#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 have sometimes found similar reversal of response when the plant is subjected to the abnormal conditions of excessively high or low temperature. #Radial E.M. variation.#--We have seen that a current of response flows in the plant from the relatively more to the relatively less excited. A theoretically important experiment is the following: A thick stem of plant stalk was taken and a hole bored so as to make one contact with the interior of the tissue, the other being on the surface. After a while the current of injury was found to disappear. On exciting the stem by taps or torsional vibration, a responsive current was observed which flowed inwards from the more disturbed outer surface to the shielded core inside (fig. 29). Hence it is seen that when a wave of disturbance is propagated along the plant, there is a concomitant wave of radial E.M. variation. The swaying of a tree by the wind would thus appear to give rise to a radial E.M.F. [Illustration: FIG. 29.--RADIAL E.M. VARIATION] FOOTNOTES: