XXXVI.— ON PHOTOTROPIC TORSION 



By 



Sir J. C. BosE, 



Assisted by 



SuRENDRA Chandra Das. 



Is addition to positive or negative curvatures light 

 induces a responsive torsion. With regard to this Jost 

 says : — 



" The mechanics of the torsions have not as yet been fully 

 explained. It has long been believed that these torsions were 

 occasioned only by the action of a series of external factors, 

 such as light, gravity, weight of the organ which individually 

 led to curvatures, but in combination induced torsions ; but 

 later investigations have shown that torsions might appear 

 when light only was the functional external factor. ... If (he 

 torsions cannot generally be regarded as due to the combina- 

 tion of tv>'0 curvatures, we are completely in the dark as to the 

 mechanics of their production."* 



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 ; as no explanation was available for this 

 movement, the suggestion has been made that the particular 

 reaction is for the advantage of the plant. I shall presently 

 show, that it is possible to reverse this normal torsion and 

 thus make the upper surface of the leaf move away from 

 light. 



* Jost -/6k/— p. 465. 



