CHAPTER XXXIII 



THE RESPONSE OF THE ROOT TO DIFFERENT 



STIMULI 



The electric response of the shoot to the stimulus of gravity 

 has been described in previous chapters ; it was shown 

 that its response to geotropic stimulation is similar to that 

 to other modes of stimulation. 



The response of the root to stimulation of various kinds 

 will be described in the present chapter. It should be 

 borne in mind that the responsive curvature in the root 

 takes place in the sub-apical growing zone which is separated 

 by a certain distance from the tip. Stimulation is there- 

 fore direct only when the stimulus is appHed at the respond- 

 ing growing region ; it is indirect when applied at the tip. 

 The intervening distance between the root-tip and the 

 responsive zone of growth is semi-conducting. 



I may briefly recapitulate the effects of indirect and 

 direct stimulation as exhibited by mechanical response of 

 the root described in greater detail in a previous chapter. 



Effect of unilateral stimulation of the root-tip. — Stimula- 

 tion of the tip induces indirect stimulation of the growing 

 region higher up on the same side. The resulting expansion 

 produces a negative curvature away from the stimulated side. 

 This occurs under modes of stimulation as diverse as photic 

 and thermal. It has been shown that this effect is not 

 pecuhar to the root but also occurs in the shoot as the conse- 

 quence of indirect stimulation of the growing region (p. 142). 



Effect of direct unilateral stimulation of the growing 

 y^aion.^ln contrast with the negative curvature induced 



