248 PLANT RESPONSE 



The stimulus, it is to be remembered, is increased by 

 increasing the voltage. But, on an undue increase of this 

 charging voltage, to about forty volts or upwards, I have 

 often found that the velocity undergoes an actual diminution. 

 This is to be ascribed to the fact, which was demonstrated in 

 the chapter on Polar Effects of Currents, that the excitatory 

 value of the kathode reaches a limit, with a certain E.M.F., 

 and that if the E.M.F. be carried far beyond this, the excita- 

 tory effect is reversed, that is to say, it is now the anode 

 that excites (p. 206). We have then the curious case of a 

 negative direction, as it were, of transmission. For whereas, 

 with moderate voltage, the excitatory disturbance travels in the 

 interpolar region from kathode to anode, it now, with exces- 

 sive voltage, travels in the opposite direction, from the anode 

 towards the kathode. 



We can now see with what great accuracy it is possible to 

 measure these changes of velocity, from which we can deduce 

 the variations of conductivity, not merely qualitatively, but 

 also quantitatively. This opens out to us the further possi- 

 bility of studying the quantitative effects of various external 

 agencies in modifying conductivity. I shall here relate a 

 simple experiment which affords an example of the method 

 to be followed in such an investigation. 



Effect of lowering of temperature on velocity of trans- 

 mission. — In order to study the effect of lowered tempera- 

 ture on conductivity, I applied ice-cold water over an area of 

 10 mm. of the conducting petiole in Biophytum. The length 

 of the conducting tissue experimented upon was 38 mm., and 

 the time taken for the stimulus of a condenser-discharge 

 (ten volts and "Oi microfarad) under normal conditions, 

 i.e. before the application of ice-cold water, to traverse this 

 length, was icri seconds, giving a velocity of 376 mm. per 

 second. But after the application of ice-cold water, the con- 

 ductivity was so diminished that the transmitted excitation 

 did not produce any response of the motile leaflet ; on 

 allowing the temperature of the cold water applied, however, 

 to rise a few degrees, the stimulus was found to be effective ; 



