390 



Sir J. C. Bose and Mr. G. Das. 



Fig. 14. — Record of effect of continuous application of light on upper half of pulvinus of 

 Mimosa leaf. Note erectile response (positive curvature) followed by neutralisation 

 and pronounced reversal into negative due to transverse conduction of excitation. 

 (Up-movement shown by down-curve and vice versd.) 



Turning to the main experiment, we find that : — 



(1) As a result of the action of light on the upper surface, there was a 

 positive phototropic effect which lasted for 50 seconds. 



(2) Owing to the internal conduction of excitation, the positive effect 

 underwent an increasing neutralisation on account of the excitatory contrac- 

 tion of the opposite side. This neutralisation depends on four factors: 

 (a) the intensity of the stimulus, (b) the conductivity of the organ in a 

 transverse direction, (c) the thickness of the intervening tissue, and {d) the 

 relative excitability of the opposite as compared with the proximal side. The 

 extent of positive curvature will also depend on the pliability of the organ. 



(3) Since the organ exhibits pronounced physiological anisotropy, and the 

 opposite side is far more excitable than the proximal, the internally diffused 

 stimulus brings about a greater contraction of the opposite side. The 

 positive phototropic curvature becomes reversed to a very pronounced 

 negative. The effect of the internally diffused stimulus is thus the same as 

 that of external diffuse stimulation. 



(4) When the stimulus is applied on the more excitable side of the organ — 

 in this case the lower half — the result is a predominant contraction of that 

 half; this cannot be neutralised by the excitation conducted to the feebly 

 excitable upper half. As the curvature is towards the stimulus, the photo- 

 tropic effect will appear to be positive. 



