The Stem and Its Functions

We have shown that the root, which absorbs water and mineral substances from the soil, and the leaf, which carries on the manufacture of food, are the primary vegetative organs of the plant. A third member, the stem, connects these two. It forms a conspicuous feature of most plants and in woody species constitutues the great bulk of their bodies. Its functions, though secondary to the major activities which we have mentioned, are nevertheless essential ones. It serves to dispose the leaves in situations favorable for photosynthesis, and provides a highway for transportation between leaf and root. In addition, the stem frequently becomes a storage-organ and may be variously modified for other special functions.

The External Structure of the Stem

The stem displays a wide range of variation in size and in external and internal structure, according to the habit or growth-form which the plant assumes. In herbs, where the whole shoot dies back to the ground during periods unfavorable to vegetative activity or at the completion of a given cycle, the stem is comparatively slender and soft in texture. In plants with perennial above-ground parts, however, it grows thicker from year to year and becomes hard and woody, forming the stout stems characteristic of shrubs and trees. In shrubs the stem is comparatively short and slender and is usually much branched, even close to the ground. In trees, it grows taller and is developed for some distance upward into a main stem or trunk which may become very thick. Woody stems transitional between these two types often occur.

Buds

The growth of the stem in length takes place only at certain definite points, where the cells are thin-walled and capable of active division. In many stems, particularly those which are perennial and woody, these growing-points are protected by leaves or scales and are then known as buds. Buds may be terminal, developing at the tip of the stem, or lateral, arising from the sides. Within the bud are not only the beginnings of the young stem but of the various structures which are borne upon it, such as leaves and flowers. The bud scales, which protect these delicate parts, are usually stout and impervious.

The terminal bud governs the elongation of the stem, and through the development of lateral buds, branches arise. The shape of the aerial portion of the plant is determined primarily by the number and arrangement of these branches and by their rate of growth relative to each other and to the main stem.

In certain herbaceous plants the terminal bud produces a flower or flower-cluster, and the growth of the stem in length usually ceases at this point. Such determinate growth is not common among woody plants, however, and their stems continue to elongate indefinitely.

Leaves

Leaves are borne throughout the length of the stem in herbaceous plants and on the twigs of the current years growth in woody forms. That point on a stem at which a leaf is attached is called a node and the region between two nodes, an internode. The position of the node also governs the position of the lateral bud, for such a bud normally arises only in the leaf axil, or upper angle between leaf and stem.

The arrangement of leaves on the stem, or its phyllotaxy, may display many different types. If but one leaf occurs at a node the next one above it arises from the other side of the stem, and the arrangement is thus an alternate one. These two leaves may be exactly half way around the stem from each other, but it is much more common for their angle of divergence to be less than 180° and for the points of attachment of a series of successive leaves thus to form a loose spiral around the stem. The closeness of this spiral and the position of the leaves thereon show great diversity, but are generally constant within any particular species. If two leaves arise from the same node they are always directly across the stem from each other and are said to be opposite in arrangement. When there are more than two leaves at a node, they are disposed about the stem in a circle or whorl.

Surface

The surface of a young stem is protected only by an epidermis, but later this is replaced in woody plants by a characteristic layer of corky cells, the bark. The necessary exchange of gases between the air and the living tissues of the stem takes place through the lenticels, small spots or strips where the bark tissue is softer and looser than elsewhere.

Other Stem Types

The typical upright, foliage-beraing stem has sometimes become radically modified for the performance of other functions than support and conduction. Many plants have abandoned the erect habit, and their weak, slender stems climb or scamble by various means over other objects or lie prostate on the ground. In certain herbs the main stem may even become subterranean, in which condition it is known as a rootstock or rhizome. Typical stems give opportunity for the storage of a certain amount of food reserves, especially in pith and cortex, but in some species this function is so greatly developed that the stem system , or certain parts of it, becomes essentially a storage organ only. This condition exists in most rootstocks, and its extreme development results in a reduction of the stem to a short, thick tuber such as we know in the potat, which is morphologically a stem but now shows little obvious resemblance to that organ. The bulb and the corm are other examples of highly modified underground stems.

