The anatomy of woody plants.

Front matter

THE BAKER & TAYLOR COMPANY NEW YORK

THE MACMILLAN COMPANY OF CANADA, LIMITED TORONTO

THE CAMBRIDGE UNIVERSITY PRESS LONDON

Preface

It is now forty years since De Bary’s classic Comparative Anatomy of the Vegetative Organs of the Phanerogams and Ferns made its appearance. In the interval much has been added to our knowledge, particularly in the paleobotanical and experimental fields. The doctrine of descent, too, has now reached a degree of prominence and importance which it did not possess in De Bary’s time. As a consequence, it is desirable that the general subject of the anatomy of the woody or so-called vascular plants should be reviewed, with special reference to its historical and experimental aspects. This is perhaps all the more desirable as an effective counterpoise to the extreme mechanistic tendencies of the time. It will accordingly serve a useful purpose to indicate how large a part of the organization of existing plants is an inheritance from their ancestors of earlier geological times.

In De Bary’s textbook both paleobotany and development are deliberately eschewed. ‘The first of these is now essential for any adequate comprehension of comparative anatomy in its all-important evolutionary aspects. It is abundantly clear that the most fruitful results from the standpoint of the doctrine of descent are to be derived from the comparative study of extinct and existing plants belonging to the same orders, families, or genera. It is, moreover, obvious that the living forms cannot be interpreted without a knowledge of their past, and that to an even greater degree the organization of fossil plants is a closed book to those who are unfamiliar with the anatomy of allied and still living types. The wide range of facts which must of necessity be covered calls for a somewhat brief and even elementary treatment. Fortunately, since De Bary’s time, it has become more and more evident that the study of the development of organs and tissues throws little trustworthy light on the processes of evolution, and consequently that aspect of our subject need receive no more attention than was vouchsafed to it by the great German anatomist nearly half a century ago.

Vv

Vi PREFACE

In the seventeenth chapter are summarized the important general principles derived from the investigation of related living and extinct organisms. The beginning of the studies leading to the formulation of these anatomical canons stands largely to the credit of French and English paleobotanists. Since they have worked mainly with Paleozoic types, their activities have been preponderantly in the direction of comparisons between the organization of the earlier cryptogams and gymnosperms and their still living survivors. It has been in some measure the good fortune of American anatomists to continue the lines of investigation thus begun and to extend them to the study of Mesozoic and still living gymnosperms. The extremely harmonious conclusions resulting from the anatomical comparison of both Paleozoic and Mesozoic forms with their surviving descendants have justified the extension of the same principles to the evolutionary investigation of other woody plants (particularly to the angiosperms), concerning the geological past of which we are still ignorant. The canons formulated in chapter xvii have as a consequence been regarded of such importance that any conclusions not in harmony with them have ordinarily not been considered in the following pages unless they have held the ground for many years or are at the present time advocated by anatomists of great eminence. On account of the large field covered in the present necessarily elementary work, this procedure has been regarded as essential, and it is hoped that, with the explanation offered, it will not appear to the reader too dogmatic.

The author has been fortunate in utilizing the services of his students in the preparation and illustration of this volume. He is particularly indebted to Mr. R. E. Torrey for his skilful and artistic execution of a large number of the figures. Help in this respect has also been supplied by Mr. R. C. Staebner and Mr. Charles Drechsler. Miss Ruth Cole has rendered invaluable aid in the onerous task of preparing the text and reading the proof. Miss Edith S. Whitaker has also assisted in the preparation of the index.

To Professor M. A. Chrysler, of the University of Maine, the author owes illustrations of secondary growth in monocotyledons,

PREFACE vii and to Miss Eloise Gerry, of the United States Forest Service, an admirable photograph elucidating the ‘‘bars of Sanio” in coniferous wood. Last but not least, the author records the valuable services of Mr. James Austin, assistant in the laboratories, in connection with the preparation of the numerous photographic illustrations. Dr. D. H. Scott and his publishers, Messrs. A. and C. Black, have very kindly permitted the reproduction of a number of figures from the admirable Studies in Fossil Botany, which are among the comparatively few illustrations in the present work which are not original. To Dr. R. T. Jackson the author is indebted for the opportunity of photographing for illustration a number of the sections of carboniferous plants under his care in the Botanical Museum.

