It was also explained that every stimulus gave rise to dual reactions, positive and negative, and that in a highly excitable tissue the positive is masked by the predominant negative. The positive, or A-effect, is generally described as a “building up” process. By choosing a sub-tonic specimen, I have been able to unmask the positive, A. In the case of sub-tonic growing organs the positive, A, is literally a building up process, giving rise to an acceleration of growth.
From these facts and others given previously it will be seen that the abnormal response of acceleration of growth under stimulus is by no means accidental or fortuitous but is a definite expression of an universal reaction, characteristically exhibited by all tissues in a condition of sub-tonicity.
CONTINUITY BETWEEN ABNORMAL AND NORMAL RESPONSES.
A given plant-tissue may exist in widely different conditions of tonicity. Let us take two extreme conditions, the optimum and the minimum. The tonic level will be at its lowest at the minimum, where growth will be at a standstill. The range between the optimum and minimum will be very extended; hence strong and long continued stimulation will be necessary to raise the tissue from the tonic minimum to the optimum level. There are innumerable grades of tonicity between the optimum and minimum. Within this wide range the characteristic response will be the abnormal positive. As we approach the optimum, the range for positive response will become circumscribed, and the intensity and duration of stimulus necessary to convert the positive to negative will be feebler and shorter. It will be very seldom that a plant is likely to be found at the optimum. Hence plants in general may be expected to give a feeble positive response under sub-minimal stimulus.
These considerations led me to look for the positive response under sub-minimal stimulation; the tracings which I have obtained with my highly sensitive Crescograph and other recorders show that my anticipations have been justified.
Fig. 83.—Acceleration of growth under sub-minimal light stimulus. Record on moving plate; stimulus applied at 5th dot, and subsequent erection of curve exhibits acceleration of growth. Last part of curve shows recovery of normal growth on cessation of stimulus.
Positive response under sub-minimal stimulus: Experiment 89.—In normal specimens, light of strong intensity induces a retardation of growth. When the source of light is placed at a distance, the intensity of light undergoes great diminution. Under the action of such feeble stimulus I obtained an acceleration of growth even in specimens which may be regarded as moderately vigorous (Fig. 83). Similar acceleration of growth was also obtained under feeble electric stimulation. The response is reversed to normal negative by increasing the intensity or duration of stimulus. Very feeble stimulus thus induces an acceleration and strong stimulus a retardation of growth. I have frequently obtained positive mechanical and electrical responses under sub-minimal stimulation. As chemical substances often act as stimulating agents, the opposite effects of the same drug in small and large doses may perhaps prove to be a parallel phenomenon.
It has been shown that stimulus induces simultaneously both A- and D-effects, with the attendant positive and negative responsive reactions, alike in pulvinated and in growing organs. A tissue, in an optimum condition, exhibits only the resultant negative response; the comparatively feeble positive is imperceptible, being masked by the predominant negative; but with the decline of its tone excitability diminishes, with it the D-effect, and we get the A-effect unmasked, resulting response then becomes diphasic. In extreme sub-tonic condition, it exhibits only the positive. The sequence is reversed when we begin with a tissue in a state of extreme sub-tonicity, which first exhibits only the positive. Successive stimulations continually exalt the tonic condition, the subsequent responses becoming, diphasic, and, with the attainment of optimum tone, a resultant negative response. As a further verification of the simultaneous existence of both A- and D-effects, it has been shown that in ordinary tonic condition a sub-minimal stimulus gives rise only to positive response; this becomes converted into normal negative under moderate stimulation.