INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 299 



been so, the responding movements would have been syn- 

 chronous with that of the hypothetical source, and all the 

 leaflets of the plant would in that case have had the same 

 period of vibration. As a matter of fact, however, different 

 leaflets have different periodicities. 



That the source of stimulation, again, is neither central 

 nor peripheral, may be shown by applying two tight liga- 

 tures, one behind and the other in front of the motile leaflets. 

 The passage of the excitatory impulse, should this be 

 transmitted from either direction, will by this means be 

 completely arrested. On carrying out this experiment, how- 

 ever, the periodic pulsation was not affected. 



Again, the leaf may be completely detached from the 

 plant, and the terminal leaflet amputated. But the periodic 

 pulsation still proceeds as before, just like the persistent beat 

 of the isolated heart of a frog. 



Thus we find that the isolated motile leaflet continues to 

 manifest an evolution of energy in the form of pulsating 

 movements, which cannot be derived from either of the 

 hypothetical sources, whether central or peripheral. It 

 maintains this activity for a long time, when kept under 

 favourable conditions. It thus becomes clear that in the case 

 of Desmodium the power of maintaining rhythmic pulsation 

 .is local, and resides in the tissue of the motile leaflets. 



This does not exclude the possibility of other periodically 

 moving organs obtaining the pulsating excitatory impulses 

 from a distant point. Such a case may well happen when, 

 the conductivity of the intervening tissue being very great, 

 and the excitability of the motile organ high, stimulation, 

 though enfeebled by transmission through a long tract, yet 

 remains above that critical intensity which would cause 

 effective response. 



In the previous chapter, I showed that if Desmodium be 

 kept in the dark for a sufficiently long period — but not so 

 long as to produce permanent rigor — or if a specimen be 

 taken in the unfavourable season of the year, its autonomic 

 movements would be found to have come to a stop for the 



