TEMPERATURE ON PHOTOTROPIC CURVATURE 301 



to light is line to the variation of transverse conductivity of 

 the organ. In a semi-conducting tissue, while the excitatory 

 effect of feeble stimulus remains localised at the proximal 

 side, the effect of stronger stimulus is conducted to the distal 

 side. This explains the positive phototropic curvature of 

 tendrils of Vitis and Ampelopsis under feeble light, and 

 its reversal into negative curvature under intense light. 



As the conducting power is increased with rise of tem- 

 perature it is evident that at a certain temperature the 

 tropic effect will be exactly neutralised by transverse con- 

 duction. Lowering of temperature, by reducing the trans- 

 mission of excitation to the distal side, will restore the 

 positive curvature. Enhancement of conduction under rise 

 of temperature will, on the other hand, increase the anta- 

 gonistic reaction of the distal side and give rise to 

 a negative curvature. 



I shall in verification of the above, describe experiments 

 which I have carried out on the phototropic response of 

 the tendril of Passljiora., supposed to be insensitive to 

 the action of light. 



Phototrojiic response of the tendril of Passifiora : 

 Experiment 145. — The tendril was cooled by keeping it 

 for a long time in a cold chamber, maintained at 15'^C. 

 The effect of unilateral light on the cooled specimen was 

 found to be positive ; the tendril was next allowed to 

 assume the temperature of the room which was 30^0. 

 The response was now found to have undergone a change 

 into negative. The positive and negative phototropic 

 curvatures of an identical organ at different temperatures is 

 seen in the two records given in figure 145. Neutralisation 



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