482 LIFE MOVEMENTS IN PLANTS 



the electric iudicatioa of excittition will undergo decline and 

 final abolition. The characteristic effects described above 

 are to be found only under the .action of gravitational 

 stimulus ; they will be absent when the organ is held in 

 a vertical position and thus freed from geotropio excitation. 



I have hitherto spoken of the excitatory effect of the 

 upper layer ; there must be some physiological reaction 

 on the lower perceptive layer, though of a different 

 character, represented diagrammatically by vertical shading. 

 Had the physiological reaction on the lower side of a 

 radial organ been the same as on the upper, geotropic 

 curvature would have been an impossibility, for similar 

 reactions on opposite sides would, by th«ir antagonistic 

 effects, 'have neutralised each other. 



After this preliminary explanation, I shall give a detailed 

 account of the experiments and results. It is to be borne 

 in mind that the investigation I am going to describe pre- 

 supposes no hypothesis of geotropic action. I start with the 

 observed fact that an organ under the stimulus oi gravity, 

 exhibits responsive movement. I ascertain the nature of 

 the underlying reaction by electric tests ; I have, in my 

 previous works, fully demonstrated that the excitatory con- 

 tractile reaction is detected by electro-motive change of 

 galvanometric negativity, and the opposite expansive reaction 

 by a change of galvanometric posifcivity. With the electric 

 probe I ascertain whether geotropic irritation is diffuse, or 

 whether it is localised at any particular depth of the organ. 

 I map out the contour lines of physiological reaction with 

 its heights and depths of excitation. 



I shall now proceed to describe the results of electric 

 exploration into the interior of the organ. The trouble I 

 foresaw, related to the irritation caused by the passage of 



