292 CHAP. XXVI. GEOTROPISM 



the organ curves towards the stimulus. According to this 

 definition, the geotropic response of the stem is positive. 



The geotropic response is delayed by the bending due 

 to the weight of the horizontally laid shoot. Reduction of 

 weight consequently shortens the latent period ; in the case 

 of the petiole of Tropaeolum in a highly sensitive condition, 

 it is less than 20 seconds. 



The complete geotropic curve shows characteristics 

 which are similar to those of tropic curves in general. 



A dorsiventral organ is anisotropic ; the moto-excita- 

 bihties of the upper and lower sides are different. In the 

 pulvinus of Mimosa the excitabihty of the lower half is 

 greater than that of the upper half, and this differential 

 excitabihty of the dorsiventral organ determines its position 

 of geotropic equilibrium. 



