Stipules
(from stipula, an upright leaf). Bodies, almost always two in number, and quite alike, situated one at each side of the base of each leaf, in many flowering plants, and in one or two Ferns. Stipules vary much in appearance and size; but the two sides of each are unlike one another. They often resemble leaflets, e.g., in Pea, and in Lotus, and are free from everything but the stem. In these cases, they do the work of leaflets; and they may even wholly replace the leaves in their action on gases, the leaves serving other uses--e.g., tendrils in Lathyrus Aphaca (a British weed). In many plants, e.g., Rose, the Stipules are small, and are fixed along each side of the base of the leafstalk (this arrangement is called "adnate"); in others, they resemble bud-scales, and serve the same purpose as the latter organs, protecting the tender structures in buds, e.g., Beech, Oak. In a few plants, Stipules of each pair are united, and form a single body, opposite the leaf (Platanus), or between it and the stem (Potamogeton), or form a sheath or "ochrea" around the stem (Polygonum and Rumex). In the genus Galium, they are so like the true leaves that the latter are distinguished only by having the buds in their axils; and the Stipules and leaves together resemble a whorl of true leaves. Frequently, Stipules are so small as to be readily overlooked; and in a good many plants they fall off early, and may thus not be detected. They are very constantly present in certain classes of plants; hence, their presence and their nature afford important characters in the definition of many natural orders. In many orders, they are not constantly met with, and in many others do not occur at all. Occasionally, e.g., Mimosa, they are metamorphosed into spines.