CHAPTER XXIX 

 GEO-ELECTRIC RESPONSE 



Theory of Hydrostatic Pressure and Theory of Statoliths Question regarding 

 active factor of curvature in geotropic response, whether contraction or ex- 

 pansion Crucial experiment by local application of cold Reasons for delay 

 in initiation of true geotropic response Geo-electric response of shoot 

 Due to active contraction of upper side, with concomitant galvanometric 

 negativity Geo-electric response of an organ physically restrained. 



IN the case of the action of external stimulus on plant 

 organs it is possible, given the direction of incident uni- 

 lateral stimulus, to predict the nature of the responsive 

 movement. I have shown elsewhere, in my work on Plant 

 Response, that all the actual movements of a plant organ 

 can be deduced from the simple law that it is the more 

 excited side that becomes concave. But though this law is 

 sufficient guide in dealing with the action of a known ex- 

 ternal stimulus, yet the problem becomes much more obscure 

 when we have to account for any movement which occurs in 

 response to a stimulus whose seat is internal. An example 

 of this class is afforded in the responsive curvature associated 

 with gravity. Thus, a shoot laid horizontally will curve up- 

 wards till the free end becomes vertical. In connection with 

 this subject there are two different points to be elucidated. 

 First, is the question as to the mode in which gravity exer- 

 cises stimulation ; and second, that of the method by which, 

 in answer to this stimulus, a definite responsive curvature 

 takes place. As regards the first of these, it may be said 

 that the only conceivable way in which gravity could produce 

 stimulation is by some differential effect of weight acting on 

 the responding cells. According to this, the necessary dif- 

 ferential weight-effect may be due to the weight of cell- 



