Page:EB1911 - Volume 16.djvu/344

Rh parallel lines, as in fan palms (fig. 6), or with veins coming off from it throughout its whole course, and running parallel to each other in a straight or curved direction towards the margin of the leaf, as in plantain and banana. In these cases the veins are often united by cross veinlets, which do not, however, form an angular network. Such leaves are said to be parallel-veined. The leaves of Monocotyledons have generally this kind of venation, while reticulated venation most usually occurs amongst Dicotyledons. Some plants, which in most points of their structure are monocotyledonous, yet have reticulated venation; as in Smilax and Dioscorea. In vascular acotyledonous plants there is frequently a tendency to fork exhibited by the fibro-vascular bundles in the leaf; and when this is the case we have fork-veined leaves. This is well seen in many ferns. The distribution of the system of vessels in the leaf is usually easily traced, but in the case of succulent plants, as Hoya, agave, stonecrop and mesembryanthemum, the veins are obscure. The function of the veins which consist of vessels and fibres is to form a rigid framework for the leaf and to conduct liquids.

In all plants, except Thallophytes, leaves are present at some period of their existence. In Cuscuta (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);

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.

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.

8.—Runcinate leaf of Dandelion. It is a pinnatifid leaf, with the divisions pointing towards the petiole and a large triangular apex.

9.—Pinnatifid leaf of Valeriana dioica.

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