• Some plants have no leaves, their functions being performed by the green juicy rind of the stalks, as in cactacea and some of the genus euphorbia; or by the general surface of the thallus (q.v.) in many acrogenous plants.
It is in the leaves of plants that the elaboration of the sap chiefly takes place, and when a tree is deprived of its leaves, no wood is formed until they are again developed. The incessant removal of leaves as they are formed destroys a plant, and this method is sometimes advantageously adopted as to weeds haviug'deep or spreading perennial roots, and otherwise difficult of extirpation.
Leaves exhibit more or less decidedly a periodical alternation in their direction and expansion, generally corresponding with the alternation of day and night. Some leaves exhibit a peculiar irritability under various influences, and those of two or three species of plants, by their closing together, catch and kill insects which alight on them, a thing, however, of which no relation to the vegetable economy is known. See IRRITABILITY ENT PLANTS, SLEEP Ole PLANTS, and DION2EA.
The forms of leaves arc extremely various. Simple leaves vary froM a form almost perfectly circular, br even broader than long, to an extreme elongation, as linear or fliform (thread-like).• The breadth of some increases towards the apex, and this is indi cated by the terms °borate, obcordate, etc., and sometimes by the word inversely prefixed to the term which describes the form. Simple leaves are either entire, or they are more or less deeply toothed or serrate; or they are cut or lobed by divisions extending from the margin towards the base; or the division may extend towards the midrib of the leaf, when the leaf is pinnatifid, or sinuate, or runcinate, etc. Terms similar to those employed to describe simple leaves are applied to the leaflets of compound leaves, but the variety of forms is not nearly so great. Compound leaves exhibit two chief varieties
of form, according as the divisions which form the leaflets extend towards the base of the blade, or towards the midrib. Of the former class are ternate, guaternate, guinate leaves, etc.; the latter are called pinnate leaves. But the same mode of division maybe repeated in the leaflets, and thus a leaf may be biternate, or, if again divided, triternate, etc., and very many leaves are bipinnate, tripinnate, etc. When the division is often repeated, the leaf is called decompound. A pinnate leaf, terminating in a pair of leaf lets, is called or abruptly pinnate; but a pinnate leaf very often terminates In an odd leaflet, and is then called The blade of a leaf is generally in the same plane with the stalk, but is sometimes at right angles to it, as in orbicular and peltate leaves.
The vernation (q.v ) of leaves, or the manner in which they are folded in bud, is, like the activation of flowers, very characteristic of different plants and tribes of plants.
are generally larger than but are only present in herbaceous plants, and are generally the first to fade. The upper stem-leaves are generally smaller and less divided than the lower, those nearest the flowers often passing into bracts. By metamorphosis of leaves, all bracts, involucres, etc., are produced, and all the different parts of flowers, as calyx, corolla, stamens, carpels, and therefore even fruits; and the mode of their arrangement relatively to the axis corresponds with that of leaves. All organs formed by metamorphosis of leaves are called See are the cotyledons of the seed, raised above ground after germination, and serving the purposes of leaves to the young plant, although generally very unlike its future leaves. This, however, only takes place in some plants.