#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 strong stimulus. From fig. 33, _b_, above, it is seen that there was an actual reversal of response in the lower portion of the curve. It is therefore not improbable that there may be more than one point of reversal. In physical phenomena we are, however, acquainted with numerous instances of reversals. For example, a common effect of magnetisation is to produce an elongation of an iron rod. But Bidwell finds that as the magnetising force is pushed to an extreme, at a certain point elongation ceases and is succeeded, with further increase of magnetising force, by an actual contraction. Again a photographic plate, when exposed continuously to light, gives at first a negative image. Still longer exposure produces a positive. Then again we have a negative. There is thus produced a series of recurrent reversals. In photographic prints of flashes of lightning, two kinds of images are observed, one, the positive--when the lightning discharge is moderately intense--and the other, negative, the so-called 'dark lightning'--due to the reversal action of an intensely strong discharge. In studying the changes of conductivity produced in metallic particles by the stimulus of Hertzian radiation, I have often noticed that whereas feeble radiation produces one effect, strong radiation produces the opposite. Again, under the continuous action of electric radiation, I have frequently found recurrent reversals.[13] #Diminution of response under strong stimulus traced to fatigue.#--But there are instances in plant response where the diminution effect can be definitely traced to fatigue. The records of these cases are extremely suggestive as to the manner in which the diminution is brought about. The accompanying figures (fig. 34) give records of responses to increasing stimulus. They were made with specimens of cauliflower-stalks, one of which (_a_) showed little fatigue, while in the other (_b_) fatigue was present. It will be seen that the curves obtained by joining the apices of the successive single responses are very similar. [Illustration: FIG. 34.--RESPONSES TO INCREASING STIMULUS OBTAINED WITH TWO SPECIMENS OF STALK OF CAULIFLOWER In (_a_) fatigue is absent, in (_b_) it is present.]