40 RESPONSE IN THE LIVING AND NON-LIVING 



a minute, instead of one, while the stimuli were 

 maintained at the same intensity as before. It will be 

 noticed (fig. 20, b) that these responses appear much 

 feebler than the first set, in spite of the equality of 

 stimulus. An inspection of the figure may perhaps 

 throw some light on the subject. It will be seen that 

 when greater frequency of stimulation was introduced, 

 the tissue had not yet had time to effect complete 

 recovery from previous strain. The molecular swing 

 towards equilibrium had not yet 

 abated, when the new stimulus, with 

 its opposing impulse, was received. 

 There is thus a diminution of height 

 in the resultant response. The origi- 

 nal rhythm of one minute was now 

 restored, and the succeeding curves 

 (fig. 20, c) at once show increased 

 response. An analogous instance may 

 be cited in the case of muscle re- 

 sponse, where ' the height of twitch 

 diminishes more rapidly in proportion as the excitation 

 interval is shorter.' l 



From what has just been said it would appear 

 that one of the causes of diminution of response, or 

 fatigue, is the residual strain. This is clearly seen 

 in fig. 21, in a record which I obtained with celery- 

 stalk. It will be noticed there that, owing to the 

 imperfect molecular recovery during the time allowed, 

 the succeeding heights of the responses have under- 

 gone a continuous diminution. Fig. 22 gives a 



1 Biedermann, loc. tit. 



FIG. 21. FATIGUE IN 

 CELERY 



Vibration of 30 at inter- 

 vals of half a minute. 



