674 PLANT RESPONSE 



towards reversal or untwining. This is analogous to the 

 effect of inversion on autonomous torsion, in which, as we 

 have seen, the original torsion tends to be reversed. 



The effect of gravity is known to be a very important 

 factor in the induction of twining. It is believed that we 

 have here a third type of geotropic action, which is neither 

 positive nor negative, but lateral in its character. Under 

 lateral geotropism, a curvature is induced in the twining stem 

 in a horizontal plane. Since we have found, however, that 

 even such diverse effects as positive and negative geotropic 

 actions are not to be regarded as due to different specific 

 sensibilities, it would be interesting to inquire whether lateral 

 geotropism also is not simply a case of ordinary apogeo- 

 tropism in combination with some other tendency, say that 

 of autonomous torsion. 



At any rate, that this erectile or apogeotropic action is 

 always an element in the process, is shown by the fact that the 

 coils in the older portion of the stem become drawn out, show- 

 ing that the ascending movement predominates in that region. 



The whole subject is, however, extremely complicated, 

 owing to the presence of many factors and their different 

 relative intensities. And each of these may again be subject 

 to modification, under the action of external stimulus, as was 

 seen to be the case with autonomous torsion under the in- 

 fluence of light, gravity, and the variation of internal energy. 



Summary 



The observed effect of torsion, by which the upper 

 surface of the leaf is turned to face the light, is not due to 

 any specific dia-heliotropic sensibility. 



Similar torsional response is induced by the lateral 

 application of any other form of stimulus, such as thermal 

 or chemical. 



Such torsional response is also obtained from a com- 

 pound strip made of two unequally contracting inorganic 

 materials, such as ebonite and stretched india-rubber, when 

 stimulus is applied to one of the lateral flanks. 



