Fig. 761.—Simple leaf.
Leaves are distinguished according to position and duration. Some leaves have very simple forms, others are compound, so to speak. Some are plain and rounded, others are toothed, like the holly. The skeleton of a leaf is a very interesting study, and it will show the beautiful structure of these common objects. The delicate lines of the green leaf are “veins,” or sap-vessels, which convey the necessary nourishment. The leaves are called the “embryonic” (seed-like), the radicle, or root-leaf, the stalk-leaf, and the stipules, which grow at the base of stem-leaves, and the floral leaves, which bear the flowers or fruit buds. Leaves which are developed at the end of a chief axis are termed blossoms. Of course it must be understood that all the different kinds of leaves do not occur upon the same plant. The leaf may be accepted to mean the stem-leaf.
Fig. 762.—Net-veined leaf.
Leaves are folded up in various ways, and the manner in which this is accomplished is termed the vernation of the plant. The leaves of endogens and exogens differ in their veining. The former veins do not touch; there is none of that beautiful interlacing which we find in the exogenous leaves. In the former the veins rise from base to apex, curving as they advance, as in the well-known lily of the valley. This “nervous system” of the leaf is its “venation,” and the veins distributed in the blade or lamina of the leaf are twofold,—as remarked,—ascending in curves, or diverging from a central nerve called the “mid-rib.” These lateral nerves are either parallel or “reticulate”—that is, net-like.
Fig. 763.—Linden tree.
We will now examine the forms of leaves which are regulated by the divergence and extension of the divisions of the mid-rib. Thus we get an orbicular, or peltate leaf; palmate, digitate, and pedate forms also occur, as may be seen in the illustrations, pages 672 and 673, where all the varied shapes can be studied. The leaf consists of a petiole or stalk, and the lamina or blade. The petiole is composed of bundles of vascular tissue; the lamina is formed by their extension, the interstices being filled with cellular tissue. So we perceive that the leaves and stems are composed of similar materials. To defend the tissue a skin, or epidermis, is placed upon the surface of the leaf, and this epidermis is full of breathing holes, or pores, called stomates (compare page 664). There are also cells filled with chlorophyl, which gives the leaf its green tint.
Fig. 764.—Stomates, highly magnified.