GERMINATION OF SEEDS 



95 



FIG. 89a. First stage of the 

 series shown in Fig. 89; one 

 cotyledon removed to show 

 the relation of parts, and the 

 arch developed by the first 

 in tern ode. 



phototropism. It is 

 interesting to note 

 that a tap-root be- 

 ing positively geo- 

 tropic, positively hy- 

 drotropic, and nega- 

 tively phototropic, all 

 of its responses under 

 ordinary conditions 

 combine to direct it 

 into the soil. 



56. Other seeds. 

 It must not be sup- 



versely phototropic. The adjust- 

 ment of the leaf-blades to the 

 new direction of the light may 

 be seen in Fig. 87. 



The root also is phototropic, 

 turning directly away from the 

 source of light; that is, it is 

 negatively phototropic. Fig. 88 

 shows a seedling of white mus- 

 tard so arranged that both stem 

 and root are exposed only to 

 weak light, the former showing 

 positive, the latter negative 



FIG. 90. Seedling of castor-bean, showing large 

 and green cotyledons. 