The Internal Structure of the Stems

In the cross section of a typical young stem there may be distinguished the same three types of tissue which are present in the root, but they are arranged somewhat differently. Outside the whole is the epidermis, consisting of a single cell-layer, and often replaced entirely, at an early stage, by a zone of corky bark. Beneath this is the cortex, varying in thickness but rarely occupying as prominent a a place in the stem as it does in the root. Beneath the cortex lies the fibro-vascular cylinder which, unlike its counterpart in the root, is arranged in the form of a hollow tube. The core of this tube is occupied by the pith, a tissue much resembling the cortex. A more detailed account of the character of the cells composing these tissues may be appropriately undertaken now, for although all the tissues here mentioned are present in root, stem, and leaf, they reach their greatest differentiation and complexity in the stem, and in this region of the plant they can therefore most profitably be studied. The structure of the fibro-vascular tissues of a woody dicotyledonous plant can well be seen in Figs. 49 and 50 (TODO: Add all figures from the book), a transverse and a radial longitudinal section through a portion of the stem shown in Fig. 48.

Protective Layers

The epidermal cells resemble those of the leaf epidermis and require no special comment. In stems which are growing in thickness, however, the epidermis is soon sloughed off and its protective function is assumed by a layer of corky cells formed directly under it and constantly renewed. In these cells the protoplasm soon disappears and the normal cellulose wall becomes corky or suberized and is thus rendered almost impermeable to air or water. The lenticels, which we have already mentioned, are spots in this corky layer where the cells are somewhat loose and spongy and thus allow the passage of gases.

Cortex and Pith

The cortex and the pith are very similar in constitution. Their cells usually remain alive, are roughly spherical in shape, retain their cellulose walls and function chiefly in the storage of food. To such undifferentiated tissues the term parenchyma is often applied. In older woody stems the pith often dries up and collapses; and the cortex, crushed by the expansion of the wood underneath it, is finally sloughed off.

Bast

The fibro-vascular cylinder is composed of two distinct tissues. On the outside is the bast or phloem, the function of which is to transport the elaborated foods - the carbohydrates, fats, and proteins - from one part of the plant to another, especially from regions of manufacture to those of storage or consumption. The cells concerned in this process are the sieve-tubes, living cells with thin cellulose walls but unique in their lack of a nucleus. They are elongated parallel to the main axis of the stem and their end walls (more rarely their sides) are provided with sieve-plates or definite groups of small perforations. Through these perforations extend threads of cytoplasm from one cell to another, so that the living substance of each sieve-tube is directly continuous with that of the adjacent ones. In the highest seed plants there is next to each sieve-tube a small companion cell, provided with an abundance of cytoplasm and a nucleus. In addition to these two types, groups of long and very thick-walled cells, the bast-fibers, characteristically occur in the phloem, and some parenchyma is usually present there also.

Wood

The inner portion of the fibro-vascular cylinder consists of the wood or xylem, which provides mechanical rigidity for the stem and transports the stream of water and dissolved substanced from root to leaf. As essential elements of the xylem we find cells which are much elongated parallel to the main axis of the stem and in which the cellulose walls have become very thick and woody. Such walls are said to be lignified. As soon as one of these cells is fully developed, it dies and its protoplasmic contents disappears, so that only the thick, woody cell-wall is left. Definite thin areas or pits occur at frequent intervals along this wall and facilitate the rapid movement of water. In simpler types of wood, such a cell is able to provide both the necessary rigidity and conductive capacity and is known as a tracheid. In the higher types, however, this simple element has become specialized in two directions and has given rise to long and very heavy-walled cells, the wood-fibers, in which almost no cavity remains and which contribute a high degree of mechanical strength to the wood; and the vessel-cells or tracheal cells, much shorter, with wide cavities, and walls which are comparatively thin and are provided with large perforations in their ends. These cells, laid end to end in vertical rows, constitute the ducts or vessels, so characteristic of the wood of many plants, which carry the ascending stream of water through the stem. Parenchyma cells sometimes occur among the lignified elements and like them may be elongated vertically. Other parenchyma cells are elongated at right angles to the stem and dispersed among the woody cells in horizontal bands or ribbons running out htrough the xylem along the radii of the stem. These structures are known as the woody-rays, and in somewhat modified form extend also into the bast. They facilitate the horizontal transfer of materials in the stem and are of particular importance as centers of food-storage.

Cambium

A narrow layer of thin-walled cells, the cambium, separates the wood from the bast. Through its activity new cells are added to the outside of the wood and the inside of the bast, and the thickness of the stem is thereby increased. Among woody plants, such growth continues from year to year and each seasons increment, or annual ring, is easily recognizable.

