CHAPTER XXVIII 



GEOTROPIC TORSION 



I HAVE explained that lateral application of certain stimuli 

 to the flanks of a dorsiventral organ induces a responsive 

 torsion by which the less excitable side is made to face the 

 stimulus (p. 256). I now show that the reaction to the 



Fig. 182. Diagram of arrangement for recording torsional re- 

 sponse of leaf of Mimosa to geotropic stimulation. The less 

 excitable upper half of the pulvinus is to the left. 



stimulus of gravity is in every respect similar to that to 

 other forms of stimulation. 



As the direction of force of gravity is fixed, it is necessary 

 that the dorsiventral organ should be movable, that it may 

 be so placed that the geotropic stimulus may act upon its 