Professor John M. Coulter has been good enough to read the proofs as the present volume passed through the press. To my friend Mr. R. W. Sayles, Director of the Geological Museum, I am indebted for valuable criticisms in the field of paleoclimatology ‘and also for other essential aid. The author is alone responsible for the views expressed and for any errors.

BOTANICAL LABORATORIES, HARVARD UNIVERSITY June, 1917 maa.

Contents

CHAPTER

LHe CELL . THE TISSUE SYSTEMS . . THE FIBROVASCULAR TISSUES: WOOD—GENERAL

. THE FIBROVASCULAR TISSUES: SECONDARY Woop—TRA-

Chapter I The Cell

It has been recognized since the seventeenth century that living beings, particularly plants, are composed of cells. The English investigators Hooke and Grew in the latter half of that century noted the fact that the body of vegetable organisms was often constituted of minute chambers, and to these Hooke first gave the name of cells. Grew was apparently the originator of the term ““tissue,’ and he compared the organization of plants with the woven texture of lace. A clear conception of the cellular structure of living beings was not, however, reached until nearly two centuries later. Plants differ from animals in the fact that their gross internal organization is of relatively slight scientific importance compared with the more obvious bony and muscular structures of the animal body. The anatomy of plants is thus essentially a matter for microscopic investigation. Some knowledge of the general features of the structure of the cell in plants is accordingly necessary as a preliminary to the more detailed pursuit of anatomy.

The cell in vascular plants has certain features which bring it into sharp contrast with the corresponding unit of structure in the higher animals. The essential substance of all living cells is protoplasm, simply distinguished from inanimate matter by the possession of a capacity for change and reproduction, which does not characterize matter devoid of life. In the higher plants the protoplasm does not show the large degree of solidity which is a feature of the animal cell, but is ordinarily reduced to a thin vesicle surrounding a larger or smaller central cavity known as the vacuole. The bladder-like protoplasmic vesicle is rendered possible in plants by a containing, supporting, and likewise more resistant envelope called the cell wall. This wall is not nitrogenous in its chemical composition, as is true of the living protoplasmic body, but is, primitively at least, a ternary compound, containing carbon, oxygen, and hydrogen, the first-named element being the most abundant.

I

Fig. 1 will make clear the organization of cells in an immature seed pod or pericarp of an iris. Each element obviously consists of a large central cavity as noted above, variously known as hydroplastid, hydroleucite, tonoplast, and vacuole. The last designation seems best for descriptive purposes. Surrounding the median space, which in life contains water with various substances in solution, is the protoplasmic utricle. The nature of the latter varies in different cells. In the elements to the left side of the illustration the protoplasm appears as a somewhat minutely granular substance, while in the cells to the right the structural organization of the protoplasm is marked by the presence of oval bodies, the chloroplastids, which in life .iI— i f ericarp o Fic. 5 Transverse section of young pericarp BAY onc green. A Iris species, showing organization of cells of epidermal and fundamental tissues. dark body of larger size is to be seen somewhere against the cell wall and imbedded in the protoplasm. This is the nucleus, one of the most important organs of the cell, which appears to preside over all its changes and activities. Another body, generally larger and lighter than the nucleus, is frequently present in the cells under discussion—the oilplastid or eleoplast. This is conspicuously spongy in its structure. We may now turn with advantage to the organization of the cell wall. On the left side of the illustration the common outer wall is limited by a distinct membrane, the cuticle, which is chemically different from the rest of the wall substance and is viable to

THE CELL 3 gases but not to fluids. Below the cuticle lies the cell wall proper, in this case composed of cellulose. The cuticle and its underlying wall are continuous except in the region of certain apertures, the stomata, one of which is represented in the figure. From the stoma there passes inward an air space, which quickly divides into fine canals lying in the angles between the internal cells. These are intercellular spaces and are a practically unfailing accompaniment of the living elements in vascular plants.

