INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 305 



In Desmodium, the autonomous movement is initiated at 

 a certain more or less definite temperature, which is about 

 1 7 C. This we may call the critical thermo-tonic condition. 

 Below this critical degree, Desmodium ceases to be autonomic, 

 and becomes an ordinarily responding plant. In Biophytum, 

 similarly, the critical thermo-tonic point is about 29°- C. 

 Above this, the young leaflets are autonomic, and below it, 

 ordinarily responding. The difference between Desmodium 

 and Biophytum in this respect lies, therefore, in the fact that 

 their critical thermo-tonic points are about twelve degrees 

 apart. 



In the case of Biophytum, when the temperature is main- 

 tained at a uniform favourable degree, the periods of the 



Fig. 124. Induction of Autonomous Response in Biophytum, 

 at Moderately High Temperature of 35 G. 



Note the diminution of amplitude of response with the gradual loss of 

 latent energy, consequent on falling temperature. The pulsations 

 came to a stop below 29 C. 



autonomous pulsations become very regular. It has been 

 said that these pulsatory movements are maintained by 

 means of energy absorbed, and with Biophytum I found an 

 added opportunity of demonstrating this fact. A particular 

 plant had been kept at a uniform temperature of 35 C, 

 under which the young leaflets gave autonomous responses. 

 The heating current, by which this temperature was main- 

 tained uniform, was now stopped, and the chamber gradually 



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