PLANT BIOLOGY 



and the plant straightens and the 

 cotyledons expand. These coty- 

 ledons, or " halves of the bean," 

 persist for some time (b, Fig. 

 19). They often become green 

 and probably perform some 

 function of foliage. Because of 

 its large size, the Lima bean 

 shows all these parts well. 



Germination of Castor Bean. 

 In the castor bean the hilum 

 and micropyle are at the smaHer end 

 (Fig. 20). The bean " comes up " with a 

 loop, which indicates that the hypocotyl 

 greatly elongates. On examining germi- 

 nating seed, however, it will be found 

 that the cotyledons are contained inside a fleshy body, 

 or sac (a, Fig. 21). This sac is the endosperm, 

 its inner surface the thin, veiny coty- 

 ledons are very closely pressed, ab- 



FIG. 19. GEKMINATION OF 

 BEAN. 



FIG. 20. SPROUT^ 

 ING OF CASTOR 

 BEAN. 



Against 



FIG. ai. GERMINA- 

 TION OF CASTOR BEAN. 



Endosperm at a. 



FIG. 22. CASTOR 

 BEAN. 



Endosperm at a, a; coty- 

 ledons at b. 



FIG. 23. GERMINATION 

 COMPLETE IN CASTOR 

 BEAN. 



sorbing its substance (Fig. 22). The cotyledons increase 

 in size as they reach the air (Fig. 23), and become func- 

 tional leaves. 



