CHAPTER XXIII 



PHOTOTROPIC TORSION 



In addition to positive or negative curvature induced by 

 light, a torsional response also occurs under certain con- 

 ditions. A leaf when struck laterally by light undergoes 

 a twist, so that the upper surface is placed more or less at 

 right angles to the incident rays. It has been supposed 

 that such torsions were produced by the action of a number 

 of external factors, such as light, gravity, and weight of the 

 organ, which individually led to curvature but which in 

 combination induced torsion. Later investigations have, 

 however, shown that torsion actually occurs when light 

 alone is the external factor. No satisfactory explanation 

 has, however, been given of the mechanics of the torsional 

 movement. 



The experiments here described were planned to throw 

 light on this obscure phenomenon. They show : 



1. That the torsional response is not dependent on the 



combination of two curvatures ; 



2. That it is independent of the effect of weight ; 



3. That it can be induced not merely by the stimulus 



of light, but by all forms of stimulation ; 



4. That the direction of the torsional response depends 



on two factors : the direction of the incident stimulus, 

 and the differential excitability of the organ ; and 



5. That there is a definite law which determines the 



direction of the responsive movement of torsion. 



