STEM, the main axis of a plant. Stems usually bear foliar organs, such as leaves, hut in the carrion-flower (Stapelia) and in some other plants no leaves are produced; and in cacti the leaves are rarely observed because they are early deciduous. Though there are stem-like organs among the alga: and fungi (Thallophytes) and liverworts and mosses (Bryophytes), true stems are found only among the club-mosses, horse-tails and ferns (Piers dophytes) and the flowering plants (Sperma tophytes). In the last-named group the stems are produced by the development of the embry onic plumule, and reach their maximum size in the California redwood which becomes more than 350 feet tall and may exceed 20 feet in diameter.
Stems are very diverse in general habit; they may be erect and unbranched as in most palms; erect and branched as in pine and many other trees; scrambling as in blackberries and rasp berries; twining as in morning-glory and hop; climbing as in grape and Boston ivy; creeping as in dewberry and periwinkle; and with many modifications of these general types. They are also definable according to duration as annual and Perennial; and according to structure as herbaceous and woody. These are also modified in nature.
Botanists favor a classification which deals with the character of foliar organs borne, thus: (1) leaf-bearing stems, geherally most evident since they bear leaves, and consequently aerial and frequently branched; (2) flower-bearing stems; and (3) scale-bearing stems, which bear rather small, more or less leaf-like but not green organs, and may be situated above or below the surface of the soil. The last group runs into many modifications, such as rhizomes, tubers and bulbs below ground, and the buds of trees and shrubs above ground. They are also grouped according to their anatomical structure, as described later.
The functions of stems are usually to lift the leaves and flowers off the ground and ex pose them to light and air, or to act as storage organs in which reserve food is placed until needed. While the epidermis remains green, aerial stems can, and do, perform the functions of leaves in assimilating food, a very important office in such plants as cacti. In function the stems may be sharply contrasted with roots or descending axes; but none of these are con stant, there being exceptions. Thus, most stems have chlorophyll (q.v.) which is almost never present in roots, except a few aerial ones; stems usually bear leaves, roots do not; the stem-tip is almost invariably a naked growing point, a root-tip is generally protected by a root-cap; stem-branches are superficial outgrowths (exog enous buds), root-branches develop endoge nously from an internal layer ; and usually stems and roots take opposite directions with respect to gravitation. In describing aerial stems the positions of leaves (nodes) are distinguished from parts which are leafless (internodes); the buds which appear in the axils of the leaves (axillary) from those which are terminal (apical) or which appear arbitrarily (adven titious), etc. The method of branching is also important in classification, as lateral, false di chotomous, dichotomous, etc.
The internal structure of a stem may be un derstood by examining twigs of a dicotyledon ous tree such as oak. The bark peels off and leaves a white woody cylinder. At the place where bark and wood separate is a more or less mucilaginous layer of cells (cambium) which are readily ruptured. This layer is re sponsible for growth in girth, since its cells are actively dividing during the growing season, forming bark layers externally and wood layers internally. It is also from this layer that the
healing of wounds in the stem is conducted. The formation of the °callus° in cuttings and the union of grafts is also a function of this layer; hence its importance from the forester's and horticulturist's standpoint. When a ring of bark, which is usually found in three layers (outer, middle and inner), is removed, but the wood is uninjured, the leaves will not wither; hence the conclusion that the water ascends through the wood. If the ring be through the young wood the leaves will wither; hence the deduction that the ascent of water is through the young wood. This conclusion is corroborated by the fact that trees will flourish after the heart-wood has decayed. The descent of food to the roots and to other places where it is needed for growth or for storage is through the layers just outside the cambium (the inner bark). By means of the microscope and thin sections of stems a great variety of structures are revealed. Com mencing with the growing-point there is found an external epidermis and an almost homo geneous internal structure of ((fundamental tis sue.* This latter becomes more and more differentiated in parts more remote from the tip. Firm strands (fibro-vascular bundles) are usually first observed arranged radially in dicotyledonous plants. They consist of an in ternal woody part (xylem) and an external bast part (phloem) and between each pair a persistent layer of cambium. Each bundle ter minates in a leaf above, becomes connected with others lower down and terminates below in the little rootlets. In the internal structure of the stems of dicotyledonous trees they consti tute the wood. Between them are plate-like structures extending, in cross section, like the spokes of a wheel from various points at, near or remote from the centre to or toward the cir cumference. The fundamental tissue continues undifferentiated in the cambium, slightly modified in the pith, if there be any pith, and in the medul lary rays. In monocotyledonous plants the fibro-vascular bundles are scattered in the funda mental tissue, pursue a curved course (inward and outward) and are collateral. The cambium being consumed at an early period of growth accounts for the usually uniform or very slightly tapering stems of plants belonging to this group — bamboos, grasses, etc.— and also for the non separation of the bark from the generally thin layer of wood which surrounds a mass of pith or a hollow space. The development of growth in these plants is not marked 'by annual rings as in the dicotyledons.
From an economic standpoint, stems are of wide importance. Various underground stems are used for food, for example, potato, sweet potato, Jerusalem artichoke, onion, asparagus, kohlrabi, etc. Others are used as condiments such as ginger, licorice, cinnamon, sassafras, etc. Many furnish valuable fibres such as hemp, flax, jute, ramie, etc. Others are employed in tan ning, as the barks of oak, chestnut and hemlock. The pith of many, such as sago, are used for food. The juices of others furnish rubber, palm wine, pulque, arrack and various oils. But the most important commercial uses made of stems are probably as timber and lumber, for which the dicotyledonous trees are most widely em ployed, as will be sufficiently clear from the above discussion. Consult any treatise on botany, as Lang's English edition of 'A Text book of Botany,' by Strasburger and others (New York 1903).