STEM, in popular language the stalk of a plant or trunk of a tree. In botany a stem may be defined as an axis bearing leaves. The stem with its leaves is known as the shoot. Structurally it dif fers from a root in having no cap (root cap) over the growing point and in terminating in a bud. Under the term caulome (stem structure) are included all those parts of a plant morphologically equivalent in bearing leaves. The stem generally ascends, seeking air and light, and has therefore been termed the ascending axis. Stems have usually considerable firmness and solidity, but some times they are weak, and either lie prostrate on the ground, thus becoming procumbent, or climb on plants and rocks by means of rootlets, like the ivy, being then called scandent, or twist round other plants in a spiral manner like woodbine, when they are twining. Twining plants turn either anti-clockwise, as the French bean, convolvulus, dodder and gourd; or clockwise, as honey suckle, twining polygonum, hop and black bryony. In other cases climbing plants are supported by tendrils, which may be modified stems, as in vine, bryony, pas sion-flower, or by the tendril-like leaf-stalks, as in clematis and Tropaeolunt. In warm climates twining plants (lianas) often form thick woody stems, while in temperate regions they are generally herbaceous. Some stems are developed more in diameter than in height, and present a pe culiar shortened and thickened aspect, as Testudinaria or tor toise-plant, cyclamen, Melocactus, Echinocactus and other Cac taceae; while in many orchids the stem assumes an oval or rounded form, and is called a pseudobulb.
Names are given to plants according to the nature and duration of their stems. Herbs, or herbaceous plants, have stems which die down annually. In some of them the whole plant perishes after flowering; in others, the lower part of the stem forming the crown of the root remains, bearing buds from which the stem arises next season. In bien nial herbs the whole plant per ishes after two years, while in perennial herbs the crown is cap able of producing stems for many years, or new annual products are repeatedly added many times, if not indefinitely, to the old stems. The short permanent stem of herbaceous plants is covered par tially or completely by the soil, so as to protect the buds. Plants producing permanent woody stems are called trees and shrubs.
The latter produce branches from or near the ground, while the former have conspicuous trunks. Shrubby plants of small stature
are called under-shrubs or bushes. The limits between these differ ent kinds of stem are not always well defined; and there are some plants occupying an intermediate position between shrubs and trees, sometimes called arborescent shrubs.
The stem is not always conspicuous. Plants with a distinct stem are caulescent; those in which it is inconspicuous are acaulescent, as the primrose, cowslip and dandelion. A similar term is given in ordinary language to plants whose stems are buried in the soil, such as cyclamen or sowbread. Some plants are truly stemless, and consist only of expansions of cellular tissue representing stem and leaf, called a thallus, and hence are denominated Thallophytes. (See ALGAE, FUNGI.) Development of Stem.—The first rudiment of the young shoot of the embryo appears from the seed after the radicle (young root) has protruded. It is termed the plum ule, and differs from the radicle in the ab sence of a root-cap and in its tendency to ascend. The apical growing portion con stitutes the terminal bud of the plant, and by its development the stem increases in height ; projections appear at regular inter vals, which are the rudimentary leaves, and in addition there is a provision for the production of lateral buds, which de velop into lateral shoots more or less re sembling the parent stem, and by these the branching of the plant is determined. These buds are found in the axil of previously formed leaves ; or, in other words, in the angle formed between the stem and leaf. They are hence called axillary. They are pro duced like the leaves from the outer portion of the stem, and at first consist entirely of cellular tissue, but in the progress of growth vascular bundles are formed in them continuous with those of the stem, and ultimately branches are produced, which in every respect resemble the axis whence the buds first sprang. The place of origin of the leaf is called a node; the intervals between nodes are called internodes. The stem, although it has a tendency to rise upwards when first developed, in many instances becomes pros trate, and either lies along the ground partially covered by the soil, or runs completely underneath its surface, giving off roots from one side and buds from the other. Some stems are therefore subterranean, and are distinguished from roots by the provision made for regular leaf-buds.