332 LIFE MOVEMENTS IN PLANTS 



(2) Owing to the internal conduction of excitation the 



positive effect underwent neutralisation by the 

 excitatory contraction of the distal side. This 

 neutralisation depends on four factors : (a) on the 

 intensity of the stimulus, (b) on the conductivity 

 of the organ in a transverse direction, (c) on the 

 thickness of the intervening tissue, and (d) on the 

 relative excitability of the distal as compared to 

 the proximal side. The extent of positive curva- 

 ture also depends on the pliability of the organ. 



(3) In anisotropic organs where the distal side is 



physiologically the more excitable than the pro- 

 ximal, the internally diffused excitation brings 

 about a greater contraction of the distal, and 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 stimulus. 



(4) When the stimulus is applied on the more excitable 



half of the organ, the result is a predominant 

 contraction of that half, which cannot be 

 neutralised by the excitation conducted to the 

 less excitable half of the organ. As the curva- 

 ture is towards the stimulus, the phototropic 

 curvature thus remains positive, even under 

 continued stimulation. 



The positive curvature is due to the differential action 

 of unilateral stimulus on the proximal and distal sides. Fut 

 when a strong light is made to act continuously on one side 

 of an organ, the excitation becomes internally diffused, and 

 the differential effect on the two f^ides *s reduced in amount 

 or vanishes altogether. Owing to the weak transverse 

 conductivity of the tissue, while the effect of a feeble 

 stimulus remains localii^ed, that of a stronger stimulus is 

 conducted across it. 



