GEOTROPISM 441 



Tropic reactions are said to be positive, when the 

 directly stimulated side undergoes contraction with the 

 result that the organ moves to meet the stimulus. Accord- 

 ing to this test, the geotropic response of the stem is 

 positive. 



The geotropic response is delayed by the bending down 

 of the horizontally laid shoot. Reduction of weight is found 

 to shorten the latent period ; in the case of the petiole of 

 Tropceolum this is shorter than 20 seconds. The latent 

 period of geotropic response is found to be of the same 

 order as the " migration period " of the hypothetical stato- 

 liths. 



The complete geotropic curve shows characteristics which 

 are similar to tropic curves in general. 



In a dorsiventral organ the geotropic excitabilities of 

 the upper and lower sides are different. In the pulvinus 

 of Mimosa the geotropic excitability of the lower half is 

 greater than that of the upper half. The differential exci- 

 tabilities of a dorsiventral orgau modifies its position of 

 geotropic equilibrium. 



3;i A 



