Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/125

LEAVES.] BOTANY 115 appear as a continuation of the leaf-stalk, they are called petinlary, as in Lathyrus Aphaca, in which the stipules perform the function of true leaves. In Flagellaria Indica, Qloriosa superba, Antlierieiun cirrhatum, and Albuca cir- rhata, the midrib of the leaf ends in a tendril. In Smilax there are two stipulary tendrils, while in the Cucumber tribe there is a single one at the base of each leaf. In the Passion-flower the lateral leaf-buds, and in the Vine the terminal ones, become tendrils. In the Vanille plant ( Vanilla aromatica) the tendrils are produced opposite the leaves, until the plant gains the top of the trees by which it is supported, the upper tendrils being then developed as leaves. The midrib is sometimes prolonged in a cup-like or funnel-shaped form ; this is occasionally seen in the common Cabbage, and seems to depend on the vascular bundles of the midrib spreading out at their extremity in a radiating manner, and becoming covered with parenchyma in such a way as to form a hollow cavity in the centre. The vascular bundles and cellular tissue are sometimes developed in such a way as to form a circle, with a hollow in the centre, and thus give rise to what are called fistidar or hollow leaves, as in the Onion, and to ascidia or pitchers. Pitchers are formed either by petioles or by laminae, and they are composed of one or more leaves. In Sarracenia (fig. 121) and Heliamphora, the pitcher is composed apparently of the petiole of the leaf. In Nepenthes (fig. 120) and perhaps in Cephalotus, while the folding of a winged petiole p forms the pitcher, the lid I, which is muted by an articulation, corresponds to the lamina. This kind of ascidium is called calyptrimorphous, and may be considered as formed by a leaf, such as that of the Orange (fig. 119); the lamina I being articulated to the petiole p, which, when folded, forms the pitcher. In Dischidia IlafHesiana, a climbing plant of India, the pitchers are formed by the lamina of the leaf, and have an open orifice into which the rootlets at the upper part of the plant enter. These pitchers would seem, therefore, to contain a supply of fluid for the nourishment of the upper branches of the plant. In Utricularia the leaves form sacs called ampulla;. Some suppose that pitchers are not due to folding and adhesion, but that they are produced by a hollowing out of the extremity of the stalk. In some cases the leaves are reduced to mere scales ; they are then frequently called cataphyllary leaves. They are produced abundantly upon underground shoots. In parasites (Lathrsea, Orobanche), and in plants growing on decaying vegetable matter (Sapro phytes), in which no chlorophyll is formed, these scales are the only leaves produced. In Pinus the only leaves pro duced on the main stem and the lateral shoots are scales, the acicular leaves of the tree growing from axillary shoots. In Cycas whorls of scales alternate with large pinnate leaves. In many plants, as already noticed, phyl- lodia or stipules perform the function of leaves. The production of leaf-buds from leaves has already been noticed. .Such leaves are termed proliferous. In Bryophyllum (fig. 67) this is a common occurrence, and it is met with in many plants of the order Gesneraceae. The leaf of Venus s Fly-trap (Dionsea muscipula, fig. 46), when cut off and placed in damp moss, with a pan of water under neath and a bell-glass for a cover, has produced buds from which young plants were obtained. Some species of Saxifrage and of Ferns also produce buds on their leaves and fronds. In Nymphaea micrantha buds appear at the upper part of the petiole. Amongst Dicotyledons we have leaves which present the greatest amount of variation in structure and form. The venation is reticulated. They are frequently articulated, exhibit divisions at their margin, and become truly com pound. There are, no doubt, instances in which the veins proceed in a parallel manner, but this will be found to occur chiefly in cases where the petiole may be considered as occupying the place of the leaf. Examples of this kind are seen in Acacias (fig. 122). Dicotyledons rarely have a sheath developed, but stipules are very commonly present. In Monocotyledons the leaves do not present an angular Leaves in network of vessels, nor do they, as a rule, exhibit divisions on Monocotj- their margin. Exceptions to this occur in some plants, lecions - as Tamus and Dioscorea, which have been called Dictyo- gens by Lindley, on account of their somewhat netted venation ; and in Palms, in which, although the leaves are entire at first, they afterwards become split into various lobes. They are rarely stipulate, and very frequently have a sheath at their base. The petiole is often absent, and a sheathing ligule takes its place. The leaves are often continuous with the stem. In some aquatic Monocoty ledons the submerged and floating leaves are narrow, like petioles, while those growing erect above the water expand. This is seen in Sagittaria sagittifolia, in which the erect leaves assume an arrow-like shape. In Acotyledons the leaves vary much, being entire or Leaves in divided, stalked or sessile, often feather-veined, occasionally Acotyle- with radiating venation, the extremities of the veins being s forked. In Ferns the leaf (frond) is usually stalked, with frequently a much-divided lamina, remarkable for the prolonged growth at the apex, and in the young state usually covered with curious flattened hairs (palece). In Equisetaceae the leaves are sheathing cylinders embracing successive internodes of the stem, and subsequently splitting at the top into few or many teeth. In Lycopodiaceae the leaves vary from mere rudimentary scales, as in Psilotum, to the flattened acuminate leaf of Selaginella, with only a single fibro-vascular bundle, and then to the more com plicated form in Isoetes, in which the longest leaves in the order occur, and these have a sheathing basal part and an upper lamina. On the face of the sheathing part is a depression or fovea, in which rests the sporangium, the margin of it rising as a thin membranous outgrowth, the velum. Above the fovea, and separated from it by a saddle- like ridge, lies a smaller depression the foveola, the lower margin of which forms a lip, the labium, and from its bottom is prolonged beyond the foveola an apiculate mem branous structure, which is termed the ligule. In Mosses we have the simplest form of leaf, composed of one or more layers of cells, sometimes the central ones being more or less compacted and forming a median vein. Leaves occupy various positions on the stem and Phyllo branches, and have received different names according to taxis, their situation. Thus leaves arising from the crown of the root, as in the Primrose, are called radical ; those on the stem are cauline ; on the branches, ramal ; on flower-stalks, floral leaves. The first leaves developed are denominated seminal leaves or cotyledons, and those which succeed are primordial. The arrangement of the leaves on the axis and its appendages is called phyllotaxis. In their arrange ment leaves follow a definite order. It has been stated already that there are regular nodes or points on the stem at which leaves appear, and that the part of the stem between the nodes is the internode. Each node is capable of giving origin to a leaf. Occasionally several nodes are approximated so as to form as it were one, and then several leaves may be produced at the same height on the stem. When two leaves are thus produced, one on each side of the stem or axis, and at the same level, they are called opposite (fig. 126); when more than two are produced (fig. 127), they are vei-ticillate, and the circle of leaves is then called a verticil or whorl. When the zone of the axis which produces the circle of leaves is transverse from its origin, the whorl is a true one ; but when the zone is the result of unequal development or of displacement the whorl

is spurious. Again, in each whorl the leaves may be all