Petiole.—The petiole or leaf-stalk is the part which unites the limb or blade of the leaf to the stem. It is absent in sessile leaves, and this is also frequently the case when a sheath is present, as in grasses (fig. 3). It consists of the fibro-vascular bundles with a varying amount of cellular tissue. When the vascular bundles reach the base of the lamina they separate and spread out in various ways, as already described under venation. The lower part of the petiole is often swollen (fig. 14), forming the pul vinus, formed of cellular tissue, the cells of which exhibit the phenomenon of irritability. In Mimosa pudica (fig. 14) and other species, a sensitiveness is located in the pulvinus which upon irri tation induces a depression of the whole bipinnate leaf. A similar property exists in the pulvini at the base of the leaflets which fold upwards. The petiole varies in length, being usually shorter than the lamina, but sometimes much longer. In some palms it is 15 or 20 ft. long, and is so rigid as to be used for poles or walking sticks. In general, the petiole is more or less rounded in its form, the upper surface being flattened or grooved. Sometimes it is compressed laterally, as in the aspen, and to this peculiarity the trembling of the leaves of this tree is due. At other times it is winged, as in lemon and Dionaea, or pitcher like, as in Sarracenia. In certain of the Australian acacias, and in some species of Oxalis and Bupleurum, the petiole is flattened in a vertical direction, the vascular bundles separating immediately after quitting the stem and running nearly parallel from base to apex. This kind of petiole (fig. 15) has been called a phyllode. In these plants the laminae or blades of the leaves are pinnate or bipinnate, and are produced at the extremities of the phyllodes in a horizontal direction; but in many instances they are not developed, and the phyllode serves the purpose of a leaf. Some petioles are long, slender and sensitive to contact, and function as tendrils by means of which the plant climbs; as in the garden nasturtiums (Tropaeolum), clematis and others ; and in compound leaves the midrib and some of the leaf lets may similarly be transformed into tendrils, as in the pea and vetch.
Leaf Base.—The leaf base is often developed as a sheath (vagina), which embraces the whole or part of the circumference of the stem (fig. 3). This sheath is comparatively rare in dicoty ledons, but is seen in plants of the family Umbelliferae. It is much more common amongst monocotyledons. In sedges the sheath forms a complete investment of the stem, whilst in grasses it is split on one side. In the latter plants there is also a mem branous outgrowth, the ligule, at right angles to the median plane of the leaf from the point where the sheath passes into the lamina, there being no petiole (fig. 3).
In leaves in which no sheath is produced we not infrequently find small foliar organs, stipules, at the base of the petiole (fig. 16). The stipules are generally 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 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. i6) the true leaves are stalked and crenate, while the stipules are large, sessile and pinnatifid. In Lathyrus Apliaca 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. In other instances the stipules unite together on the side of the stem opposite the leaf forming an ochrea, as in the dock family. The stip ules 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. 13), or, as in Smilax, each stipule may be represented by a tendril. At the base of the leaflets of a compound leaf, small stipules (stipels) are occasionally produced.
Modifications.—Variations in the structure and forms of leaves and leaf-stalks 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 cel lular 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 opera tions is to increase the bulk and succulence of leaves. It is in this way that cabbages and savoys are rendered more delicate and nu tritious. The leaves of barberry and of some species of Astrag alus, 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, 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, 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 germina tion. 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 quently 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 ten drils, 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.