7l6 COMPARATIVE ELECTRO-PHYSIOLOGY 



or ( ...). Examples of these are afforded by the eye of 

 the frog. These correspondences, between the effects of 

 light in vegetable tissues and in the retina, will be clearly 

 understood from the series of figs. 253 to 261. 



The next subject to be summarised is that of the 

 electrical response of plants to gravitational stimulus. In 

 an apogeotropic organ like the stem, when laid horizontally, 

 the mechanical response is such as to make the shoot once 

 more vertical. The active factor in this curvature might 

 obviously be, either the responsive contraction of the upper 

 side, or the responsive expansion of the lower. The question 

 to be decided here was whether the response of the plant, 

 to geotropic stimulus, was or was not of the same nature as 

 its response to other effective forms of stimulation that is 

 to say, by excitatory contraction. An experiment has been 

 described (p. 436) in which this question was subjected to 

 tests. The local application of cold is known to bring 

 about the temporary abolition of the excitatory effect, and 

 in the present case, its application on the lower side of a 

 horizontally laid shoot was not seen to induce any effect on 

 the response, while, when applied on the upper, it retarded 

 and arrested response to gravitation. This shows that in 

 this response it is the contraction of the upper side which is 

 the active factor. This is independently verified by the 

 test of electrical response, where I find that the upper 

 side, when subjected to gravitational stimulus, exhibits the 

 sign of true excitation namely, by induced galvanometric 

 negativity. 



The important Theory of Statoliths offers us a suggestive 

 explanation of the manner in which gravity exercises stimu- 

 lation upon the responding tissue, by the weight of solid 

 particles. When the stem is vertical, in consequence of the 

 symmetry of distribution of the particles on all sides, there 

 is no resultant action ; and as soon as this symmetry is 

 disturbed by laying the stem horizontally, response might be 

 expected to be initiated. This, however, is not the case. 

 The shoot first bends down, and it is not until after the 