The next illustration (Fig. 2) visualizes the conditions in a harder tissue, namely, one of the wood rays of a dicotyledon. Here the mass of cells present individually somewhat rounded contours, and in the resulting angular interstices appear the intercellular spaces. The protoplasm in the cells under consideration is much denser than in the elements of the wall of the ovary figured above, Fic. 2.—Tangential section of and accordingly the nucleus lies near cells of a ray in a dicotyledon the geometrical center of the cell, as (‘Primys species), showing pitting ; : : in relation to cells and intercelluis commonly the case in highly proto- j,, mays plasmic elements, in contrast to its peripheral position in those in which the protoplasm is only a bounding utricle. The cell wall in the present instance is, relatively to the size of the cells, much thicker than in the first figure. As a consequence of this increased thickness of the wall, special devices are necessary for the purpose of permitting interchanges between the cellular elements and their environment. The wall is thin in certain definite regions for the attainment of this end, and these locally thin spots are known as pits. It is clear that the pit of one cell always coincides with a corresponding pit in a neighboring element. Further, it is obvious that, although the cells are in close relation to one another by means of their pits, these are prevented from becoming actual holes by the persistence of the middle lamella or cement substance as the membrane of the pits. Not all the pits, however, meet a corresponding depression. It will be observed that there are well-developed pits toward the intercellular aérating spaces. These are known as air pits and have no counterpart on the side of the air space. They are commonly found wherever the thickening of the cell wall makes difficult the interchanges between the living protoplasm and the outside air. In the figure under consideration the cells are united by the cement substance of the middle lamella, which appears dark and contains the air spaces within its substance. Most of the cells have had both the top and Pi eX se, ees 3 the bottom walls removed ay te ei RSS zi RES) by the plane of section, but Se FAS Qu — 5 we coaege = in one or two the cell wall

; meets the eye. Here the pits are seen in face view, and it becomes clear that they vary somewhat in size and are characteristically outlined by a single contour.

On account of this condi-

Fic: 3 Section of inneripericarp tion, which results from the (“‘stone”’) of a peach, showing cells with ex- fact that the bounding walls tremely thick and layered walls as well as = qo not overhang, pits of this numerous elongated pits. ; :

type are called simple pits.

Communicating pores of this kind are found in cells which in their functional condition contain living protoplasm.

Let us now turn to the examination of cells in which the wall is so greatly thickened that the space containing the protoplasmic body becomes much reduced in size. Fig. 3 reproduces some of the elements of a peach stone which has been softened by a prolonged sojourn in hydrofluoric acid. The protoplasmic structures have been largely consumed in the thickening of the cell wall. The latter shows well-marked indications of layering, a feature often present when much thickening has taken place. The layering doubtless has some numerical relation to the age of the wall in days. The pits are of special interest in this instance. Their correspondence in adjacent cells is as marked as in the former

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THE CELL 5 illustration. In many cases two or more near-lying pits of the same cellular element become confluent as they pass inward, and open into the cavity of the cell by a common aperture. The cement

Fic. 4.—Wood of the spruce (Picea canadensis), showing ray cells with simple pits and longitudinal water-conducting elements with bordered pits.

substance is strongly developed here and the thickened walls of the cells are lignified. The pits, in spite of the complications noted above, are to be regarded as simple pits.

The examples of cells hitherto considered have been those which in their functional condition contain living protoplasm. It is now necessary to refer to those very important histological elements of vascular plants, which in the mature condition normally contain no protoplasm and serve to conduct water. Elements of this nature are typically much elongated and are provided with a different sort of intercommunicating pitting than that found in the case of those which in the active state shelter a protoplasmic body. In the accompanying figure (Fig. 4) are seen a number of cells of this sort cemented together by the middle lamella (repre-

sented as a black framework around the cell walls). The pits which bring about intercommunication are figured as occurring on two of © ©) the four nearly parallel sides of the tracheids or water-conducting cells.

