239. General character. The leaf may be regarded as an expansion of the substance of the bark, extended into a broad thin plate by means of a woody frame work or skeleton, issuing from the inner part of the stem. The expanded portion is called the lamina or blade of the leaf, and it is either sessile, that is, attached to the stem by its base, or it is petiolate, attached to the stem by a footstalk called the petiole.

240. Stipules. But the regular petiole very often bears at its bears a pair of leaf-like appendages, more or less apparent, called stipules. Leaves so appendaged are said to be stipulate, otherwise they are ex-stipulate.

241. Therefore a complete leaf consists of three distinct parts; the lamina or blade, the petiole, and the stipules.

242. Transformations. Both the petiole, blade and stipules are subject to numerous modifications of form. Either of them may exist without the others, or they may all be transformed into other organs, as pitchers, spines, tendrils, and even into the organs of the flower, as will hereafter appear.