In nearly all plants, except Thallophyta, leaves are present at some period of their existence. In Cuscuta (Dodder) (q.v.), how ever, we have an exception. The forms assumed by leaves vary much, not only in different plants, but in the same plant. It is only amongst the lower classes of plants—Mosses, Characeae, etc. —that all the leaves on a plant are similar. As we pass up the scale of plant life we find them becoming more and more variable. The structures in ordinary language designated as leaves are con sidered so par excellence, and they are frequently spoken of as foliage leaves. In relation to their production on the stem we may observe that when they are small they are always produced in great number, and as they increase in size their number dimin ishes correspondingly. The cellular process from the axis which develops into a leaf is simple and undivided ; it rarely remains so, but in progress of growth becomes segmented in various ways, either longitudinally or laterally, or in both ways. By longitudinal segmentation we have a leaf formed consisting of sheath, stalk and blade; or one or other of these may be absent, and thus stalked, sessile, sheathing, etc., leaves are produced. Lateral seg mentation affects the lamina, producing indentations, lobings or fissuring of its margins. In this way two marked forms of leaf are produced—(i) Simple form, in which the segmentation, however deeply it extends into the lamina, does not separate portions of the lamina which become articulated with the midrib or petiole; and (2) Compound form, where portions of the lamina are sep arated as detached leaflets, which become articulated with the midrib or petiole. Of both simple and compound leaves, according to the amount of segmentation and the mode of development of the parenchyma and direction of the fibro-vascular bundles, many forms are produced.
If these divisions take place in a simple feather-veined leaf it becomes either pinnatifid (fig. 4), when the segments extend to about the middle, or pinnatipartite, when the divisions extend nearly to the midrib. These primary divisions may be again sub divided in a similar manner, and thus a feather-veined leaf will become bipinnatifid or bipinnatipartite; still further subdivisions give origin to tripinnatifid and laciniated leaves. The same kinds of division taking place in a simple leaf with palmate or radiating venation, give origin to lobed, cleft and partite forms. The name palmate or palmatifid (fig. 2) is the general term applied to leaves with radiating venation, in which there are several lobes united by a broad expansion of parenchyma, like the palm of the hand, as in the sycamore, castor-oil plant, etc. The divisions of leaves
with radiating venation may extend to near the base of the leaf, and the names bipartite, tripartite, quinquepartite, etc., are given according as the partitions are two, three, five or more. The term dissected is applied to leaves with radiating venation, having nu merous narrow divisions, as in Geranium dissectum.
When in a radiating leaf there are three primary partitions, and the two lateral lobes are again cleft, as in hellebore, the leaf is called pedate or pedatifid, from a fancied resemblance to the claw of a bird. In all the instances already alluded to the leaves have been considered as flat expansions, in which the ribs or veins spread out on the same plane with the stalk. In some cases, how ever, the veins spread at right angles to the stalk, forming a peltate leaf as in Indian cress (fig. 6).
The form of the leaf shows a very great variety, ranging from the narrow linear form with parallel sides, as in grasses or the needle-like leaves of pines and firs, to more or less rounded or orbicular—descriptions of these will be found in works on descrip tive botany—a few examples are illustrated here (figs. 8, 9, io). The apex also varies considerably, being rounded, or obtuse, sharp or acute, notched (fig. 19), etc. Similarly the shape of the base may vary, when rounded lobes are formed, as in dog-violet, the leaf is cordate or heart-shaped; or kidney-shaped or reniform (fig. io) when the apex is rounded as in ground ivy. When the lobes are pro longed downwards and are acute, the leaf is sagittate (fig. I ; when they proceed at right angles, as in Rumex Acetosella, the leaf is hastate or halbert-shaped. When a simple leaf is divided at the base into two leaf-like appendages, it is called auriculate. When the development of parenchyma is such that it more than fills up the spaces between the veins, the margins become wavy, crisp or undulated, as in Rumex crispus and Rheum undulatum. By cul tivation the cellular tissue is often much increased, giving rise to the curled leaves of greens, savoys, cresses, lettuce, etc.
Compound Leaves.—Compound leaves are those in which the divisions extend to the midrib or petiole, and the separated portions become each articulated with it, and receive the name of leaflets. The midrib, or petiole, has thus the appearance of a branch with separate leaves attached to it, but it is considered properly as one leaf, because in its earliest state it arises from the axis as a single piece, and its subsequent divisions in the form of leaflets are all in one plane. The leaflets are either sessile (fig. 12) or have stalks, called petio/u/es (fig. 13). Compound leaves are pinnate (fig. 13) or palmate (fig. 12) according to the arrange ment of leaflets. When a pinnate leaf ends in a pair of pinnae it is equally or abruptly pinnate (paripinnate) ; when there is a single terminal leaflet (fig. 13), the leaf is unequally pinnate (impari pinnate) ; when the leaflets or pinnae are placed alternately on either side of the midrib, and not directly opposite to each other, the leaf is alternately pinnate; and when the pinnae are of differ ent sizes, the leaf is interruptedly pinnate. When the division is carried into the second degree, and the pinnae of a compound leaf are themselves pinnately compound, a bipinnate leaf is formed.