STEM (AS. stcmn, 0I1G. stain, Ger. Stamm, stun; connected with 01r, tamon. stem, and ulti mately with Eng. stand). In general. stems of plants are axes distinguished from roots by bear ing leaves or leaf-like organs. Although there are stem-like structures among the thallophytes and bryophytes, it is only among vascular plants (pteridophytes and spermatophytes) that real stems appear.
Stems may be variously classified. By duration, they are annual or perennial ; by structure, her baceous or woody; by direction, erect, decumbent, prostrate, climbing. etc. Perhaps one of the most important classifications is upon the basis of character of foliage organs produced, as fol lows: Foliage-bearing stems, which generally give style to the whole plant body. arc the most conspicuous, since they display green leaves. and are necessarily aerial. As a consequence, they are inclined to branch, and the leaf-bearing joints are well separated. Scale-bearing stems, which bear the comparatively small and colorless (not green) leaf-like bodies (scales), may be subter of stems, especially those bearing foliage and flowers, is a conspicuous feature. ( For structural details of stems, see ANATOMY or• IIIS ToLoGy.) Certain broad outlines must be pre sented here as a basis of the most fundamental classification. At the tip of the stein there is a single apical cell (some pteridophytes) or a group of apical cell,. which by their power of continuous division increase the stem in length and give rise to all the tissues. Just behind this growing the three great regions be gin to be defined (Fig. 2). On the outside is a layer of cells ( derma t ogen ) that gives rise to the epidermis. In perennial stems that increase in diameter the epidermis is ranean or aerial, and the joints may be so near together that the scales overlap, as in the aerial scale bud of shrubs and trees, or the subterranean bud-like structures called bulbs, whose scales and stem become gorged with reserve food. Other prominent subterranean types are tubers (potato). comparatively short, thick stems, with much reduced and not overlapping scales, and the more slender rootstocks and rhizomes (Fig.
sloughed off on the older parts, and the region beneath develops a cork tissue of greater or less thickness, which constitutes the bulk of the bark (q.v.). Within the dermatogen is a zone con sisting of several layers of cells (periblem) that gives rise to the cortex. Within the periblem is
a central solid cylinder of cells (plerome) that 1). Stems bearing flower parts are very much modified, and constitute the so-called flowers of angiosperms (q.v.). The most essential classi fication of stems is on the basis of their an atomical structure (see below). The bracing gives rise to the stele. characterized by developing the vascular or woody bundles. The arrangement and character of the woody bundles developed in the stele determines the three general and most fundamental types of vascular stems. In the dicotyledon and conifer types (Fig. 3) the woody bundles are arranged in a hollow cylinder sur rounding a central pith. Around the woody tissue of perennial stems there is developed a layer of actively dividing cells. the cambium, that adds a new outer zone of woody bundles each growing season, in cross-section giving the ap pearance of concentric annual rings. This power of increasing in diameter is one of the prominent features of these stems. In the monocotyledon type (Fig. 4) the woody bundles are scattered irregularly through the stele, as in the corn stalk and palm, so that there is no defined cen facture of foods, as in leafless stems, such as Equisetum and the cacti. Among the external factors which influence stem development, light and moisture are perhaps most important. In general. light is thought to retard stem develop ment, as seen when stems grown in full light are compared with those grown in weak light and in the case of tubers. In some cases, however, as tral pith region. Such stems show no annual increase in diameter. In the pteridophyte type usually a single large woody bundle of peculiar structure appears in the stele. in such a way that there can be no annual increase in diameter.
The primary function of the stem is to develop foliage leaves and display them to the air and sunlight. and also to act as a great conducting region between the root system and the leaves. (See CONDUCTION.) Subsidiary functions are vegetative propagation (as in horizontal ground stems) ; the storage of foods (as in bulbs and tubers) ; and the manu in the cacti, light has been found to favor stem development. Experiments seem to show that much of the so-called influence of light is due to a moisture influence (Fig. 6).