Their openings have overhanging margins, and the membrane, the

Fic. 5.—Diagram of simple and Presence of which precludes the bordered pits in face and profile views. possibility of actual openings, is Tn’ the center 2 bordered’ pit from” ‘much thickened in the midulomes heartwood is shown. }

constitute the so-called torus. The presence of overhanging margins clearly distinguishes this type of pore from that found in the walls of living cells. The cement substance uniting the cells with one another is in general lignified (that is, has undergone that somewhat complex and obscure modification chemically known as lignification) like the cell walls which it holds together, but is likewise partially in the pectic or mucilaginous condition, a state which causes it to absorb hematoxylin strongly. In the region of the pit membranes the middle lamella becomes pectic cellulose, and here water passes through the walls much more readily than it does elsewhere. On either side of the figure is seen a ray of the wood, the cells of which are in relation with the tracheids by means of pits. It is clear that the pits which bring about intercommunication are bordered on the side of the elongated element of the wood (the tracheid) and are

THE CELL - simple on the side of the medullary-ray cells. Pits of this nature are called half-bordered.

In Fig. 5 simple and bordered pits are represented diagrammatically side by side and from both profile and face view. Obviously the simple pits in face view are single in contour, while those which are bordered have a triple concentric outline, the outermost circle corresponding to the boundary of the broad membrane of the pit, the innermost to the narrow mouth, and the intermediate representing the outline of the torus. The distinction between simple and bordered pits is an extremely important one, particularly in the lower groups of vascular plants. In higher forms the distinction is of less value, but the presence of bordered pores in vascular elements in general still indicates that an important function of the element so provided is the transport of water. It should be further noted that elements with bordered pits are usually without intercellular spaces, while those in which the pits are of the simple type normally possess such aérating cavities. Exceptions to this statement are found ordinarily only in plants which have become highly specialized in connection with resistance to drought.

Chapter II The Tissue Systems

The cells in higher plants are generally grouped into well-marked systems of elements which are known as the tissues. ‘Tissues are sometimes defined as aggregations of cells performing a stmilar function. This definition is, however, open to some objections, as is also that which describes a tissue as a mass of cells of similar origin. Characterization of tissues, either by their functions or by their mode of origin, seems less desirable than a definition which makes clear that the aggregations of cells have a common. organization. From the standpoint of evolution it is the structural features of the tissues which are of the greatest significance. In works dealing with so-called physiological plant anatomy the functions of the tissues rather than their peculiarities of structure are naturally most emphasized. From the point of view of the doctrine of descent functional features are of less significance, since it is precisely these which are the most readily modified and as a consequence furnish the least valuable indications of the course of evolutionary development in any given large group. Because the present work deals with anatomy from the outlook of evolution, structural organization in the case of the tissues stands in the foreground, although, of course, the question of the functioning of the various tissue systems cannot be left out of view.

It is an interesting general fact that the boundaries between the tissue systems are much more marked in the plants which are geologically older and lower in the scale of evolution than they are in the higher seed plants, the conifers, and the angiosperms. Further, the more conservative organs of the higher forms—namely, the root and leaf—exemplify a sharper delimitation of the tissue systems than does that most progressive of all plant parts, the stem.

For the purpose of the present work the tissues of plants may be divided into three distinct systems, which can be most easily identified by reference to the accompanying figure (Fig. 6) of a

THE TISSUE SYSTEMS 9 transverse section of the creeping stem of Pleris aquilina. 'Throughout the area of the figure are scattered oval masses, the fibrovascular bundles. These are very sharply marked off from the rest of the tissues by a boundary appearing as a dark circumscribing line. On the outside of the stem is the integumentary system, or

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Fic. 6.—Transverse section of the rootstock of Pteris aquilina, showing three categories of tissue—namely, epidermal (external), vascular, and fundamental.

epidermal tissue, consisting of a single layer of cells. The epidermis is characteristically uniseriate in the lower forms, and only in some of the higher vascular plants does it become a multiple layer. The remaining structures of the stem of the bracken fern belong to the fundamental system. The next illustration (Fig. 7) shows a part of the stem of Pteris more highly magnified, so that the details of

