LEAVES, folio, are organs of plants, springing from the sides of the stem or branches, generally more or less flat and green, never bearing flowers, and of great use in the veg, ctable economy, as exposing the sap to air and light on their extensive surfaces. It is usually in the axils (q.v.) of leaves that buds and branches are developed; and with reference to buds and branches, they are never situated otherwise than beneath them, although in the axils of many leaves no development of bud or branch ever takes place. After its full development. a leaf retains its form and size unchanged till its death. As to the duration of their life, leaves exist either for one year—that is, during a year's period.of active vegetation—in which case they are called deciduous (q.v.), or for more than one year, when they are called evergreen (q.v.).
= A leaf first appears as a little conical body pushed out from the stem or branch. Al first it consists entirely of cellular tissue, continuous with the bark, but vascular tissue afterwards generally appears in it. When fully developed, it usually consists of two parts: an expanded part, called the blade or limb; and a stalk supporting this part, and called the leafstalk, or petiole, which sometimes assumes the form of a sheath of the stem, as in grasses. The leaf-stalk, however, is often wanting, in which case the leaf is called sessile; and when the base of the leaf embraces the stein, it is called amplexicaul. A leaf -which has a leaf-stalk is called petiolate. Sessile leaves often extend in wing-like prolongations down the stem, and are then called decurrent. They are sometimes per. foliate, entirely surrounding the stem with their base, so that it seems to pass through the leaf. Opposite leaves are sometimes combined in this way. Leaves are called simple when all their parts are united into one whole by a connected cellular tissin ; they are called compound when they consist of a number of distinct, completely sep:v rated parts, which are called to the place where leaves arise from the stem, they are either radical (root-leaves), when they arise from the very base—and many plants have radical leaves only; or cauline (stem-leaves), when they arise from the devel oped stem or branches—the radical leaves really arising from the stem; or floral, when they arise from the flowering axis.—As to their arrangement on the stem, leaves
are verticillate, or whorled, or opposite, or alternate, or scattered. Opposite leaves are usu ally placed so that each pair is at right angles to those next above and beloW. All these modes of arrangement on the stem can be reduced either to the whorl or to the spiral; whilst by the tearing out of the whorl the spiral arrangement arises, and the whorl by the compression of the spiral, but so that the whorl and the spiral are essentially the same. The number of leaves requisite to form a complete cycle, or to encircle the stem, is very constant in the same species. In the common houseleek, tha cycle consists of no fewer than 13 leaves, which are grouped together to form the rosette of this plant.
Leaves consist either exclusively of cellular tissue, as in mosses, or, more generally. of cells and bundles of spiral vessels, as m the leaves of trees and most other phanerog amous plants. The stronger bundles of vessels form nerves, externally conspicuous, the finer ramifications of Which are called reins. In endogenous plants, the nerves of the leaves run mostly in straight lines, and nearly parallel; whereas, in exogenous plants, a net-like ramification of the nerves prevails.
The leaves of phanerogamous plants and ferns are covered with a well-developed separable epidermis, which extends over all their parts, and which is provided with numerous small pores—stomata (q.v.)—sometimes on one, sometimes on both sides, serving for the absorption and exhalation of gaseous substances. Submerged leaves, however, and the under side of leaves which float on the surface of water, have no sto mata, no true epidermis, and no true vascular tissue.