372 chap. xxxiii. the response of the root 



Geo-Perception at the Root-Tip 



The results given above fully confirm Charles Darwin's 

 discovery that it is the root-tip that perceives the stimulus 

 of gravity ; he found that removal of the tip abolished the 

 geotropic response of the root. The objection has been 

 raised that the shock-effect of the operation is itself the 

 cause of abolition of response. But subsequent observa- 

 tions have shown that Darwin's conclusions are in the main 

 correct. 



The experiments which have been described on the geo- 

 electric response of the root-tip and of the growing region 

 offer convincing proof of the perception of stimulation at 

 the tip, and of the indirect stimulation of the growing 

 region. These experiments exhibit, in one and the same 

 uninjured organ, the excitatory reaction at the upper side 

 of the tip, the cessation of excitation, and the excitation 

 of the opposite side of the tip, following the rotation of the 

 organ through + 90°, 0°, and — 90°. The effect at the 

 growing zone is precisely the opposite to that at the tip, 

 i.e. an expansive reaction which is the effect of indirect 

 stimulation. 



Difference in Geotropic Response of Shoot 



AND Root 



The next step is to endeavour to form some idea of the 

 difference in the conditions of geotropic stimulation of the 

 shoot and of the root, to account for the opposite responses 

 in the two organs. The reason for this difference lies in the 

 fact that in the shoot the perceptive and responding regions 

 are one and the same ; every piece of growing stem exhibits 

 the characteristic geotropic curvature. In the root the case 

 is different, since the perceptive and the responding regions 

 are separated from each other. When the perceptive root- 

 tip is removed the geotropic movement is either reduced or 

 abohshed. It must be borne in mind that this holds good 



