374 CHAP. XXXIII. THE RESPONSE OF THE ROOT 



containing the starch-grains are situated; and this would 

 account for the necessarily indirect geotropic stimulation 

 of the growing region of the root. 



The anatomical fact that the perceptive region in the 

 root is separate from the responding region, explains the 

 physiological difference between the two organs. Through 

 whatever means the stimulus of gravity may act, it is 

 inevitable, inasmuch as the stimulation of the shoot is 

 direct and that of the root indirect, that an identical 

 stimulation should induce responsive reactions of opposite 

 sign in the two cases. 



It will thus be seen that the postulation of two different 

 irritabilities in the shoot and in the root is wholly unneces- 

 sary and unwarranted by facts. Experiment has shown 

 that the irritability of the root is in no way different from 

 that of other organs. 



Summary 



When the tip of the root is subjected to the stimulus of 

 gravity, the upper half exhibits an excitatory reaction of 

 galvanometric negativity indicative of direct stimulation. 



The consequent electric response in the growing region 

 above the stimulated root-tip is positive, indicative of 

 increase of turgor and expansion. This is the effect of an 

 impulse transmitted from the stimulated tip. The stimulus 

 of gravity is perceived at the root-tip, whilst the responsive 

 movement takes place in the distant growing region. 



In contrast with the root, the growing region of the shoot 

 both perceives and responds to geotropic stimulation. 



As the effects of direct and indirect stimulation on growth 

 are antithetic, the geotropic responses of shoot and of root 

 cannot but be of opposite signs, since the stimulation is direct 

 in the one case and indirect in the other. 



There is no necessity for postulating different irrita- 

 bilities for the shoot and the root, since it has been demon- 

 strated that positive or negative curvature is dependent 

 upon whether the stimulation is direct or indirect. 



