timulus, but with increasing stimulus the response undergoes a rapid
increase, after which it tends to approach a limit (fig. 33, _a_).

[Illustration: FIG. 33.--RESPONSES TO INCREASING STIMULI PRODUCED BY
INCREASING ANGLE OF VIBRATION
(_a_) Record with a specimen of fresh radish. Stimuli applied at
intervals of two minutes. The record is taken for one minute.
(_b_) Record for stale radish. There is a reversed response for the
feeble stimulus of 5 deg. vibration.]

3. As an extreme instance of the case just cited, I have often come
across a curious phenomenon. During the gradual increase of the stimulus
from a low value there would be apparently no response. But when a
critical value was reached a maximum response would suddenly occur, and
would not be exceeded when the stimulus was further increased. Here we
have a parallel to what is known in animal physiology as the 'all or
none' principle. With the cardiac muscle, for example, there is a
certain minimal intensity which is effective in producing response, but
further increase of stimulus produces no increase in response.

4. From an inspection of the records of responses which are given, it
will be seen that the slope of a curve which shows the relation of
stimulus to response will at first be slight, the curve will then ascend
rapidly, and at high values of stimulus tend to become horizontal. The
curve as a whole becomes, first slightly convex to the abscissa, then
straight and ascending, and lastly concave. A far more pronounced
convexity in the first part is shown in some cases, especially when the
specimen is stale. This is due to the fact that under these
circumstances response is apt to begin with an actual reversal of sign,
the plant under feebler than a certain critical intensity of stimulus
giving positive, instead of the normal negative, response (fig. 33,
_b_).

#Diminution of response with excessively strong stimulus.#--It is found
that in animal tissues there is sometimes an actual diminution of
response with excessive increase of stimulus. Thus Waller finds, in
working with retina, that as the intensity of light stimulus is
gradually increased, the response at first increases, and then sometimes
undergoes a diminution. This phenomenon is unfortunately complicated by
fatigue, itself regarded as obscure. It is therefore difficult to say
whether the diminution of response is due to fatigue or to some
reversing action of an excessively s