Fig. 21.—Fatigue in Celery
Vibration of 30° at intervals of half a minute.
The rhythm was now changed to intervals of half 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 original 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 response, where ‘the height of twitch diminishes more rapidly in proportion as the excitation interval is shorter.’[11]
Fig. 22.—Fatigue in Leaf-stalk of Cauliflower
Stimulus: 30° vibration at intervals of one minute.
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 undergone a continuous diminution. [Fig. 22] gives a photographic record of fatigue in the leaf-stalk of cauliflower.