At each node a small but complete segment of the fibro-vascular ring separates from the rest and passes out through the cortex into the base of the petiole, causing a break, or leaf-gap, in the ring. Into each leaf may enter one, three, or five or more of these leaf-traces which are destined to pass upward through the petiole and to form the system of veins in the blade.

Woody and Herbaceous Stems

The perennial woody stem in which the fibro-vascular cylinder, as seen in cross section, forms a continuous and rather wide ring (except for the leaf-gaps), and which recieves additions in thickness year by year through cambial activity, is probably the most ancient stem-type among seed plants; and the herbaceous condition, where the stems are much softer and shorter-lived, has apparently been derived form it in response to climatic changes or for other reasons. In herbaceous species, the amount of fibro-vascular tissue has become proportionally very much less. This may be due simply to a decrease in activity of the entire cambium, or to the breaking up of the cylinder into separate bundles, but in general any herbaceous stem is roughly comparable to a one-year-old twig of the particular woody stem-type from which it has been evolved. The herbaceous stem in Fig. 55, with its thin but continous vascular ring, has probably arisen from some such woody form as shown in Fig. 53, where the vascular ring is similarly continuous and homogeneous. The stem in Fig. 56, however, in which the cylinder has been broken into distinct and completely separate bundles, is quite different in type and has probably arisen from a woody stem somewhat resembling that in Fig. 54, where the vascular ring is divided into segments by the development of very wide rays. Cambial activity is usually weaker opposite rays than opposite the woody segments of the cylinder, and the stouter herbs of this type, the rays therefore tend to form broad constrictions in the ring. In more delicate herbaceous stems the constrictions finally become complete, the broad rays disappear, and the cylinder is thus broken up into a ring of separate segments or fibro-vascular bundles. Each of these consists of a group of wood cells on its inner side and of bast cells on its outer, with a vestige of cambium between. Connecting the cambium layers of two adjacent bundles there may be a weak interfascicular cambium, producing a few layers of parenchyma cells. In many herbaceous stems, however, the bundles, each surrounded by a bundle-sheath of thick-walled cells, are quite distinct and widely separated from one another, with no remnant whatever of a cambial zone between them.

In still more highly specialized stems, characteristic of monocotyledonous plants, the bundles are no longer arranged in a ring but are scattered irregularly throughout the whole area of the stem (Fig. 57). The individual bundles are very distinctive in appearance (Fig. 58), each possessing a large air-space or lacuna, two large vessels, and a patch of very regularly arranged sieve tubes and companion cells. In such a stem no distinction between pith and cortex now remains. The departure of the leaf-traces here is very complex, a large number of bundles moving outward from the center of the stem and entering the sheathing leaf-base.

The Structure of Wood

In shrubs and trees (conifers and dicotyledons alone in this context) the great bulk of the stem, particularly in its older portions, consists of but one tissue, the wood. Wood is so important in the economy of the plant and of such great significance to man that we are justified in studying it a little more closely than we have the other tissues.

Through the activity of the cambium (a fuller account of which we shall reserve for the chapter on growth) a new concentric layer of wood cells is added each year to the outside of the woody cylinder. The tracheids and ducts produced at the beginning of the growth in the spring are usually of large diameter and are accompanied by comparatively new fibers, and it is apparently in this spring wood that most of the upward conduction of water takes place. In the later-formed portion of the annual ring, the water-carrying cells are fewer and narrower, and the bulk of the tissue is composed of fibers. This summer wood is responsible for most of the rigidity and strength of the stem. In large branches and trunks, the older portion of the wood, consisting of the first-formed annual rings at the center of the stem, in time becomes dead throughout and ceases to perform its functions of water-conduction and storage. It then constitutes the heart-wood (Fig. 59) and is frequently distinguished from the outer layers by its darker color. The living and functioning part of the wood is its youngest portion and is known as the sap-wood (Fig. 59). This, of course, is on the outside of the woody cylinder, and it is usually constant in width in any particular species, its outermost is added by the activity of the cambium. All of the non-woody cells here (the parenchyma cells and ray cells) are alive.

