shoot

in botany, one of the principal organs of higher plants; it consists of a stem with leaves and buds. The shoot and root systems constitute the vegetative body of ferns, horsetails, club mosses, gymnosperms, and angiosperms. For this reason, such plants are given the special designation “cormophytes” (as opposed to thallophytes).

Shoots arose in higher plants as a phylogenic adaptation to terrestrial life. The earliest terrestrial flora—the psilophytes—had neither shoots nor roots; their dichotomously branched bodies consisted of vegetative and sporebearing leafless branchlets—telomes. Shoots formed as a result of the aggregation and coalescence of telomes, whereas leaves arose either as surface outgrowths on axils (microphyllous line of evolution—club mosses) or as a result of the flattening and concrescence of entire systems of telomes (macrophyllous line—fernlike and seed plants). The intrinsic unity of the shoot lies in the common origin of leaves and stems and the unity of their vascular system. Further evidence is the ontogenetic formation of leaves and stems from the single mass of the meristem (growing point).

The development of the shoot and, hence, of cormophytes marked a particularly important stage in the development of the plant world. The photosynthesizing surface increased sharply owing to the flat shape of the leaves. The intensification of transpiration that resulted fostered the development of true roots as perfect organs for the absorption of water and mineral salts. As a result, shoot-bearing plants became distributed on the earth’s entire land surface and have dominated the plant cover since the Carboniferous.

Primordial forms of shoots—assimilating and sporebearing forms—performed only the basic functions of photosynthesis and reproduction. The functions of shoots subsequently became more diverse. In the typical assimilating shoot the most important vital functions are divided among its separate organs. The leaves are the organs of photosynthesis and transpiration. The stems are support organs on which the leaves are arranged in positions most favorable for photosynthesis and for conducting water, salts, and plastic matter. The buds are organs of growth, renewal, and vegetative reproduction. They include foci of meristem capable of ensuring apical growth of the shoot (terminal bud) and branching of the shoot, that is, the formation of shoots of consequent orders and the formation of a shoot system (lateral buds). Metamerism—the recurrence of structural parts along the longitudinal axis—is characteristic of shoots.

Common features of the shoot are a node, which has a leaf or whorl of leaves departing from it, and the internode. Buds are usually located in the axils, and the metameres that appear successively on the growing point of the shoot change regularly from the base to the apex.

In annual plants, all shoots live for only one season. In perennials, the shoots may vary in life-span, but each year the buds produce regenerative shoots that become part of the perennial shoot system and replace the shoots of preceding generations. A shoot that develops from a single focus of apical meristem may grow apically (monopodially) for a limited time or for a very long time; its growth may subsequently be interrupted by external or internal factors (winter, drought, correlations in the growth of various organs). A shoot that grows out of a bud during a single growth period is said to be elementary; one that grows during the course of a year is called an annual shoot. In temperate climates, most woody plants have only one accretion per year; hence their elementary growth is also annual. But in oaks, for example, there is often a second growth period in midsummer, when lammas shoots are formed; in such a case, the annual growth consists of two elementary periods. The shoots of citrus plants often yield three or four new growths per year; tropical trees (cacao, Para rubber tree) yield as many as seven. The perennial skeletal shoots of woody plants are formed from a series of annual shoots. The length of the internodes in elementary or annual shoots reflects changes in growing intensity: at the base of a shoot the internodes are usually short, in the middle they are longer, and toward the apex they are again short. Such growth characterizes Sorbus, honeysuckle, bird cherry, dock, and yarrow. In some plants, such as linden, elm, and hazelnut, the last internode of an annual shoot is the longest as a result of the underdevelopment and death of the apical part. Underdevelopment of internodes and approximation of leaves lead to the formation of rosette shoots. The length of the internodes also determines whether the shoots are long or short. Short shoots are often specialized in trees as flower-bearers (fruit spurs); their green leaves are few in number or underdeveloped leaves (in cherry, almond, elm, dwarf bay). In herbs, however, the flower-bearing shoots are long. In meadow grasses, long shoots constitute the principal mass of hay, and the short ones are the basis of pasturage. In fruit trees and shrubs, the long shoots form the skeleton of the crown, while the short ones yield the fruit. Depending on the purpose for which woody plants are grown, the ratio of long shoots to short shoots can be regulated by pruning.

The size and shape of leaves along the longitudinal axis change regularly: the lower ones are often scalelike, the middle ones are green and broad, and the upper ones are bracteal. This difference, known as heterophylly, is determined by change in the age of the growing point of the shoot and by conditions during leaf formation.

Two phases are distinguished in the formation and growth of a shoot: the embryonic stage, in which new organs of the shoot are laid down, and the postembryonic stage, when already formed organs unfold and grow. Sometimes new organs are formed during the postembryonic stage. If all the elements of a potential annual shoot, including the inflorescence and the flowers, are laid down in the bud of a perennial plant before the winter, then only the unfolding of organs occurs in the spring (in the majority of trees and shrubs of the temperate zone and in early-flowering perennial herbs). If, however, the shoot is only partially laid down in the wintering bud, then in the spring and summer, along with the unfolding of already formed elements of the annual shoot, new metameres form (in suckers of trees and shrubs and in late-flowering herbs). In annuals the growth of shoots results from the formation of new elements on the growing point.

Shoots may be orthotropic or plagiotropic. In the first case, the shoot grows vertically upward or, very rarely, downward; plagiotropic shoots grow in a horizontal or inclined direction. In herbaceous plants the shoots are often anisotropic, that is, they change their direction of growth. They grow horizontally at first and then bend and grow vertically. Formation of the inflorescence in an anisotropic plant begins, as a rule, only with transition of the shoot to an orthotropic position.

The development of shoots in perennial and annual herbs is usually effected by the formation of inflorescences and flowers. However, after flowering and fruiting the shoots of perennials do not die completely. Their basal sections, which bear the regenerative buds, are preserved. The developmental cycle of such a monocarpous shoot from the opening of the bud to fruiting may last for one vegetative period (monocyclic shoots—in rose bay, Solomon’s seal, figwort), two years (dicyclic shoots—in lungwort), or three or more years (tricyclic and polycyclic shoots—in Stipa, tufted hair grass, wintergreen).

In addition to typical assimilating aboveground shoots, plants form various types of metamorphosed shoots with specific structural-biological features associated with the functions of storage, regeneration, vegetative reproduction, protection (thorns), or climbing (tendrils). The formation of the flower as an organ of seed reproduction is also classified as shoot metamorphosis. The many types of shoots, which determine the life forms of plants, arose in a long process of evolution as an adaptation to various habitats and, in cultivated plants, to environmental changes caused by man.