62 RESEARCHES ON IRRITABILITY OF PLANTS 



with gradually increasing stimuli. We have already seen, 

 in fig. 12, the marked heightening of the response under 

 an increased intensity of stimulus. In muscle, in the 

 narrow range between minimal and maximal stimulation, 

 there is increasing amplitude of response with increasing 

 stimuli. But this soon attains a limit beyond which there 

 is no further increase of responsive contraction, whatever 

 be the stimulus-intensity employed. 



In order to demonstrate a similar progressive increase 

 in the response of Mimosa, I first determined the minimal 



Fig. 29.— Increasing response under increasing 

 intensity of stimulation. 



stimulus that was barely effective in inducing a feeble 

 response. Starting from the particular position of the 

 secondary which gave this minimal intensity of stimulus, 

 I very gradually increased the intensity, by moving the 

 secondary only 5 mm. at a time nearer to the primary. 

 At each step I took a corresponding record. 



In fig. 29 a series of seven such records is shown, the 

 successive responses being taken, as previously mentioned, 

 under slightly increasing stimuli and at intervals of 15 

 minutes. It will be seen how the height of response is 

 progressively increased. This increase is at first marked, 

 but towards the end we note that a limit is being ap- 

 proached, the difference between numbers 6 and 7 of the series 



