302 PLANT RESPONSE 



continuous stimulation, it is necessary to satisfy ourselves 

 that this continuous source is the sole cause of the rhythmic 

 movements. This will appear conclusive, if it is made clear 

 that the plant does not possess any intrinsic energy of self- 

 movement, but only the power to regulate rhythmically the 

 overflow of energy supplied. 



The demonstration will then be made rigorous, if we take 

 a plant which is- not manifesting automatic movement, and 

 cause it to exhibit such action by the application of constant 

 external stimulus. For the purpose of this experiment, then, 

 we may take the leaflet of Desmodium in a state of standstill, 

 or that of BiopJiytum under normal conditions, which, as we 

 have seen, may be regarded as practically equivalent to 

 Desmodium in a standstill condition. If therefore we can 

 succeed in making Biophytum exhibit rhythmic movements 

 under the continuous action of a given external energy, we 

 shall not need to look for the explanation of the so-called 

 automatic movements of Desmodium to some periodically 

 varying stimulus, any constant stimulus being then proved 

 fully competent to produce the rhythmic pulsations. 



[b) On Biophytum. — I first subjected the motile leaflet of 

 Biophytum to light of comparatively short duration, i.e. two 

 minutes, by throwing sunlight upon it from a reflecting 

 mirror. There was no immediate response, but after a latent 

 period of one minute the leaflet gave a single response. I 

 next subjected the plant to the continuous action of light 

 during ten minutes. I obtained rhythmic pulsations, there 

 being two responses in the given time. I then cut off the 

 light, and the rhythmic action was stopped. After an 

 interval of several minutes I again applied light for ten 

 minutes, and this gave rise to similar rhythmic responses. 

 The procedure was repeated once more with the same results. 

 With another specimen, I applied light for a still longer 

 period, i.e. twelve minutes, and I obtained three pulsations. 

 It will thus be seen that we have here a regulated outflow of 

 energy ; the plant absorbs energy continuously, but gives it 

 out in a pulsating manner (fig. 123). In this way, a number 