IO THE ANATOMY OF WOODY PLANTS structure may be more easily discerned. External is the single layer! of the epidermis, made up of cells with thick, heavily pitted walls. With the greater magnification employed, the cellular organization becomes very obvious. Underneath the epidermal layer is situated the fundamental system, in turn clearly outlined against the fibrovascular tissues by a striking boundary composed of a single series of cells with dark tanniniferous contents. This is the most internal layer of the fundamental system and is known either as the endodermis or as the phloeoterma (the latter term in its etymology indicating the inner layer of the cortex or fundamental tissue). The fundamental tissues are characterized locally by

Fic. 7—A portion of a transverse section of the rootstock of Pteris aquilina more highly magnified, showing the three tissue systems.

certain bands of dark-brown skeletal tissue, which are very characteristic of the lower vascular plants and subserve to a large extent the mechanical function which in the higher plants is attended to by the fibrovascular system. ‘Those regions of the fundamental system which are not mechanical in their nature take over the function of storage and are crowded with granules of starch forming a cordon around the periphery of the cells. Centrally the elements of storage show the presence of a dark-brown substance in the region of the vacuole. This is tannin-like in its nature and is commonly present in the fundamental system of ferns. The fibrovascular structures stand out sharply from the rest of the tissues and are obviously much more complicated in their organization than are those previously considered. The fibrovascular ageregations of cells are of the greatest anatomical importance, both because their very complexity of structure supplies many valuable

THE TISSUE SYSTEMS Il features for comparison which may be utilized in the study of evolution, and because their conservatism makes them on the whole the least variable of the elements entering into the composition of the higher plants. Likewise, by reason of their resistance to decay, they are more likely to be preserved as fossils. Beyond emphasizing the complexity of the fibrovascular system, it need not be further considered at the present time.

Fic. 8.—Transverse section of the leaf of the white pine (Pinus Strobus), showing the three tissue systems in a leaf.

We may next consider the tissue arrangements in another of the plant organs, namely, the leaf. In the figure (Fig. 8) is represented the transverse section of the needle of Pinus strobus. Externally the epidermis forms a boundary of a single row of cells, continuous except where interrupted by the occurrence of stomatic openings. Beneath the epidermis lies a layer which is ordinarily. known as the hypoderma and is more strongly developed in fossil

Ne than in living pines. The fibrovascular strand is sharply bounded in the median region by the endodermis, a layer circular in configuration and well developed in Pinus, though often absent in the higher conifers. The organization of the fibrovascular strand need not particularly occupy our attention at this stage, as it will be considered in detail more appropriately in the sequel. It is enough often showing bordered pits—in other words, of tracheids. These are continuous on the flanks of the strand with short-pitted elements, the transfusion cells, which are of great interest from the evolutionary standpoint.

The cellular complex lying) outside the endodermis and within the epidermis is the mesophyll, the representative of the fundamental tissues in the leaf.

The cells of the mesophyll are infolded in a manner Fic. 9.—Transverse section of the root of characteristic of most the sarsaparilla (Smilax), showing the three living pines. Tn the meso-\ systems of tissues in root organs. phyll lie He on the lane side of the leaf, conspicuous secretory spaces, the resin canals. In the root of the pine, as in the leaf, the same sharp distinction between the fibrovascular structures and the fundamental tissues

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is present. In the stem of the conifers generally, however, the limit between the tissues belonging to the central conducting cylinder (the fibrovascular system) has become obsolete and can be

Judged to have been formerly present only on theoretical grounds.

In the case of the root of vascular_plantsin-general, from the lowest to the highest, the limit between conducting or fibrovascular tissues and the fundamental system is usually very distinct and is one of the features which so clearly and universally mark the root

_ as the most conservative of the organs of plants. Fig. 9 illustrates

THE TISSUE SYSTEMS ie the situation in this respect for the monocotyledons, which may on strong grounds be considered as the highest of the seed plants. Externally is the piliferous layer, from which the root hairs are derived, and which may be considered in a general way as the equivalent of the epidermis of the stem and the leaf. The central region is occupied by the fibrovascular system, sharply limited by the endodermis, composed of cells ordinarily thick-walled. Between the endodermal limiting membrane and _ the piliferous layer lies 4 _ the cortex of the root, and this corresponds to the fundamental “category of tissues in the case of the stem and leaf.