Wood is usually cut along one of three distinct planes, and the cut surface in each case presents a very different appearance (Fig. 60). In describing a given wood it is therefore customary to consider its characteristics as they are shown in these three cuts or sections. An ordinary “cross cut”, at right angles to the length of the log, is known as a transverse section, and shows the annual rings as a series of concentric circles, with the wood rays running out from the center as narrow lines along the radii. Where the cut is longitudinal and made exactly along the radius of the log, a radial section results. This presents the annual rings as vertical straight lines and the wood rays as horizontal stripes or markings. Where the rays are fairly wide, as in the oak, these markings are prominent and furnish the much-prized “silver gain” so readily seen in quartered oak. Other longitudinal sections, which do not lie in a plane passing through the center of the log, are known as tangential. If the structure is exactly regular and the cut exactly true, the annual rings are here seen as straight lines somewhat unequally distant, running up and down along the wood. The irregularities which almost always occur, however, cause the rings in such a cut to appear as wavy lines which produce the common “grain” of most wood surfaces. The rays are very inconspicuous in a tangential section, for only their cut ends are visible. The relations between these three sections, and the characteristic appearance of the various wood structures as seen in them, is shown in the segment of an oak log (Fig. 61) and the magnified cube of the pine wood (Fig. 62).

Woods of verious species differ from one another markedly in such gross characters as color, weight, hardness, chemical composition, width of annual rings, width of rays, and number, size and arrangement of vessels; and in such microscopic features as the size, shape, character, and location of the different classes of wood-elements, the type and distribution of pits, and the various markings on the cell-walls. The structure of two distinct and their important woods, those of pine and of oak, are well shown in their transverse, radial and tangential sections in Figs. 63 and 64. The various details of wood structure remain so constant that they may often be used to identify the plant species from which a piece of wood has been derived. The great diversity which wood displays, together with its abundance and the ease with which it can be manipulated, have led to its use in numberless ways, and there is consequently no other plant tissue, aside from those used as food, which is of such great economic importance.

The Ascent of Sap in Stems

We can determine by experiment that water and dissolved substances absorbed by the roots are carried upward in the wood of the stem. As to what causes this movement, however, there is still much question. To explain the ascent of water in low-growing herbaceous plants might be fairly simple, but the factors which bring about the lifting of water in large quantites to the tops of tall trees, sometimes more than 300 feet (100m) above the ground, are very hard to determine. An upward osmotic pull is of course furnished by the increased sap concentration in the leaf-cells which follows the water-loss therefrom in transpiration, but even granting a strong pull at the leaf, the rise obviously cannot be due to simple “suction” or atmospheric pressure. Nor is capillarity probably concerned to any great extent in the process, for, although water may be lifted very high in exceedingly small capillary tubes, its movement is so slow under these conditions that capillarity certainly could not provide the large amounts of water which we know much ascend the trunk daily. Root-pressure, if it were strong enough, might perhaps be important, but root-pressure is mainfest in woody plants only during the early spring and is therefore lacking at the season when transpiration is most active. It has been suggested that the living ray and wood parenchyma cells may be concerned in the upward movement of water in some way, perhaps furnishing a continuous series of osmotic pumps. These cells may be of some such service, but we know that for a considerable time, at any rate, water may ascend through a stem where all the living cells have been killed. The post plausible hypothesis yet put forward is based on the very high cohesive power exhibited by water under certain conditions. In very thin water columns, such as must occur in the conducting cells of the wood, this cohesive power is perhaps so strong that a pull at the top - in this case the osmotic pull at the leaf - will lift the column bodily, as a rope might lifted. There are certain objections to this explanation, too, but they are not as serious as in the other hypotheses. Possibly several of the factors mentioned may be concerned together in the ascent of sap. We must admit that this problem, like so many others in biology, is as yet far from a satisfactory solution.

The Translocation of Foods

The plant must possess means not only for insuring the passage of a plentiful supply of water to the leaves through the wood of the stem, petioles and veins, but also for transporting the product of the leafs activity - the manufactured food in the form of carbohydrates; fats and proteins - to any region of the plant where food is used or stored. This function of translocation is performed chiefly by the sieve-tubes of the bast. The movement of organic substances by diffusion from cell to cell is a comparatively slow process, but is the only means available in regions remote from the vascular system. Movement of food for long distances, as from the leaf to the storage regions of stem and root, seems to take place almost entirely in the bast. Here the protoplasmic connectoins from sieve-tube to sieve-tube through the sieve-plates do away with the necessity for diffusion through a long series of membranes and thus facilitate the rapid transfer of substances from place to place. This importance of the bast has repeatedly been demonstrated by experiments involving “ringing” or “girdling”, in which there is removed from around the stem a continuous encircling strip of tissue, including all bark and bast. It is a matter of common observation that a tree which the trunk has been girdled in this way will ultimately die. Although small in amount, therefore, and rather inconspicuous when compared with wood, the bast is a vitally necessary tissue in the economy of the plant.