PHOTONASTIC CURVATURES 385 



positive ami negative sigus, represents the amplitude 

 and sign of curvature. 



a. Radial thick organ, in which transverse conduction 

 is absent. Curvature is positive, i.e., movement to- 

 wards light. The result will be similar when light 

 strikes in an opposite direction, i.e., from right 

 to left. 



h. Radial thin organ. There is here a possibility of 

 transverse conduction. Sequence of curvature : 

 positive, neutral, and negative. Reversal of 

 direction of light gives rise to similar sequence 

 of responses as before (e.g., seedling of Sinapis). 



c. Anisotropic thick organ ; transverse conduction 



possible. Thick line represents the more excitable 

 distal side. Sequence of curvature : positive, 

 neutral and pronounced negative. When light 

 strikes from opposite direction on the more 

 excitable side the curvature will remain positive, 

 since the pronounced reaction of the more excit- 

 able side cannot be neutralised or reversed by 

 transmitted excitation to the less excitable distal 

 side {e.g., leaf of Mimosa). 

 In the absence of transverse conduction, the curvature 

 remains positive (e.g., leaflet of Erythrina). 



d. Anisotropic thin organ with high transverse con- 



ductivity. Sequence of curvature : transient posi- 

 tive, quickly masked by predominant negative. 

 Light striking on the more excitable side will 

 give rise only to positive. The response in rela- 

 tion to the plant, will apparently be in the same 

 direction whether light strikes the organ on one 

 side or the opposite (e.g., leaflets of Mimosa 

 Averrhoa and Biophytum). 



I have shown that tissues in sub-tonic condition exhibit 

 an acceleration of the rate of growth under stimulus (p. 224) 



