ess
(1) the injured end be completely killed and rendered quite
irresponsive, or (2) there be an effective block between A and B, so
that the disturbance due to stimulus can only act on one, and not on the
other.

I have found cases where, even when there was a perfect block, a
positive after-effect occurred. It would thus appear that if molecular
distortion from stimulus give rise to a negative variation, then during
the process of molecular recovery there may be over-shooting of the
equilibrium position, which may be exhibited as a positive variation.

#Positive variation.#--The responses given by muscle or nerve are,
normally speaking, negative. But that of retina is positive. The sign of
response, however, is apt to be reversed if there be any molecular
modification of the tissue from changes of external circumstances. Thus
it is often found that nerve in a stale condition gives positive,
instead of the normal negative variation, and stale retina often gives
negative, instead of the usual positive.

[Illustration: FIG. 27.--ABNORMAL POSITIVE RESPONSES IN STALE LEAF-STALK
OF TURNIP CONVERTED INTO NORMAL NEGATIVE UNDER STRONG STIMULATION[12]
The relative intensities of stimuli in the two cases are in the ratio
of 1:7.]

Curiously enough, I have on many occasions found exactly parallel
instances in the response of plants. Plants when fresh, as stated, give
negative responses as a rule. But when somewhat faded they sometimes
give rise to positive response. Again, just as in the modified nerve the
abnormal positive response gives place to the normal negative under
strong and long-continued stimulation, so also in the modified plant the
abnormal positive response passes into negative (fig. 27) under strong
stimulation. I was able in some cases to trace this process of gradual
reversal, by continuously increasing the intensity of stimulus. It was
then found that as the stimulus was increased, the positive at a certain
point underwent a reversal into the normal negative response (fig. 28).

[Illustration: FIG. 28. ABNORMAL POSITIVE PASSING INTO NORMAL NEGATIVE
IN A STALE SPECIMEN OF LEAF-STALK OF CAULIFLOWER
Stimulus was gradually increased from 1 to 10, by means of
spring-tapper. When the stimulus intensity was 10, the response
became reversed into normal negative. (Parts of 8 and 9 are out of
the plate.)]

The plant thus gives a reversed response under abnormal conditions of
staleness. I ha