UNIFORM, FATIGUE, AND STAIRCASE EFFECTS I I 3 



exceed a certain minimum value in order to produce any 

 movement at all. If, again, the escape of water should be 

 resisted, owing to the peculiar condition of the tissue, which 

 has already been described (p. 49), then even a strong 

 stimulus would be unable to bring about movement. 



Analysis of the different phases in the response of 

 Mimosa under continuous stimulation. — Having now con- 

 sidered in detail some of those changes of protoplasmic pro- 

 perties which are brought about by the action of stimulus, we 

 are enabled to study the effect of long-continued stimulation 



Fig. 60. Ineffectiveness of Stimuli, owing to Increasing Fatigue, 

 in Mimosa 



In the left-hand figure stimuli were applied at intervals of 3-5 minutes. 

 These evoked feeble responses. In the right-hand figure stimuli were 

 applied at intervals of two minutes. Response now became incon- 

 spicuous. Where stimuli were applied at intervals of one minute no 

 effect was produced. The leaf was refractory. 



from a somewhat different point of view. We saw that with 

 incomplete recovery, protoplasmic excitability was pro- 

 gressively diminished. In order to demonstrate this, in the 

 case of Mimosa, I obtained responses at intervals of 3*5 

 minutes, the uniform stimulus of condenser discharge being 

 employed. The responses, which had been uniform, when 

 stimulus was applied after complete recovery, at intervals of 

 about eight minutes, were now found to be very much reduced 

 (fig. 60). In a second series of experiments on the same 

 specimen, the intervening periods of rest were still further 



I 





