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° 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 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.