TEMPERATURE ON PHOTOTROPIC CURVATURE 393 



conduction of excitation in the petiole of Mimosa is as 

 high as 30 mm. per second in summer, it is reduced to 

 about 4 mm. in late autumn and early \vinter. 



ANTAGONISTIC EFFECTS OF LIGHT AND OF RISE 

 OF TEMPERATURE. 



I have explained the complex effect of rise of temperature 

 on phototropic curvature. Rise of temperature, within 

 limits, enhances the excitability, and therefore the positive 

 curvature under light. Its expansive reaction, on the other 

 hand, opposes the contraction of the proximal side, which 

 produces the normal positive curvature. Rise of tempera- 

 ture, as previously stated, introduces another element of 

 variation by its effect on conductivity. Transverse conduc- 

 tion favoured by rise of temperature promotes neutralisation 

 and reversal ; the resultant effect will thus be very 

 complicated. I give below account of an experiment where 

 the induced positive curvature under light underwent a 

 reversal during rise of temperature. 



Reversal of tropic curvature under rise of temperature : 

 Experiment 146. — The specimen employed for this experiment 

 was a seedling of pea, enclosed in a glass chamber, the 

 temperature of which could be gradually raised by means of 

 an electric heater. Provisions were made to maintain the 

 chamber in a humid condition. The temperature of the 

 chamber was originally at 29°C., and application of light on 

 one side of the organ gave rise to positive curvature, followed 

 by complete recovery on the cessation of light (Fig. 14t)a). 

 The next experiment was carried out with the same specimen ; 

 while the plant was undergoing increasing positive curvature 

 under the continued action of light, the temperature of the 



30 a 



