RELATION BETWEEN STIMULUS AND RESPONSE 99 



increasing frequency of stimulation, becomes more and more 



J complete (fig. 49). 



As regards the mechanical response of plants, I obtained 



: similar tetanic effects with the longitudinal contractions of 

 the pistil of Datura alba (fig. 50). Stimuli were here applied 

 at intervals of ten seconds, which was too short an interval, 

 when compared with the natural period of recovery, lasting 

 about two minutes. Hence we obtained incomplete tetanus. 

 This incomplete tetanus became more complete when the 

 stimulation- frequency was increased, 

 successive stimuli being now applied at 

 intervals of five seconds. It may be 

 noted here that, in the tetanus both of 

 muscle and of plant, the effects of 

 individual stimuli, when rapidly suc- 

 ceeding, become so merged as to appear FlG . 50< Photographic 

 continuous. It is only after the maxi- Record of Genesis of 



. Tetanus in Mechanical 



mum effect has been attained, that the Response of Plants 

 individual effects of stimuli sometimes {Style oi Datura aOa) 

 become distinguishable by slight oscillatory movements of 

 the curve. In the case of rhythmic cardiac muscle, how- 

 ever, there is no tetanus ; and similarly, as described in 

 Chapter XXVII., we find no tetanus in the rhythmic 

 vegetable tissue of Desmodium. 



Summary 



There is a minimal intensity of stimulus necessary to 

 initiate response. 



A stimulus, singly ineffective, becomes effective on repe- 

 tition. 



Increasing intensity of stimulus produces increasing 

 response, which, however, tends to approach a limit. 



The effects of rapidly succeeding stimuli, in plant-tissues, 

 as in animal, become fused, individual effects being then in- 

 distinguishable. A maximum contractile effect is then pro- 

 duced depending on the intensity of stimulus. 



h 2 



