29O PLANT RESPONSE 



antecedent cause. In some of these cases the origin of the 

 impulse can be traced, but there are others in which no 

 external source of stimulus can be assigned. And here we 

 find ourselves passing imperceptibly into the obscure region 

 of automatism. 



There is much resemblance between such semi-automatic 

 phenomena in Biophytum and the well-known instance of 

 spontaneous movements exhibited by Desmodium gyrans, the 

 successive mechanical responses of which exhibit all the 

 peculiarities of multiple response as seen in Biophytum. In 

 Desmodium as in Biophytum we have similar cyclic changes 

 of period and of amplitude. Just as in the case of Biophytum, 

 when we cannot determine the source of stimulus, we are 

 tempted to regard the phenomenon as automatic, so in the 

 case of Desmodium gyrans it is our own inability to trace out 

 the origin of stimulus that leads us to regard the periodic 

 movements of the plant as automatic. 



Biophytum, then, under ordinary circumstances, exhibits 

 a single response to a single stimulus ; but when the stimulus 

 is strong, a single stimulation will produce multiple responses, 

 and these will persist long after the cessation of the primary 

 stimulus. Under exceptionally favourable circumstances 

 periodic movements occur, apparently without any exciting 

 cause. I have again found, as will be described more fully in 

 Chapter XXIV., that Biophytum itself under favourable cir- 

 cumstances of light arid warmth exhibits persistent and long- 

 continued autonomous pulsations, which are in no way 

 distinguishable from those of Desmodium. Even the periods 

 of vibration are, generally speaking, similar, inasmuch as in 

 both these plants, under different circumstances, I have 

 recorded pulsations, the periodicities of which vary from 

 one minute or less to four or five minutes. Thus Biophytum 

 forms a connecting link between those plants which exhibit 

 only ordinary response (single stimulus, single response) and 

 those in which mechanical movements appear to be auto- 

 matic. Biophytum is also particularly interesting, be- 

 cause in its case we find the same plant, under different 



