Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/124

Rh 114 perfect; others having the petiole slightly expanded or winged, and the lamina imperfectly developed ; and others in which there is no lamina, and the petiole becomes large and broad. Some petioles, in place of ending in a lamina become changed into a delicate filiform body, a tendril or cirrus, so as to enable the plant to climb. In many leaves interposed betwixt the petiole and the stem, or, in sessile leaves, betwixt the lamina and the stem, an expan sion of the foliar tissue takes place, so that a sheath (vagina] is formed, which embraces the whole or part of the circumference of the stem (fig. 102, gv}. This sheath is comparatively rare in Dicotyledons, but is seen in Plate VII. Umbelliferous plants, where it constitutes the peridadium, and in the Rhubarb order, where it is large and membranous, and has received the name of ochrea or boot. It is much more common amongst Monocotyledons. In Palms it forms a kind of network, to which the name of reticulum has been given. In Sedges the sheath forms a complete investment of the stem, whilst in Grasses (fig. 102) it is split on one side. In the latter plants there is also a membranous outgrowth at right angles to the median plane of the leaf from the point where the sheath passes into the lamina (there being no petiole). To this structure the name of ligule (fig. 102, gl) has been given. It is of various dimensions, and thus gives a character to the plants. Stipules. In leaves in which no sheath is produced we not unfre- quently find small foliar organs at the base of the petiole. These have been denominated stipules (fig. 123, s). The stipules are often two in number, and they are important as supplying characters in certain natural orders. Thus they occur in the Pea and Bean family, in Rosaceous plants, and the Cinchona bark family. They are not common in Dicotyledons with opposite leaves. Plants having sti pules are called stipulate ; those having none are exstipu- late. Stipules are formed by some of the vascular bundles diverging as they leave the stem, and becoming covered with parenchyma, so as to resemble true leaves. Like leaves they are large or small, entire or divided, decid uous or persistent, articulated or non-articulated. They are not usually of the same form as the ordinary foli age leaves of the plant, from which they are distinguished by their lateral position at the base of the petiole. In the Pansy (fig. 123) the true leaves I are stalked and cre- nate, while the stipules s are large, sessile, and pinnatifid. In Lathyrus Aphaca, and some other plants, the true pinnate leaves are abortive, the petiole forms a tendril, and the stipules alone are developed, performing the office of leaves. When stipules are attached separately to the stem at the base of the leaf, they are called caidinary, as in Salix aurita. When stipulate leaves are opposite to each other, at the same height on the stem, it occasionally happens that the stipules on the two sides unite wholly or partially, so as to form an interpetiolary or inter foliar stipule, as in Cinchona and in Ipecacuan. In the case of alternate leaves, the stipules at the base of each leaf are sometimes united to the petiole and to each other, so as to form an adnate, adherent, or petiolary stipule, as in the Rose, or an axillary stipule, as in Houttuynia cordata. In other instances the stipules unite together on the side of the stem opposite the leaf, arid become synochreate, as in Astragalus. This so-called union or adhesion of stipules is not an accidental adhesion taking place after they have been developed. In these cases the parts never were separate; from the first they are developed as one por tion. In the development of the leaf the stipules fre quently play a most important part. They begin to be formed after the origin of the leaves, but grow much more rapidly than the leaves, and in this way they arch over the young leaves and form protective chambers wherein the parts of the leaf may develop. In Ficus, Magnolia, and Potamogeton they are very large and completely envelop the young leaf-bud. The stipules are sometimes so minute as to be scarcely distinguishable without the aid of a lens, and so fugacious as to be visible only in the very young state of the leaf, They may assume a hard and spiny character, as in Robinia Pseudacacia, or maybe cirrose, as in Smilax, where each stipule is represented by a tendril; while in Cucurbitacese there is only one cirrose stipule. At the base of the leaflets or foliola of a compound leaf, small stipules are occasionally produced, to which some have given the name of stipels, Variations in the structure and forms of leaves and leaf- Abnor- stalks are produced by the increased development of cellular malities. tissue, by the abortion or degeneration of parts, by the multiplication or repetition of parts, and by adhesion. When cellular tissue is developed to a great extent, leaves become succulent and occasionally assume a crisp or curled appearance. Such changes take place naturally, but they are often increased by the art of the gardener, and the object of many horticultural operations is to increase the bulk and succulence of leaves. It is in this way that Cabbages and Savoys are rendered more delicate and nutritious. By a deficiency in development of parenchyma and an increase in the fibro-vascular tissue, leaves are liable to become hardened and spinescent. The leaves of Bar berry and of some species of Astragalus, and the stipules of the False Acacia (Robinia) are spiny. To the same cause is attributed the spiny margin of the Holly-leaf. In the Gooseberry, the swelling (pulvinus) at the base of the petiole, and below the leaf, assumes a spinose character. Changes in the appearances of leaves are produced by adhesions and foldings of various kinds. When two lobes at the base of a leaf are prolonged beyond the stem and unite (fig. 124), the leaf is perfoliate, the stem appearing to pass through it, as in Bupleurum perfoliatum and Chlora perfoliata ; when two leaves unite by their bases they become connate (fig. 125), as in Lonicera Caprifolium ; Fig. 125. FIG. 124. Perfoliate leaf of a species of Hare s-ear (Hiiplevrum rotwndifMuiri). The two lobes at the case of the leaf are united, so that the stalk appears to come through the leaf. FIG. 1 25. Connate leaves of a species of Honeysuckle (Lonicera CaprifoHum). Two leaves are united by their bases. and when leaves adhere to the stem, forming a sort of winged or leafy appendage, they are dtcurrent, as in Thistles. The formation of peltate and orbicular leaves has been traced to the union of the lobes of a cleft leaf. In the leaf of the Victoria regia the transformation may be traced during germination. The first leaves produced by the young plant are linear, the second are sagittate and hastate, the third are rounded-cordate, and the next are orbicular. The cleft indicating the union of the lobes remains in the large leaves. The parts of the leaf are fre- Tendrils, quently transformed into tendrils (cirri), with the view of enabling the plants to twine round others for support. In Leguminous plants (the Pea tribe) the pinnse are frequently cirrose. When tendrils occupy the place of leaves, and