Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/123

Rh COMPOUND LEAVES.] BOTANY 113 tissue. The vessels are enclosed in an epidermal cover ing, with few stomata, and are more or less compressed. When the vascular bundles reach the base of the lamina they separate and spread out in various ways, as already described under venation. At the place where the petiole joins the stem there is frequently an articulation, or a con striction with a tendency to disunion, and at the same time there exists a swelling (fig, 117,p), called pulvinus, formed of cellular tissue, the cells of which exhibit the phenomenon Fig. 117. Fig. 118. FIG. 117. Branch and leaves of the Sensitive plant (Mimosa pudica), showing the petiole in its erect state, a, and in its depressed state, 6; also the leaflets closed, c, and the leaflets expanded, d. Irritability resides in the pulvinus p, and in the struma. FIG. 118. A portion of the branch and leaf of the Moving plant of India (fferfy- sarum or Dexmodium gyratis). The leaf is impari-pinnate, and often pinnately- trifoliate. The large leaflet or pinna, a; the smaller leaflets, b, of which there are either one or two pairs. The leaflets are in constant motion. of irritability (figs. 117, 118). At the point where the petiole passes into the lamina, or where the midrib joins the leaflets of a compound leaf, there is occasionally a cellu lar dilatation called struma, and an articulation. In Mimosa pudica a sensitiveness is located in the pulvinus (fig. 117, p), which upon irritation induces a depression of the whole bipinnate leaf; a similar property exists in the struma at the base of the leaflets which fold upwards. In Desmodium gyrans (fig. 118) the pulvinus and struma have a similar power of causing movements of the large terminal leaflet and the two smaller lateral ones. In other cases the petiole is not articulated, but is continuous either with the stem, or with the sheath (vagina). The articulation or joint is by many considered as indicating a compound leaf, and hence the leaf of the Orange is considered as such, although it has an undivided lamina (fig. 119, I). In articulated leaves, the pulvinus may be attached either to the petiole or to the axis, and may fall with the leaf, or remain attached to the stem. When articulated leaves drop, their place is marked by a cicatrix or scar, seen below the bud in fig. 63. In this scar the remains of the vascular bundles c are seen ; and its form furnishes characters by which particular kinds of trees may be known when not in leaf. In the case of many Palms and Tree-ferns the scars or cicatrices of the leaves are very conspicuous. In fossil plants important characters are founded on them. When there is no articulation between the petiole and the stem, as is the case with many Monocotyledons, the leaf is continuous with the axis, and is not deciduous, but withers on the stalk. In many Liliaceous plants the leaves during their decay continue attached to the plants. The petiole varies in length, being usually shprter than the lamina, but sometimes much longer. In some Palms it is 15 or 20 feet long, and is so firm 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 attributed. In aquatic plants the leaf-stalk is sometimes distended with air, as in Pontederia and Trapa, so as to float the leaf. At other times it is winged, and is either leafy, as in the Orange (fig. 119, p), Lemon, and Dioucea (fig. 40, p), or pitcher-like, as in the Pitcher-plant Fig. 119. Fig. 120. Fig. 121. FIG. 119. Leaf of Orange (Citrus Aurantium), showing a winged leafy petiole p, which is articulated to the lamina I. It is considered a compound leaf, having only one leaflet. FIG. 120. -Pitcher of a species of Pitcher-plant (Nepenthes distiUatoria). It is supposed to be formed by a folded petiole p, the edges of which are united. The lid I at the top is supposed to represent the lamina, united by articulation to the pitcher. FIG. 121. Pitcher (aicidium) of a species of Side-saddle plant (Sarracenia pur- pwea). The pitcher is supposed to be formed by the folded petiole, which is prolonged. (fig. 120, p) and Sarracenia (fig. 121). Leafy petioles are occasionally united to the axis for some extent, and thus become decurrent. In some 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 run ning nearly parallel from base to apex. This kind of petiole (fig. 122, p) has been called phyllodium. In these plants the laminae or blades of the leaves are pinnate, bipinnate, or ternate, and are produced at the extremities of the phyllodia in a horizontal direction ; but in many instances they are not developed, and the phyllodium serves the pur pose of a leaf. Hence some Acacias are called leafless. These phyllodia, by their vertical position and their peculiar Fig. 123. Fig. 122. FIG. 122. Leaf of an Acacia (Acacia heterophyJla), showing a flattened leaf-like- petiole p, called a phyllodium, with straight vei.ation, and a bipinnate lamina /. FIG. 123. Leaf of Pansy, I, separated from the stem; the lyrate-pinnatifid stipules j are distinctly visible, and their lateral position is seen. form, give a remarkable aspect to vegetation. On the same Acacia there occur leaves with the petiole and lamina IV. 15