276 PLANT RESPONSE 



In view of certain other phenomena not altogether dis- 

 connected, it seems unfortunate that the term ' refractory 

 period ' should be used with its present significance. For 

 this term might be held to imply that the tissue refuses to 

 take any account of the superfluous energy that is impressed 

 upon it. It is more likely, however, that by some peculiar 

 mechanism, the superfluous stimulus — i.e. what is over and 

 above the amount necessary for producing maximal re- 

 sponse — is prevented from overflowing. This excess of 

 energy may, then, at least in some cases, remain latent, 

 to be manifested at a later period in the form of excitatory 

 impulses. Such impulses, again, attuned by some regulating 

 process, may give rise to periodic or rhythmic overflow. 

 That this is actually the case will be demonstrated in the 

 next chapter. 



Summary 



The latent period of response is protracted by cold. 

 It is also protracted by fatigue. It is shorter under strong, 

 than under moderate stimulation. 



In vigorous specimens of Biophytum leaflet, the mini- 

 mally effective stimulus is also maximal ; under normal 

 conditions, this minimally effective stimulus has a definite 

 value. 



There is also a definite refractory period in the response 

 of Biophytum. If a second stimulus fall within this refractory 

 period, it appears to produce no effect. 



The response of the leaflet of Biophytum resembles in 

 many respects that of cardiac muscle. In both, response is 

 on the 'all or none' principle, and both exhibit a relatively 

 long refractory period. 



