a " 



io6 BOTAXY AND PHARMACOGNOSY. 



tuberous roots, as those of the sweet potato and jalap, for these 

 latter have the morphological characters of roots. 



Instead of the node, or internode, or both, becoming excess- 

 ively thickened, they may be reduced in size and crowded upon 

 each other, the leaves at the same time becoming thickened and 

 filled with nutriment. Such a modified stem and leaves, as in the 

 onion, is called a bulb. Bulbs are sometimes produced in the axils 

 of the leaves of overground stems, as in some lilies, and are then 

 called bulbils or bud-bulbs, or they may be produced at the ends 

 of branches replacing the flowers, as in Allium. Bulbs and tubers 

 serve as storage-organs and carry the life of the plant over from 

 one season to another or they may in some instances tide over 

 unfavorable conditions. The thickened fleshy stems of Cactaceje 

 are also regarded as storage organs. 



A CORM is intermediate between a true tuber and a bulb; 

 it is more in the nature of a thickened internode, being surrounded 

 in some cases by thin membranous scales, as in Crocus and Col- 

 chicum. 



III. THE LEAF. 



Leaves arise as lateral developments on branches, being 

 derived from primordia which have their origin in the meristem- 

 atic tissue at the vegetative point, and young leaves are accord- 

 ingly at the ends of the branches. 



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, 

 which are known as stipules (Fig. 70). The principal function 

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

 tulip poplar {Liriodendron) (Fig. 144), although they may 

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

 the ^s.nsy {Viola tricolor) (Fig. 70). 



Light 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 



