720 PLANT RESPONSE 



under a still higher E.M.F., there is a complete reversal, 

 inasmuch as the anode here excites at make, and the kathode 

 at break. This reversal, further, is facilitated by fatigue of 

 the tissue (p. 215). 



These polar effects may also, as I have shown, be demon- 

 strated in the case of animal tissues by means of the glow- 

 response of the firefly. An excitatory reaction is here shown 

 by an increase of the intensity of luminescence, and a 

 depressing reaction by its diminution. 



Multiple response. — When response is observed by 

 means of the electromotive or electrotactile method, we 

 obtain a single response to a single moderate stimulus ; but 

 on the application of strong stimulus a multiple series of 

 responses is found to be evoked. In the case of the retina, 

 similarly, a single intense stimulation by light gives rise to 

 recurrent visual impulses. 



In the same way, in the leaf of Biophytum, while a single 

 moderate stimulus gives rise to a single mechanical response, 

 a strong stimulus gives rise to a multiple series of responses. 

 In this case certain other peculiarities may also be observed ; 

 for instance, a certain minimal intensity of stimulus induces 

 the maximal mechanical response, which is not increased by 

 any increase of intensity of the stimulus. The excess of such 

 stimulus is held latent by the tissue for the time being, to find 

 subsequent expression as rhythmic multiple response. These 

 multiple responses are evoked by all forms of stimulation, 

 mechanical, thermal, chemical, photic, and electrical. In 

 this respect, of the minimally effective stimulus inducing 

 maximal response, we have an important point of resem- 

 blance between the actions of a rhythmic plant-tissue and 

 the cardiac muscle of the animal. Both, again, are charac- 

 terised by the exhibition of a long refractory period, which is 

 an expression of fatigue, or temporary loss of excitability 

 after excitatory discharge. The periodic oscillation of excit- 

 ability which is thus induced, imparts a rhythmic character 

 to the mechanical expression of the excess of stimulus which 

 is held latent in the tissue. 



