Page:EB1911 - Volume 16.djvu/346

Rh in the pea and bean family, in rosaceous plants and the family Rubiaceae. They are not common in dicotyledons with opposite leaves. Plants having stipules are called stipulate; those having none are exstipulate. Stipules may be large or small, entire or divided, deciduous or persistent. They are not usually of the same form as the ordinary foliage leaves of the plant, from which they are distinguished by their lateral position at the base of the petiole. In the pansy (fig. 24) the true leaves are stalked and crenate, 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 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 interfoliar stipule, as in members of the family Rubiaceae. 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 forming an ocrea, as in the dock family (fig. 25).

In the development of the leaf the stipules frequently play a most —Leaf of an Acacia (Acacia heterophylla), showing a flattened leaf-like petiole p, called a phyllode, with straight venation, and a bipinnate lamina. 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 the figs, magnolia and pondweeds 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 (fig. 19), or may be cirrose, as in Smilax, where each stipule is represented by a tendril. At the base of the leaflets of a compound leaf, small stipules (stipels) are occasionally produced.

Variations in the structure and forms of leaves and leafstalks are produced by the increased development of cellular 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 mechanical tissue, leaves are liable to become hardened and spinescent. The leaves of barberry and of some species of Astragalus, and the

stipules of the false acacia (Robinia) are spiny. To the same cause is due the spiny margin of the holly-leaf. When two lobes at the base of a leaf are prolonged beyond the stem and unite (fig. 26), 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. 27), as in Lonicera Caprifolium; and when leaves adhere to the stem, forming a sort of winged or leafy appendage, they are decurrent, as in thistles. The formation of peltate 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 frequently transformed into tendrils, with the view of enabling the plants to twine round others for support. In Leguminous plants (the pea tribe) the pinnae are frequently modified to form tendrils, as in Lathyrus Aphaca, in which the stipules perform the function of true leaves. In Flagellaria indica, Gloriosa superba and others, the midrib of the leaf ends in a tendril. In Smilax there are two stipulary tendrils.

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 fistular 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. 22) and Heliamphora the pitcher is composed of the petiole of the leaf. In the pitcher plant, Nepenthes, the pitcher is a modification of the lamina, the petiole often plays the part of a tendril, while the leaf base is flat and leaf-like (fig. 28).

In Utricularia bladder-like sacs are formed by a modification of leaflets on the submerged leaves.

In some cases the leaves are reduced to mere scales—cataphyllary leaves; they are produced abundantly upon underground shoots. In parasites (Lathraea, Orobanche) and in plants growing on decaying vegetable matter (saprophytes), in which no chlorophyll is formed, these scales are the only leaves produced. In Pinus the only leaves produced 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, phyllodia or stipules perform the function of leaves. The production of leaf-buds from