390 LIFE MOVEMENTS IN PLANTS 



will relatively be greater than expansion ; above the 

 critical point, expansion will be the predominant effect. 

 The critical temperature will obviously be different in 

 different organs. The positive curvature may thus- be 

 increased by a slight rise, while it may be neutralised, or 

 even reversed by a greater rise of temperature. 



The induced variation of excitability due to change of 



temperature is not the only factor in modifying tropic 



curvature, for variation of conductivity also exerts a 

 marked effect. 



EFFECT OF TEMPERATURE ON CONDUCTION. 



The conducting power of an organ is greatly enhanced 

 with rise of temperature. Thus in Mimosa the velocity of 

 transmission of excitation is doubled by a rise of tempera- 

 ture through 9^C. (p. 100). An organ which is practically 

 non-conducting at a low temperature will become conducting 

 at a higher temperature. 



Thus at a low temperature the organ may be non- 

 conducting, and the excitatory contraction under unilateral 

 stimulus will remain localised at the proximal side ; this 

 will give rise to a positive curvature. But under rising 

 temperature, the power of transverse conduction will be 

 increased and the excitation will be conducted to the 

 distal side. The result of this will be a neutralisation or 

 reversal into negative curvature (p. 139). A positive 

 curvature is thus reversed into negative by change of 

 excitability and conductivity, induced by rise of tempera- 

 ture ; examples of this will be given presently. 



PHOTOTROPIC RESPONSE OF TENDRILS. 



I shall here adduce considerations which will show that 

 the apparent anomalies regarding the response of tendrils 



