ime, we may expect the responses to regain their former
height after a period of rest. In order to verify this, therefore, I
renewed the stimulation (at intensity 45 deg.) after fifteen minutes. It
will at once be seen from record D how far the fatigue had been removed.

One peculiarity that will be noticed in these curves is that, owing to
the presence of comparatively little residual strain, the first response
of each set is relatively large. The succeeding responses are
approximately equal where the residual strains are similar. The first
response of A shows this because it had had long previous rest. The
first of B shows it because we are there passing for the first time to
increased stimulation. The first of C does _not_ show it, because there
is now a strong residual strain. D again shows it because the strain has
been removed by fifteen minutes' rest.

#Fatigue under continuous stimulation.#--The effect of fatigue is
exhibited in marked degree when a tissue is subjected to continuous
stimulation. In cases where there is marked fatigue, as for instance in
certain muscles, the top of the tetanic curve undergoes rapid decline. A
similar effect is obtained also with plants (fig. 24).

[Illustration: FIG. 24.--RAPID FATIGUE UNDER CONTINUOUS STIMULATION IN
(_a_) MUSCLE; (_b_) IN LEAF-STALK OF CELERY]

The effect of rest in producing molecular recovery, and hence in the
removal of fatigue, is well illustrated in the following set of
photographic records (fig. 25). The first shows the curve obtained with
a fresh plant. The effect is seen to be very large. Two minutes were
allowed for recovery, and then stimulation was repeated during another
two minutes. The response in this case is seen to be decidedly smaller.
A third case is somewhat similar to the second. A period of rest of five
minutes was now allowed, and the curve obtained subsequently, owing to
partial removal of residual strain, is found to exhibit greater
response.

[Illustration: FIG. 25.--EFFECT OF CONTINUOUS VIBRATION (THROUGH 50 deg.) IN
CARROT
In the first three records, two minutes' stimulation is followed by two
minutes' recovery. The last record was taken after the specimen had
a rest of five minutes. The response, owing to removal of fatigue by
rest, is stronger.]

The results thus arrived at, under the simple conditions of vegetable
life, free as they are from all possible complications and
uncertainties, may perhaps throw