XXXIV.— ON PHOTONASTIC CURVATURES 



By 



Sir J. C. BoSE, 



Assisted hy 



GURUPRASANNA DaS. 



PhototropiC response, positive or negative, is determined 

 by the directive action of light. But photonastic reaction 

 is supposed to belong to a different class of phenomenon, 

 where the movement is independent of the directive action 

 of light. I shall, however, be able to establish a continuity 

 between the tropic response of a radial and the nastic 

 movement of a dorsiventral organ. The intermediate link 

 is supplied by organs originally radial, but subsequently 

 rendered anisotropic by the unilateral action of stimulus of 

 the environment. In a dorsiventral organ, owing to ana- 

 tomico-physiological differentiation, the responsive movement 

 is constrained to take place in a direction perpendicular to 

 the plane of separation of the two unequally excitable 

 halves of the organ. Even in such a case, it will be shown, 

 that light does exert a directive action ; the direction of 



