348 



A TEXT-BOOK OF BOTANY. 



III. THE OUTER MORPHOLOGY OF THE LEAF. 

 Leaves are lateral formations upon the stem and their growth 

 is definite. They never occur on other portions of the plant than 

 stems, from the surface of which they are developed. Leaves 

 appear in acropetal succession, so that the youngest leaves occur 

 nearest the apex of the stem. Terminal leaves are extremely 

 rare, but arise in some instances from the flowers of certain 

 Euphorbiaceae. 



Fig. 201. A, leaf of violet (Viola tricolor) showing broad lamina, long petiole, and one 

 of the palmately-lobed stipules at the base of the petiole. B, C, stages in the development 

 of the leaf. The lobes of the stipules (s) develop before the lamina (1). 



A Simple Leaf consists of a lamina or blade, which is usually 

 membranous and of a green color, and a petiole or stalk, which, 

 however, may be wanting when the leaf is said to be sessile. 

 Leaves may also possess a pair of leaf-like structures at the base, 

 known as stipules (Figs. 201, 204). The principal function of 

 the latter appears to be that of protecting the buds, as in the 

 tulip poplar (Liriodendron) (Fig. 204), although they may be- 

 come leaf-like and assist in the functions of the lamina, as in 

 Viola tricolor (Fig. 201). 



