ht 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]

[Illustration: FIG. 21.--FATIGUE IN CELERY
Vibration of 30 deg. at intervals of half a 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.

[Illustration: FIG. 22.--FATIGUE IN LEAF-STALK OF CAULIFLOWER
Stimulus: 30 deg. vibration at intervals of one minute.]

It is evident that residual strain, other things being equal, will be
greater if the stimuli have been excessive. This is well seen in
fig. 23, where the set of first three curves A is for stimulus intensity
of 45 deg. vibration, and the second set B, with an augmented response, for
stimulus intensity of 90 deg. vibration. On reverting in C to stimulus
intensity of 45 deg., the responses are seen to have undergone a great
diminution as compared with the first set A. Here is seen marked
fatigue, the result of overstrain from excessive stimulation.

[Illustration: FIG. 23.--EFFECT OF OVERSTRAIN IN PRODUCING FATIGUE
Successive stimuli applied at intervals of one minute. The intensity of
stimulus in C is the same as that of A, but response is feebler
owing to previous over-stimulation. Fatigue is to a great extent
removed after fifteen minutes' rest, and the responses in D are
stronger than those in C. The vertical line between arrows
represents .05 volt. (Turnip leaf-stalk.)]

If this fatigue be really due to residual strain effect, then, as strain
disappears with t