relatively small size. The Oak, in addition to numerous small medullary rays, has other larger rays which may be as much as a millimetre broad and a decimetre high. In the Poplar, Willow, and Box the medullary rays are so extremely small that they are scarcely visible, even with the aid of a magnifying glass. The height of the broad primary rays of many lianes, on the other hand, may be equal to that of a whole internode. In certain Conifers, resin-ducts occur not only in the wood, but also in the broader medullary
142
BOTANY
rays. These radial resin-ducts are in communication with the vertical ducts. It is due to this fact that such a large amount of resin exudes from wounds in Pine or Fir trees.
Secondary Thickening of the Root in Gymnosperms and Dicotyledons. — The ROOTS of Gymnosperms and Dicotyledons, in which the stems increase in thickness, show a similar GROWTH IN
THICKNESS (12S). When secondary growth begins in a root with its xylem and phloem strands alter- nating with each other (Figs. 1 24, 131), layers of cambium arise on the inside of the phloem strands, through the division of the funda- mental tissue : these give off wood elements towards the centre of the root, and bast towards the periphery. These cambium layers soon meet in the pericycle, just outside the xylem strands, and so form a complete cambial ring. In Fig. 151 A, this process is diagrammatically repre- sented. As a result of the activity of its cells the cambium ring soon loses its sinuous form, and becomes circular. In front of the primary vascular strands (g'), the cambium produces medullary