SENSITIVITY IN PLANTS AND ANIMALS 71 



Let us now study another stimulus, that of light. 

 Since light is of such transcendent importance to the 

 plant it is manifestly of the highest advantage that 

 the shoot should learn to grow towards the source Light, 

 of light, just as it is equally important that the root 

 should learn 

 to grow away 

 from it. If 

 some mustard 

 seed be grown 

 on damp moss 

 on a window- 

 sill we shall find 

 that the shoots 

 grow towards 

 the window 

 (heliotropism), 

 while the roots, 

 if exposed, 

 grow a way from 

 it (apheliotrop- 

 ism). This is 

 still better seen 

 if the plants 

 be grown in 

 culture solu- 

 tions. Now if 



other mustard seedlings be cultivated in the same 

 way, but if they be placed during cultivation on a 

 horizontally revolving disc, it will be seen that the 

 roots and shoots obey the stimulus of gravity only, 

 their shoots show no tendency to turn towards the 

 light, and their roots show no inclination to turn 

 away from it. 



Another illustration of the sensitivity of vegetable 



FIG. 28. Heliotropic curvature of 

 mustard seedlings. 



