Stem
That part of a plant which supports the leaves and (in flowering plants) the flowers. It is always present in Vascular plants, though occasionally so extremely ill-developed that the plants are called acaulescent or stemless; but, in such plants, the top of the so-called root is, in truth, the stem. In the Algae, Fungi, and Lichens there is no real distinction into Stems and leaves, the two being replaced by the thallus. In most plants, the Stem usually rises upwards towards light, contains chlorophyl, is covered with true epidermis, with stomata, and bears leaves and buds at the nodes--standing in marked contrast, in all these points, to roots, though exceptions occur to all of the characters mentioned. Stems vary from 1/2in. or less (Centunculus minimus) to 470ft. in height (Eucalyptus amygdalina), and from extreme slenderness (e.g., in Radiola millegrana) to the enormous trunks of the Sequoias in Western America, whose girth is many yards. It would occupy too much space to enter into details of the wide differences in habit met with in Stems, due to variation in thickness, branching (whether with or without a main stem), direction, and many other characters. A reference to the figures under such headings as Agave, Cactus, Coreopsis, Fragaria, Hedera, Palmae, Pinus, &c., will give a better idea of these differences than any brief description could.
Internal structure of Stems also varies greatly; but there are two well-marked types in woody Stems, after the first year of growth. The one is characteristic of Dicotyledons and Conifers, and the other of Monocotyledons. In the former, there is the appearance, in cross-section, of as many rings of wood as the Stem is years old, due to the wood formed in autumn being closer in texture than that of spring, and therefore differing from it in appearance. There is also a well-marked bark, separated from the wood by a well-developed cambium, or layer, where growth in thickness is effected. In most Dicotyledons there is also pith in the centre of the Stem, and lines of cellular tissue, named medullary rays, run towards the circumference from the girth, and cut the wood into wedge-shaped masses. In the Monocotyledons, on the contrary, the Stems, in transverse section, show no appearance of layers, the bundles of woody fibres lying imbedded amongst cellular tissue, in which no separation into pith and bark is observable. In these Stems there are no annual rings, no medullary rays, no cambium, and no true bark; and the Stem, after it is once formed, does not increase in thickness, but only in length. Certain forms of Stems have received special names, of which the following are among the more important: Caudex, applied to the columnar, erect Stems of Palms and Tree-ferns, marked with the bases of the leafstalks, or with their scars; Culm, the cylindrical, jointed Stems of Grasses and allied plants, which usually have hollow internodes; Scape, a leafless, erect stem, ending in a flower or group of flowers. Decumbent lateral stems, or branches, receive the following names: Runner, when slender, lying on the surface of the soil, and emitting roots at the nodes; Stolon, resembling a runner, but forming erect Stems from the terminal buds, which tend to form new plants; Offset, a short stolon; Sucker, an underground stolon.
Stems are occasionally much modified to serve peculiar functions. Among the most remarkable of such modifications are Spines and Tendrils. The former are branches or main Stems that have remained short, but have the woody tissues largely developed, so that they are very strong and rigid, and, ending in sharp points, form an efficient protection against injury from animals. Examples of Stem-spines are seen in the Blackthorn, the Hawthorn, and many other shrubs. Tendrils, in a few plants (e.g., Passion-flower and Vine), are very slender Stems that have the power of twining round firm supports in a continuous spiral from right to left, or from left to right. Twining Stems form a transition between tendrils and ordinary Stems.