10 GROWTH AND WORK OF PLANTS 



water. In germination the swelling of the embryo and endo- 

 sperm by the absorption of water bursts the rigid seed coat 

 causing it to crack lengthwise, so that a portion of the endosperm 

 can be seen through the crack. The radicle emerges from the 

 end where the caruncle is located. The hypocotyl, which is very 

 short in the embryo stage, now elongates. The seed being bulky 

 is not readily pushed up through the soil. The hypocotyl part 

 of the stem arches upward forming a loop and as it elongates and 



Fig. 12. , 



Germination of castor-oil bean. 



endeavors to straighten up it pulls the "seed" from the ground. 

 Figure 1 2 shows different stages. The hard seed coat gradually 

 slips off. The white endosperm is now very distinct and is 

 seen to cover up the cotyledons which remain closed. It can be 

 seen, however, that the endosperm is "wasting away," that it is 

 being absorbed through the outer faces of the cotyledons. By 

 this means, they are exposed to the light and take on a green 

 color. The mass of the endosperm becomes less and less, until 

 finally there is but a thin film or remnant which dries. The food 

 stored in the seed has, therefore, been completely used up by the 

 embryo during germination, and the cotyledons have served as 

 the absorptive organs for this food. 



13. The embryo of the castor-oil bean presents a type very 

 different from that of the bean as we can readily see by a com- 

 parison. Yet the two types so different in form and structure 



