PLANT BIOLOGY 



and the plant straightens and the 

 cotyledons expand. These coty- 

 ledons, or " halves of the bean," 

 persist for some time (/;, 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 smaller 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. Against 

 its inner surface the thin, veiny coty- 

 ledons are very closely pressed, ab- 



FlG. 19. 



• Germination of 



Bean. 



Fig. 20. — Sprout- 

 ing of Castor 

 Bean. 



Fig. 21. — Germina- 

 tion of Castor Bean. 



Endosperm at a. 



Fig. 22.— Castor 

 Bean. 



Endosperm at a, a; coty- 

 ledons at i. 



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. 



