46 MOVEMENTS OF PLANTS 



able form of seedling. When the grain germinates 

 it does not send up a simple cylindrical sprout like 

 an oat, but a delicate stem terminating in a pointed 

 swollen part which looks like a little spear-head. 

 When a group of Setarias is illuminated from one 

 side they bend strongly over, with their spear- 

 heads all pointing straight at the light. The spear- 

 heads do not bend ; the whole movement is carried 

 out by the stalk on which the head is supported. 

 And what is remarkable is, that the spear-head 

 and not the stalk perceives the light. This is 

 easily proved by covering the heads of a few 

 Setarias with opaque caps. For the result is that 

 the blindfolded seedHngs remain vertical while their 

 companions are pointing to the light. Thus the 

 part which bends is unaffected by illumination, and 

 the part which is affected does not bend. The 

 spear-head is the percipient organ, the shaft or 

 stalk is the motor region, and from head to shaft 

 an influence has clearly been transmitted. 



My father and I made an attempt to prove the 

 same thing for the gravitation-sense of roots, that is, 

 to prove that the tip of the root is the region in 

 which the force of gravity is perceived by the plant. 

 Our method of proof does not hold good, but our 

 conclusions are true after all. When gravitation is 

 the stimulus, the experiment is much more difficult 

 than when light is in question, because now that 

 fairy godmothers are extinct we must not hope for 

 a substance opaque to gravitation, a substance 

 with which we might shelter the root-tips from the 

 force of gravity as the tips of the Setaria seedlings 

 were sheltered from light. 



