DISPERSAL AND GERMINATION OF SEEDS 183 



into stalks of varying lengths. In the castor-bean (Fig. 149) 

 and squash, the cotyledons not only escape, but become green 

 and work like ordinary leaves. In corn, as in all cereals, the 

 embryo lies close against one side of the seed, so that it be- 

 comes completely exposed by the splitting of the thin skin 

 that covers it (Fig. 150). In this case the single cotyledon is 

 never freely expanded, but remains as an absorbing organ 

 in contact with the starch-containing endosperm. 



With the establishment of roots in the soil and the exposure 

 of green leaves to light and air, germination is over, for the 

 young plant is now able to make its own food. 



115. Phototropism. The stem of the seedling is sensitive 

 to the direction of rays of light, and therefore it is said to be 



FIG. 151. A bean seedling that was placed in a horizontal position and after two 

 hours photographed. 



phototropic (" light-turning "). It is not light in general 

 that acts as a stimulus, but the direction of the rays of light. 

 The response of the stem to this stimulus is to curve directly 

 towards the source of the light rays, and therefore it is posi- 

 tively phototropic. Figure 151 shows a bean seedling placed 

 in a horizontal position and after two hours photographed ; 

 while Figure 152 shows the same seedling completely inverted 

 and photographed after two days. 



The root is also -photo tropic, turning directly away from 

 the source of light; that is, it is negatively phototropic. 

 Figure 153 shows a seedling of a white mustard so arranged 

 that both stem and root are exposed only to weak light, the 



