OEOTROPISM 421I 



test of excitatory reaction uuiler geotropic stimulus is 

 furnishetl l)y investigations ou geo-electric response. When 

 a shoot is (lisplacetl from vertical to Jiorizontal position, 

 the upper side of the oryati is found to undergo an excita. 

 tori/ electric change of galvanometric negativity indicative 

 of diminution of turgor and contraction. The electric 

 change induced on the lower side is one of galvanometric 

 positivity, which imlicates an increase of turgor and expan- 

 sion. The tropic effect of geotropic stimulus is thus 

 similar to that of any other mode of stimulation, i.e., a 

 contraction of the uj)per (which in the present case is the 

 proximal) and expansion of the lower or the distal side. 

 The vertical liue-^ of gravity impinge on the upper side of 

 the organ which undergoes effective stimulation. 



In order to show that the concavity of the upper side 

 i& not due to the passive yielding to the expansion of 

 the lower half, I restrained the organ from any movement. 

 I have explained that excitatory electric response is mani- 

 fested even in the absence of mechanical expression of 

 excitation ; and under geotropic stimulus, the securely held 

 shoot gave the response of galvanometric negativity of 

 the upper side. Hence the fundamental reaction under 

 geotropic stimulus is excitatory contraction as under other 

 modes of stimulation. 



Finally, I employed the additional test of induced para- 

 lysis by application of intense cold. Excitatory physiolo- 

 gical reaction is, as we know, abolished temporarily by the 

 action of excessive cold. 



Experiment 163. — I obtained records of mechanical res- 

 ponre to determine the side which undergoes excitation 

 under geotropic stimulus, the method of discrimination being 

 local paralysis induced by cold. I took the flower-scapes 

 of AmarayUis and of Uriclis, and holding them vertical 

 applied fragments of ice on one of the two sides. I then 



