THE APPLICATION OF QUANTITATIVE STIMULUS 39 



under successive thermal shocks, imparted at intervals of one 

 minute. We have hitherto studied the responses caused by 

 uniform stimuli. We shall next observe the increase of 

 responsive effects brought about by increase of stimulus. In 



28. 



Photographic Record of Uniform Response in Petiole of Fern 

 to transmitted excitation 



animal tissues it is found, speaking generally, that increasing 

 stimuli induce increasing effects, but that this process has a 

 limit ; and in plant tissues the same is found to be the case. 

 In order to obtain effects of the simplest type, not compli- 

 cated by any secondary phenomena, 

 it is necessary to choose specimens 

 which exhibit little fatigue. In the 

 first of these the stimulus was ap- 

 plied by means of the spring-tapper. 

 The first stimulus was given by a 

 fall of the striking-lever from the 

 height h ; the second from 2h ; and 

 the third from 3h. The response- 

 curves (fig. 29) clearly show the in- 

 crease of effect due to this increasing 

 stimulus. 



In the second series, the stimulus applied was vibrational, 

 and increased from 2-5 to 12-5 by steps of 2-5 at a time. 

 Fig. 30 shows how the intensity of response tends under 

 these conditions to approach a limit. The following table 

 gives the absolute values of the responsive electro-motive 

 variations. 



FIG. 29 



Taps of increasing strength 

 1:2:3:4 producing in- 

 creased response in leaf- 

 stalk of turnip. 



