114. =A Scaly Bulb= (like that of the Lily, Fig. 107, 108) is one in which the scales are thick but comparatively narrow.

[Ill.u.s.tration: Fig. 107. Bulb of a wild Lily. 108. The same divided lengthwise, showing two forming buds of the next generation.]

[Ill.u.s.tration: Fig. 109. A ground leaf of White Lily, its base (cut across) thickened into a bulb-scale. This plainly shows that bulb-scales are leaves.]

115. =A Tunicated or Coated Bulb= is one in which the scales enwrap each other, forming concentric coats or layers, as in Hyacinth and Onion.

116. =Bulblets= are very small bulbs growing out of larger ones; or small bulbs produced above ground on some plants, as in the axils of the leaves of the bulbiferous Lilies of the gardens (Fig. 110), and often in the flower-cl.u.s.ters of the Leek and Onion. They are plainly buds with thickened scales. They never grow into branches, but detach themselves when full grown, fall to the ground, and take root there to form new plants.

[Ill.u.s.tration: Fig. 110. Bulblets in the axils of leaves of a Tiger Lily.]

117. =Consolidated Vegetation.= An ordinary herb, shrub, or tree is evidently constructed on the plan developing an extensive surface. In fleshy rootstocks, tubers, corms, and bulbs, the more enduring portion of the plant is concentrated, and reduced for the time of struggle (as against drought, heat, or cold) to a small amount of exposed surface, and this mostly sheltered in the soil. There are many similar consolidated forms which are not subterranean. Thus plants like the Houseleek (Fig. 91) imitate a bulb. Among Cactuses the columnar species of Cereus (Fig. 111, _b_), may be likened to rootstocks. A green rind serves the purpose of foliage; but the surface is as nothing compared with an ordinary leafy plant of the same bulk. Compare, for instance, the largest Cactus known, the Giant Cereus of the Gila River (Fig. 111, in the background), which rises to the height of fifty or sixty feet, with a common leafy tree of the same height, such as that in Fig. 89, and estimate how vastly greater, even without the foliage, the surface of the latter is than that of the former. Compare, in the same view, an Opuntia or p.r.i.c.kly-Pear Cactus, its stem and branches formed of a succession of thick and flattened joints (Fig. 111, _a_), which may be likened to tubers, or an Epiphyllum (_d_), having short and flat joints, with an ordinary leafy shrub or herb of equal size. And finally, in Melon-Cactuses, Echinocactus (_c_), or other globose forms (which may be likened to permanent corms), with their globular or bulb-like shapes, we have plants in the compactest shape; their spherical figure being such as to expose the least possible amount of substance to the air. These are adaptations to climates which are very dry, either throughout or for a part of the year. Similarly, bulbous and corm-bearing plants, and the like, are examples of a form of vegetation which in the growing season may expand a large surface to the air and light, while during the period of rest the living vegetable is reduced to a globe, or solid form of the least possible surface; and this protected by its outer coats of dead and dry scales, as well as by its situation under ground. Such are also adapted to a season of drought. They largely belong to countries which have a long hot season of little or no rain, when, their stalks and foliage above and their roots beneath early perishing, the plants rest securely in their compact bulbs, filled with nourishment and retaining their moisture with great tenacity, until the rainy season comes round.

Then they shoot forth leaves and flowers with wonderful rapidity, and what was perhaps a desert of arid sand becomes green with foliage and gay with blossoms, almost in a day.

[Ill.u.s.tration: Fig. 111.]

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 a.s.similates 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 a.s.similated, 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 a.s.similation, 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.

[Ill.u.s.tration: Fig. 112. Leaf of the Quince: _b_, blade; _p_, petiole; _st_, stipules.]

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 princ.i.p.al 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, Ba.s.swood, Plane or b.u.t.tonwood (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 gra.s.s, or that of the Lily of the Valley (Fig. 113) will furnish a good ill.u.s.tration. Such parallel veins Linnaeus 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 cla.s.ses,--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.

[Ill.u.s.tration: 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 are said to be _Palmately_ or _Digitately_ veined, or (since the ribs diverge like rays from a centre) _Radiate-veined_.

133. Since the general outline of leaves accords with the framework or skeleton, it is plain that _feather-veined_ (or _penni-veined_) leaves will incline to elongated shapes, or at least to be longer than broad; while in _radiate-veined_ leaves more rounded forms are to be expected.

A glance at the following figures shows this.

[Ill.u.s.tration: Fig. 115-120. A series of shapes of feather-veined leaves.]

134. =Forms of Leaves as to General Outline.= It is necessary to give names to the princ.i.p.al shapes, and to define them rather precisely, since they afford easy marks for distinguishing species. The same terms are used for all other flattened parts as well, such as petals; so that they make up a great part of the descriptive language of Botany. It will be a good exercise for young students to look up leaves answering to these names and definitions. Beginning with the narrower and proceeding to the broadest forms, a leaf is said to be

_Linear_ (Fig. 115), when narrow, several times longer than wide, and of the same breadth throughout.

_Lanceolate_, or _Lance-shaped_, when conspicuously longer than wide, and tapering upwards (Fig. 116), or both upwards and downwards.

_Oblong_ (Fig. 117), when nearly twice or thrice as long as broad.

_Elliptical_ (Fig. 118) is oblong with a flowing outline, the two ends alike in width.

_Oval_ is the same as broadly elliptical, or elliptical with the breadth considerably more than half the length.

_Ovate_ (Fig. 119), when the outline is like a section of a hen's egg lengthwise, the broader end downward.

_Orbicular_, or _Rotund_ (Fig. 132), circular in outline, or nearly so.

[Ill.u.s.tration: Fig. 121, oblanceolate; 122, spatulate; 123, obovate; and 124, wedge-shaped, feather-veined, leaves.]

135. A leaf which tapers toward the base instead of toward the apex may be

_Oblanceolate_ (Fig. 121) when of the lance-shaped form, only more tapering toward the base than in the opposite direction.

_Spatulate_ (Fig. 122) when more rounded above, but tapering thence to a narrow base, like an old-fashioned spatula.

_Obovate_ (Fig. 123) or inversely ovate, that is, ovate with the narrower end down.

_Cuneate_ or _Cuneiform_, that is, _Wedge-shaped_ (Fig. 124), broad above and tapering by nearly straight lines to an acute angle at the base.

[Ill.u.s.tration: Fig. 125, sagittate; 126, auriculate; and 127, halberd-shaped or hastate leaves.]

136. =As to the Base=, its shape characterizes several forms, such as