306 PLANT RESPONSE 



cooled down. One of the young leaflets had been attached 

 to the Optical Lever, and records were now taken con- 

 tinuously. It was very interesting to observe how, with the 

 gradual loss of absorbed energy, the pulsating movements of 

 the plant became diminished in amplitude (fig. 124) till they 

 came to a stop. We shall find exactly the same thing in the 

 case of Desmodium. 



I referred in the last chapter to certain observations of 

 my own, in which the leaflets of Biophytum were found, under 

 favourable circumstances of light and temperature, to give 

 rise to apparently spontaneous movements, which could not 

 be traced to any definite varying external cause. From the 

 experiment just described, however, we see that it was the 

 continuous stimulus of favourable temperature and light com- 

 bined that caused this rhythmic movement, which appeared 

 as automatic. 



(b) Resumption of automatic movements in Desmo- 

 dium. — Similarly in Desmodium, if the isolated leaflet in which 

 all movement has been brought to a stop, but not to a condi- 

 tion of permanent rigor, be raised in temperature, and if the 

 tissue be maintained uniformly at this higher point, the 

 pulsatory movement will be found to commence and persist 

 for a long time. The initiation and maintenance of the 

 responses here, again, is undoubtedly due to the renewed 

 supply of energy. 



A new difficulty now arises, however, from our habit 

 of regarding stimulation as dependent upon some sudden 

 variation of external conditions. For we are here confronted 

 with a case in which uniform and continuous application of 

 heat produces stimulation. It may be urged that there has 

 been some variation of environmental conditions, in the fact 

 of the rise of temperature before the constant point was 

 reached. But it has to be remembered, that this rise was 

 purposely made very gradual, and even if a single stimulation 

 had been caused by this preliminary thermal variation, it 

 would have evoked only a single response, or at most a few 

 multiple responses But how are we to account for these 