It will be clear from the account given above that there are three tissue systems in plants which are very distinct in lower forms and in the less changeable and more conservative parts. Of these the epidermal tissues are always clearly limited both toward the outside and also in relation to the tissue system which lies inside. The boundaries dividing the fibrovascular from the fundamental] tissues are often less plainly indicated, and in the case of the higher groups of plants, particularly those in which the secondary growth is strongly developed, may disappear altogether; in such cases the limits of the tissues can only be inferred from comparative and developmental anatomy.

Chapter III The Fibrovascular Tissues: Wood—general

Since the fibrovascular tissues are on the whole the most important in the organization of the higher plants both from the evolutionary and from the physiological standpoint, it will be well to begin with their anatomy. The most conspicuous and_bestdeveloped portion of the fibrovascular system in land plants is the wood. The aggregation of elements assembled under this heading affords also the best exemplification of the process of evolution in the higher plants as the result of the progressive development of the principle of division of labor and the gradual adaptations of plants to the more complicated conditions of life obtaining in later geological times. The resistance of the wood to the organisms of decay is greater than that of any other common plant tissue except those possessing cutinized or suberized cell walls. We have, consequently, in the woody structures past and present an almost perfect biological document, carrying back the history of plants in relation to their changing conditions of environment into remote epochs of our earth’s history.

In beginning the discussion of the organization of wood it will be well to direct our attention in the first instance to the contrasts in structure presented by woods of ancient and modern types. The first illustration (Fig. 10) shows us the situation in the oak. The wood is conspicuously marked into areas by boundaries running at right angles. Crossing the figure from top to bottom are rows } of large openings which represent the vessels or water-conducting | tubes of our modern forest trees. It is clear that these are large’ only along the lines which mark the beginning of each year’s growth. Farther out the vessels become suddenly of much smaller caliber. Not only is the ligneous structure of the oak transversely banded by reason of the strikingly larger size of the vessels which signalize the spring development, but also by an equally significant change in the diameter and the distribution of other more or less highly

FIBROVASCULAR TISSUES: WOOD 15 differentiated elements, to be described in a later chapter. At right angles to the annual zones of growth and crossing these are the large wood rays. ‘These are extremely conspicuous structures, and as a result of a variation in the rate of growth due to their presence they bring about very evident depressions on the faces of the annual rings. The intervals between the large wood rays are occupied by more numerous linear storage bands, which are but a single row of cells in thickness and are known as uniseri- ALeoLAyS.

A marked contrast _to the wood of the oak is presented Dyethe ligneous organization of the Paleozoic gymnosperm Cordattes, illus- Peaecdein Fig. 11, Imereti heannua | rings, SO clearly DIEeS- Fic. 10.—Transverse section of the wood of the red ent in the oak as qa 02k (Quercus rubra), showing annual rings and highly differentiated structure which characterizes the organization of the woody cylinder in modern trees.

result of a differentiation in the size and character of the elements corresponding to regularly recurring annual changes, are conspicuous by their absence. This situation is directly correlated with the more equable annual cycle of remote geological times. We find illustrated in the case of Cordaites absolutely no indication of seasonal changes in temperature or variations in other important conditions of a periodic or seasonal nature. Not only is the wood monotonously the same as one passes from the inner regions to the exterior layers, but it likewise shows slight differentiation in the direction from left to right. Large rays of the oak type are quite absent, and the radial storage strands are entirely linear in their nature.

Having noted the varieties involved in wood structure in correlation to the more variable conditions of environment present in modern times, we may now with advantage direct our attention to other features of organization which characterize the evolution of the woody cylinder in the higher plants.

Fig. 12 illustrates the transverse section of the wood of a lepidodendrid, an ancient

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