INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 309 



condition, which is determined by the sum total of all these 

 exciting factors. 



In connection with these facts, it is well to bear in mind 

 that the excitability imparted by a stimulus does not always 

 increase continuously with the intensity. On the contrary, 

 there may be an optimum intensity beyond which excitability 

 may be diminished. 



We have now seen that the energy which expresses itself 

 in pulsatory movements may be derived by the plant, either 

 directly from immediate external sources ; or from the excess 

 of such energy, already accumulated and held latent in the 

 tissue, aided by the incidence of external stimulation ; or from 

 an excessive accumulation of such latent energy alone. In 

 the last case, however, if the plant were kept isolated from all 

 external supply of energy, it is clear that its reserve would 

 become exhausted, and its automatic movements would cease. 

 I have already described an experiment in which this arrest 

 of pulsation took place in the case of a specimen of Desuiodiuui 

 kept in a dark room. We then saw that revival of response 

 was only brought about when fresh stimulus was applied. 

 And we have also seen the converse, namely, the ordinarily 

 responding Biophytum, when supplied with excess of energy, 

 become automatically responding. 



Cause of rhythmicity. — Having, then, seen that it is a 

 constant source of energy, external or internal — the latter 

 being really derived from a previous absorption of external 

 stimulus — which maintains the so-called automatic movements 

 in plants, we have still to determine how it is that a latent 

 or constantly acting external stimulus can find only periodic 

 expression ? In connection with this we have seen how, 

 when the minimal factor of stimulating intensity is exceeded, 

 there is a manifestation of the fact by visible response. I 

 have also shown (p. 245) that after each excitatory discharge 

 there is a marked diminution of both conductivity and 

 excitability ; and a new stimulus, or existing excess of 

 stimulus, owing to the loss of these properties, is retarded 

 for a time from producing any effect on the motile tissue. It 



