MORPHOLOGY OF ANGIOSPERxMS. 



107 



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

 ally 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. 70, 74). The principal function of 

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

 tulip poplar {Liriodcndron) (Fig. 74), although they may 



Fio. 



Fig. ti- 



Fic. 70. A, leaf of violet (Viola tricolor) showing broad lamina, long petiole, and one 



of the palmately-lobed stipules at the base of the petiole. 

 Fig. 71. B, C, stages in the development of the leaf. The lobes of the stipules (s) de- 

 velop before the lamina fl). 



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

 the pansy {Viola tricolor) (Fig. 70). 



Right Relation of Leaves. While the lamina of the leaf 

 appears to assume a more or less horizontal position, it usually 

 inclines at such an angle as to receive the greatest amount of dif- 

 fused daylight. Wiesner has shown, for instance, that when 

 plants are so situated that they receive direct sunlight only for a 

 time in the morning, and diffused daylight during the rest of the 

 day, the position of the upper surface is at right angles to the 



