CONDUCTIVITY AND EXCITABILITY 227 



this that the excitability at 35 C. is higher than at 30 C. 1 

 Hence we arrive at two conclusions directly opposed to each 

 other. 



This apparent anomaly completely disappears, however, 

 in the light of the distinction between the receptive and 

 motile excitabilities ; for it was said that it was the 

 mobility of the superficial particles which determined the 

 receptivity, and this is evidently enhanced by rise of tempera- 

 ture. The amplitudeof mechanical response, however, by which 

 we measure the motile excitability, is not solely dependent on 

 molecular mobility. This mechanical response is, as we have 

 seen, brought about by diminution of turgor, and any agent 

 which produced increase of turgor would act antagonistically, 

 and thus diminish the motile expression of excitation. For 

 example, we have seen that a pulvinus of Mimosa, when 

 highly turgid, failed to show any motile response, though 

 excited (p. 49). Now, it will be shown (p. 400), that rise 

 of temperature has the effect of increasing turgor. Hence 

 the diminution of mechanical response with increasing 

 temperature does not indicate diminution of excitability in 

 general, but rather the setting in of an antagonistic force, 

 whose influence will be to increase the force of recovery from 

 molecular distortion. It should be mentioned, however, 

 that there is a limit to the enhancement of excitability by 

 rise of temperature ; for the molecular disturbance caused 

 by heat will when excessive be detrimental to response. 



Excitability versus conductivity. — The same considera- 

 tions which have thus enabled us to distinguish between 

 receptivity and motile excitability, will also enable us to see 

 the difference between motile excitability and conductivity. 

 We have seen, for example, that at 35 C. the conductivity in 

 a given specimen of Biophytum was almost three times as 

 great as at 30 C. in spite of the fact that, as just explained, 

 contractile response is considerably diminished at high 



1 It will be found in Chapter XXX. that growth, which is a phenomenon of 

 excitatory response, is, in the case of many plants, at its maximum at or near 

 35° C. 



Q 2 



