LEAF (AS. Cote. laufs, OTIG. ionic, Ger. Laub. leaf). A lateral usually green outgrowth from the stem of a plant. whose principal func tion is to elaborate food. The most conspicuous form is the foliage leaf ; that is. one which eon tains green tissue and is prominently concerned in food manufacture. (See PuozosvNTHEsts.) Very simple expansions of green tissue. which are commonly called leaves, occur among the algae, especially the more complex marine forms, and in the ease of the leafy liverworts and the But the highly organized foliage leaf is found only in the fern-plants (pteridophytes) and seed-plants (spermatophytes), associated with the vascular systemic. Such a leaf may develop several distinct regions, the most conspicuous of which is the expanded portion or blade. In many cases where the blade arises directly from the axis. and no other region appears, the leaf is called sessile. Very frequently. however, the blade has a stalk of greater or less length, called the petiole. In many cases, also, the petiole bears at or near its junction with the axis a pair. of appendages of various form. called stipules. The stipules may be conspicuous and leaf-like or merely minute bract-like bodies: they may be distinct from one another or united in various ways. in the smartweeds they unite and form a conspicuous sheath about the stem just above the insertion of each leaf. Still another leaf region which may appear conspicuously in grasses is the sheath, which more or less surrounds the stem. The angle formed by the petiole with the stem is called the axil.
Foams OF LEAVES. The form of the blade varies greatly, and has given rise to a long list of descriptive terms, which are of service only to the specialist in classification. These terms apply to the general outline of the leaf, as linear, lanceolate, ovate, cordate, etc.; or to the char acter of its margin, as entire, serrate, toothed, lobed, etc.: or to the character of its apex or base, as acute, obtuse. etc. The greatest modi fication in the form of the blade arises from its branching, in which case the general blade be comes divided up into smaller blades called leaf lets. Such branching leaves are usually compound. and the compounding may occur twice or thrice or even more times, resulting in a gen eral blade made up of very numerous leaflets..
The stalks of the leaflets are called petiolules, and their stipules are stipels. Closely associated with the contour and branching of leaves is the sys tem of veining or venation (q.v.). Two general
types of venation are recognized, called the parallel and the reticulate (or net-veined) types. In a parallel-veined leaf the prominent veins run approximately parallel from the base to the apex of the blade, resulting in a comparatively nar row and elongated outline and an entire margin, as in grass-blades. This type of venation is characteristic of the monocotyledons, although all of them do not possess it, nor is it absolutely restricted to them. A more significant phrase for this type, perhaps, is closed venation, implying that the veins do not end freely in the margin. As a result of this, such leaves do not become toothed or lobed. nor do they branch. In a reticulately veined leaf the veins branch freely in various directions, and there is usually evident, especially on the lower surface, a distinct and often conspicuous network of veins. Such leaves may be characterized as exhibiting open venation, as many of the veins have free ends, especially in the margins, resulting in a tendency to toothing, lobing, and branching. The dicotyledons exhibit such leaves in the greatest profusion. The ferns also have open venation, hut the veins fork re peatedly. that is. they are dichotomous. (See DICHOTOMY.) Reticulately veined leaves exhibit two prominent types. the palmate and the pin nate. in the former, three or more main ribs diverge from the base of the blade, resulting in a broad outline. When such leaves are lobed or branched they are said to be palmately lobed or compound. In pinnate leaves there is a main rib (midrib) which runs through the blade from base irregular hut continuous intercellular passage ways. This region is called the spongy meso phyll. The third histological region of the foliage leaf is the vascular region, represented by the veins which traverse the mesophyll in every direc tion. The epidermis does not hermetically seal the mesophyll tissue from the outside air, but in leaves exposed to air it is perforated by numerous very small openings. called stomata (Fig. 3), popu to apex, and gives rise to lateral ribs of secondary importance, resulting in a comparatively narrow outline. \\len such leaves are lobed or branched they are said to be pinnately lobed or compound. Fern-leaves ( `fronds') are very comnionly pin nately compound, and a special terminology is applied to the branches. the main branches being called pinme, and the ultimate branches pinnules.