GEOTROPTSM 439 



the next chapter. In the ordinary method of experiment- 

 ation with mechanical response the organ is rotated in a 

 vertical plane. The geotropic movement is found increased 

 as the directive angle is increased from zero to 90^. 



DIFFERENTIAL GEOTROPIC EXCITABILITY. 



It has been shown that geotropic stimulus acts more 

 effectively on the upper side of the organ. The intensity of 

 geotropic reaction is, moreover, modified by the excitability 

 of the responding tissue. It is easy to demonstrate this 

 by application of depressing agents on the more effective 

 side of the organ. The rate of geotropic up-movement will 

 be found reduced, or even abolished by the local applica- 

 tion of cold, anaesthetics like chloroform, and of poison- 

 ous potassium cyanide solution. 



The different sides of a dorsiventral organ are 

 unequally excitable to diff'erent forms of stimuli. I 

 have already shown (p. 85) that the lower side of the 

 pulvinus of Mimosa, is about 80 times more excitable to 

 electric stimulus than the upper side. Since the effect of 

 geotropic stimulus is similar to that of other forms of 

 stimuli, the lower side of the pulvinus should prove to be 

 geotropically more excitable than the upper side. This 

 I have been able to demonstrate by different methods of 

 investigation which will be described in the following 

 chapters. 



Under ordinary circumstances, the upper half of the 

 pulvinus is, on account of its favourable position, more 

 effectively stimulated by geotropic stimulus ; in consequence 

 of this the leaf assume a more or less horizontal position 

 fif " dia-geotropic " equilibrium. But when the plant is 

 inverted the more excitable lower half of the organ now 

 occupies the favourable position for geotropic excitation. 



38 



