Stipule

Stipule, a basal appendage sometimes present in leaves. They are almost invariably in pairs and are most frequent among Dicotyledons. When they are present the leaf is stipulate; when absent, exstipulate. The stipules are commonly leafy, and are exceptionally large in cases where the blade of the leaf is reduced, as in many tendril-bearing Leguminosae. They are rarely associated with a leaf-sheath; but are so in the rose. They often serve as b~d-scales, as in the alder, oak, beech, and linden, and they are then generally membranous and deciduous. In most acacias they are represented by spines, and in Smilax (q.v.) by tendrils (q.v.). Various cases of union occur among leafy stipules. They may cohere by their outer margins, as in Astragal1us, and are then termed opposite, as they form a leaf-like structure opposite their leaf; or they may cohere by their inner margins, as in Melianthus, when they are termed axillary; or by both margins, as in the Polygonaceae, when they are called ocreate, forming a tubular sheath round the internode. When the leaves are two or more in a whorl, stipules may cohere, forming interperiolar stipules, as in Galium Cruciata and in the epicalyx of the Strawberry. Compound leaves sometimes have stipules below each leaflet, which are termed stipels.