334 



PLANT RESPONSE 



quicker pulsations, but of an amplitude so small as usually 

 to pass undetected. It is only under prolonged exposure 

 to this relatively high temperature, or after exposure to a 

 temperature above 50 C, that true 

 heat-rigor sets in. This maximum 

 temperature varies in individual cases 

 with the tonic condition of the plant. 

 Effect of the reduction of 

 temperature to the thermo-tonic 

 minimum. — It has been said that, 

 generally speaking, the amplitude of 

 response increases with the lowering 

 of temperature. It is evident, how- 

 ever, that this process must have a 

 limit. For we know that pulsation 

 vanishes at the thermo-tonic mini- 

 mum ; before this reduction of ampli- 

 tude to zero it is clear that there must 

 be some point where the increase of 

 amplitude due to continuous cooling 

 must undergo reversal, or diminution. 

 And this is what we should theoreti- 

 cally expect, for since it is the absorbed 

 thermal energy that maintains the pulsation, it follows that, 

 when this is diminished below par, the vibrational energy 



FlG. 138. Record of Pulsa- 

 tions of Desmodium at 

 Different Temperatures 



ElG. 139. Record of Pulsations of Frog's Heart at Different 

 Temperatures|(Pembrey and Phillips) 



should also undergo diminution. We had an illustration of 

 this (p. 305), when the automatically vibrating Biophytum 



