GERMINATION OF SEEDS 



91 



To the inward sloping surface is pinned a seedling whose 

 root has well started. The photograph (Fig. 84) shows 

 that the root, con- 

 tinuing to grow, has 

 turned from the ver- 

 tical direction under 

 the stimulus of the 

 moisture in the bibu- 

 lous paper, and is 

 pursuing a general di- 

 rection that is a re- 

 sultant between the 

 two stimuli. A more 

 detailed observation 

 of such an experi- 

 ment shows that the 

 root - tip sometimes 

 turns toward and 

 sometimes away from 

 the moist paper. 



54. Escape of the 

 cotyledons and the 

 plumule. After the 

 root with its branches 

 has anchored the 

 plantlet to the soil, 

 the hypocotyl begins to elongate rapidly; and since the 

 cotyledons are still within the testa this elongation results 

 in the development of an arch, the hypocotyl arch (Fig. 

 85). As the arch constantly seeks to straighten itself, the 

 upward pull on the cotyledons finally draws them out of 

 the testa and the hypocotyl straightens. The cotyledons, 

 however, have done their work, and although they may 

 become green and persist for some time, in the bean they 

 are of no further importance. It is the escape of the 



FIG. 84. A bean seedling showing the response 

 of the root when the stimulus of gravity is 

 from one direction and that of moisture from 

 another. 



