Section VII. LEAVES.
118. Stems bear leaves, at definite points (nodes, [13]); and these are produced in a great variety of forms, and subserve various uses. The commonest kind of leaf, which therefore may be taken as the type or pattern, is an expanded green body, by means of which the plant exposes to the air and light the matters which it imbibes, exhales certain portions, and assimilates the residue into vegetable matter for its nourishment and growth.
119. But the fact is already familiar ([10-30]) that leaves occur under other forms and serve for other uses,—for the storage of food already assimilated, as in thickened seed-leaves and bulb-scales; for covering, as in bud-scales; and still other uses are to be pointed out. Indeed, sometimes they are of no service to the plant, being reduced to mere scales or rudiments, such as those on the rootstocks of Peppermint (Fig. [97]) or the tubers of Jerusalem Artichoke (Fig. [101]). These may be said to be of service only to the botanist, in explaining to him the plan upon which a plant is constructed.
120. Accordingly, just as a rootstock, or a tuber, or a tendril is a kind of stem, so a bud-scale, or a bulb-scale, or a cotyledon, or a petal of a flower, is a kind of leaf. Even in respect to ordinary leaves, it is natural to use the word either in a wider or in a narrower sense; as when in one sense we say that a leaf consists of blade and petiole or leaf-stalk, and in another sense say that a leaf is petioled, or that the leaf of Hepatica is three-lobed. The connection should make it plain whether by leaf we mean leaf-blade only, or the blade with any other parts it may have. And the student will readily understand that by leaf in its largest or morphological sense, the botanist means the organ which occupies the place of a leaf, whatever be its form or its function.
[§ 1.] LEAVES AS FOLIAGE.
121. This is tautological; for foliage is simply leaves: but it is very convenient to speak of typical leaves, or those which serve the plant for assimilation, as foliage-leaves, or ordinary leaves. These may first be considered.
[122.] The Parts of a Leaf. The ordinary leaf, complete in its parts, consists of blade, foot-stalk, or petiole, and a pair of stipules.
[123.] First the Blade or Lamina, which is the essential part of ordinary leaves, that is, of such as serve the purpose of foliage. In structure it consists of a softer part, the green pulp, called parenchyma, which is traversed and supported by a fibrous frame, the parts of which are called ribs or veins, on account of a certain likeness in arrangement to the veins of animals. The whole surface is covered by a transparent skin, the Epidermis, not unlike that which covers the surface of all fresh shoots.
124. Note that the leaf-blade expands horizontally,—that is, normally presents its faces one to the sky, the other to the ground, or when the leaf is erect the upper face looks toward the stem that bears it, the lower face away from it. Whenever this is not the case there is something to be explained.
[125.] The framework consists of wood,—a fibrous and tough material which runs from the stem through the leaf-stalk, when there is one, in the form of parallel threads or bundles of fibres; and in the blade these spread out in a horizontal direction, to form the ribs and veins of the leaf. The stout main branches of the framework are called the Ribs. When there is only one, as in Fig. [112], [114], or a middle one decidedly larger than the rest, it is called the Midrib. The smaller divisions are termed Veins; and their still smaller subdivisions, Veinlets. The latter subdivide again and again, until they become so fine that they are invisible to the naked eye. The fibres of which they are composed are hollow; forming tubes by which the sap is brought into the leaves and carried to every part.