up its orifice, as in Sarracenia 100 and Nepenthes 104, it is called a pitcher or ascidium; if it is unclosed, and is a mere sac, as in Utricularia, it is called ampulla 101. 202. Sometimes the petiole has no lamina, or is lengthened beyond the lamina, and retains its usual cylindrical or taper figure, but becomes long, and twists spirally; such a petiole is called a tendril (Vrille, Fr.) 102. 111 The petiole is usually either taper, or channelled; and it has often a struma 203. The petiole is sometimes articulated transversely, as in the Orange. 204. The LAMINA of a leaf is an expansion of the parenchyma of the petiole, and is traversed by veins which are ramifications or extensions of the bundles of vascular tissue of the petiole, or, when there is no petiole, of the stem. 205. Sometimes one, sometimes both the surfaces of a leaf are furnished with stomates. 206. The veins either branch in various directions among the parenchyma, anastomosing and forming a kind of network, or they run parallel to each other, being connected by single transverse unbranched veins. 207. The former is characteristic of Exogenous, the latter of Endogenous plants. 208. The principal vein of a leaf is a continuation of the petiole, and runs in a direct line from the base to the apex of the lamina; this vein is called the midrib. It usually produces other veins from its base or sides, or from both such veins are called ribs, if very strong, and proceeding from the base to the apex; under other circumstances, they are frequently named nervures. 209. There are two strata of veins, the one belonging to the upper, and the other to the under surface. 210. The upper stratum conveys the juices from the stem into the lamina, for the purpose of being aërated and elaborated; the under returns them into the bark. 211. The veins are interposed among cellular substance, called diachyma, diploë, or mesophyllum; which is often stratified differently below the two surfaces of the leaf; the upper stratum being more compact than the lower, and having its cells perpendicular to the plane of the leaf: in such cases, the cells of the lower stratum are commonly more or less parallel with the under surface. 212. The lamina is variously divided and formed; it is usually thin and membranous, with a distinct upper and under surface; but sometimes becomes succulent, when the surfaces are often not distinguishable. 213. The upper surface is presented to the sky, the lower to the earth; this position is rarely departed from in nature, and cannot be altered artificially, except by violence. 214. A leaf is simple when its lamina is undivided, or when, if it is separated into several divisions, those divisions do not reach the midrib; Ex. Lime-tree, Oak. 215. The form of the simple leaf is extremely variable, and the terms employed to denote the variations are numerous in proportion. 216. Some leaves have the margin so continuous, that the outline is scarcely interrupted, except by small toothings. Of such leaves the following are among the more common forms: ; ovate 121 Orbicular 120 ; lanceolate 122; oval 118; oblong 117 ; roundish oblong 113 ; cordate ovate 114 ; cordate acuminate 119 ; reniform 123 ; auriculate 139. ; ; 217. In other leaves the margin is produced here and there into manifest angles; in which cases the following terms are commonly in use: 218. In other cases the margin is repeatedly interrupted in a definite manner along its whole course; and then such terms as the following are employed: Palmate 138 132 ; seven-lobed 134 ; pinnatifid 133; sinuated 132; panduriform 135. 219. A leaf is compound when the divisions pass down to the midrib so as to subdivide the leaf into smaller and distinct leaves, or leaflets (foliola). 220. When leaves are compound, their mode of division is expressed by such terms as the following: Ternate 144; biternate 146 or triternate; digitate 140; pedate 142; pinnate 145 ; interruptedly pinnate 147; lyrate 143; bipinnate 150 ; decompound or tripinnate 141 bijugate 148; conjugato-pinnate 149. ; 221. In speaking of the margin, we say that it is 222. The point of the leaves gives rise to other terms, such as the following: Acute 118; obtuse 139 ; retuse 113; emarginate 113; acuminate 119; mucronate truncate 129 132 ; 223. STIPULES are attached to each side of the base of the petiole. They have, if leafy, veins, the anatomical structure of which is the same as that of the veins of the leaves. 224. Sometimes only one stipule is formed, the other being constantly abortive, as in Azara. 225. Stipules are sometimes transformed into leaves: they sometimes have buds in their axils; and may be, therefore, considered rudimentary leaves. 226. Whatever arises from the base of a petiole, or of a leaf, if sessile, occupying the same place, and attached to each side, is considered a stipule. The appearance of this organ is so extremely variable, some being large and leaflike, others being mere rudiments of scales, that botanists are obliged to define it by its position, and not by its organization. 227. The stipules must not be confounded with cellular marginal appendages of the petiole, as in Apocynaceæ. 228. Stipules, the margins of which cohere in such a way that they form a membranous tube sheathing the stem, are called ochrea; Ex. Rhubarb. 229. All leaves are originally continuous with the stem ; as they grow, an interruption of their tissue at their junction with the stem takes place, by which a more or less complete articulation is formed sooner or later. 230. The articulation between a leaf and stem being completed, the tissue of the former becomes gradually incrusted by the foreign matter deposited by the sap in the process of secretion and digestion, and at last is incapable of further action, when it dies. When the stem continues to increase in diameter, as a dead leaf will not increase with it, the latter is eventually thrown off; this is the fall of the leaf. But in some Endogens the articulation is so slight, and the stem increases so little in diameter, that the leaf is never thrown off, but simply withers and decays. |