The Elements of Botany, For Beginners and For Schools. Gray Asa
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.
126. Venation is the name of the mode of veining, that is, of the way in which the veins are distributed in the blade. This is of two principal kinds; namely, the parallel-veined, and the netted-veined.
127. In Netted-veined (also called Reticulated) leaves, the veins branch off from the main rib or ribs, divide into finer and finer veinlets, and the branches unite with each other to form meshes of network. That is, they anastomose, as anatomists say of the veins and arteries of the body. The Quince-leaf, in Fig. 112, shows this kind of veining in a leaf with a single rib. The Maple, Basswood, Plane or Buttonwood (Fig. 74) show it in leaves of several ribs.
128. In parallel-veined leaves, the whole framework consists of slender ribs or veins, which run parallel with each other, or nearly so, from the base to the point of the leaf—not dividing and subdividing, nor forming meshes, except by minute cross-veinlets. The leaf of any grass, or that of the Lily of the Valley (Fig. 113) will furnish a good illustration. Such parallel veins Linnæus called Nerves, and parallel-veined leaves are still commonly called nerved leaves, while those of the other kind are said to be veined—terms which it is convenient to use, although these "nerves" and "veins" are all the same thing, and have no likeness to the nerves and little to the veins of animals.
129. Netted-veined leaves belong to plants which have a pair of seed-leaves or cotyledons, such as the Maple (Fig. 20, 24), Beech (Fig. 33), and the like; while parallel-veined or nerved leaves belong to plants with one cotyledon or true seed-leaf; such as the Iris (Fig. 59), and Indian Corn (Fig. 70). So that a mere glance at the leaves generally tells what the structure of the embryo is, and refers the plant to one or the other of these two grand classes—which is a great convenience. For when plants differ from each other in some one important respect, they usually differ correspondingly in other respects also.
Fig. 113. A (parallel-veined) leaf of the Lily of the Valley. 114. One of the Calla Lily.
130. Parallel-veined leaves are of two sorts—one kind, and the commonest, having the ribs or nerves all running from the base to the point of the leaf, as in the examples already given; while in another kind they run from a midrib to the margin, as in the common Pickerel-weed of our ponds, in the Banana, in Calla (Fig. 114), and many similar plants of warm climates.
131. Netted-veined leaves are also of two sorts, as in the examples already referred to. In one case the veins all rise from a single rib (the midrib), as in Fig. 112, 116-127. Such leaves are called Feather-veined or Penni-veined, i.e. Pinnately-veined; both terms meaning the same thing, namely, that the veins are arranged on the sides of the rib like the plume of a feather on each side of the shaft.
132. In the other case (as in Fig. 74, 129–132), the veins branch off from three, five, seven, or nine ribs, which spread from the top of the leaf-stalk, and run through the blade like the toes of a web-footed bird. Hence these