In all plants, except Thallophytes, leaves are present at some period of their existence. In Cuscuta (Dodder) (q.v.), however, 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, &c.—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 considered 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 diminishes 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, &c., leaves are produced. Lateral segmentation affects the lamina, producing indentations, lobings or fissuring of its margins. In this way two marked forms of leaf are produced—(1) 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 separated as detached leaflets, which become articulated with the midrib or petiole. In 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.
Simple Leaves.—When the parenchyma is developed symmetrically on each side of the midrib or stalk, the leaf is equal; if otherwise, the leaf is unequal or oblique (fig. 3). If the margins are even and present no divisions, the leaf is entire (fig. 7); Simple leaves. if there are slight projections which are more or less pointed, the leaf is dentate or toothed; when the projections lie regularly over each other, like the teeth of a saw, the leaf is serrate (fig. 3); when they are rounded the leaf is crenate. If the divisions extend more deeply into the lamina than the margin, the leaf receives different names according to the nature of the segments; thus, when the divisions extend about half-way down (fig. 8), it is cleft; when the divisions extend nearly to the base or to the midrib the leaf is partite.
If these divisions take place in a simple feather-veined leaf it becomes either pinnatifid (fig. 9), when the segments extend to about the middle, or pinnatipartite, when the divisions extend nearly to the midrib. These primary divisions may be again subdivided 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 divisions 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. 4) 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, &c. The divisions of leaves with radiating venation may extend to near the base of the leaf, and the names bipartite, tripartite, quinquepartite, &c., are given according as the partitions are two, three, five or more. The term dissected is applied to leaves with radiating venation, having numerous narrow divisions, as in Geranium dissectum.
| Fig. 7. | Fig. 8. | Fig. 9. |
Fig. 7.—Ovate acute leaf of Coriara myrtifolia. Besides the midrib there are two intra-marginal ribs which converge to the apex. The leaf is therefore tricostate.
Fig. 8.—Runcinate leaf of Dandelion. It is a pinnatifid leaf, with the divisions pointing towards the petiole and a large triangular apex.
Fig. 9.—Pinnatifid leaf of Valeriana dioica.
| Fig. 10.—Five-partite leaf of Aconite. | Fig. 11.—Pedate leaf of Stinking Hellebore (Helleborus foetidus). The venation is radiating. It is a palmately-partite leaf, in which the lateral lobes are deeply divided. When the leaf hangs down it resembles the foot of a bird, and hence the name. |
When in a radiating leaf there are three primary partitions, and the two lateral lobes are again cleft, as in hellebore (fig. 11), 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, however, the veins spread at right angles to the stalk, forming a peltate leaf as in Indian cress (fig. 12).
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 descriptive botany—a few examples are illustrated here (figs. 7, 13, 14, 15). The apex also varies considerably, being rounded, or obtuse, sharp or acute (fig. 7), notched (fig. 15), &c. 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. 16), when the apex is rounded as in ground ivy. When the lobes are prolonged downwards and are acute, the leaf is sagittate (fig. 17); 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 cultivation the cellular tissue is often much increased, giving rise to the curled leaves of greens, savoys, cresses, lettuce, &c.