_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. I have described the action of stimulus on tissues in which, on account of sub-tonicity, growth has become enfeebled. I shall next take up the question of effect of stimulus on tissues in which growth, on account of extreme sub-tonicity, has been brought to a state of standstill. SUMMARY. The modifying influence of tonic condition on response is similar in pulvinated and growing organs. The motile organ of _Mimosa_ in a condition of sub-tonicity, exhibits a _positive_ response, by expansion, increase of turgor, and erection of the leaf. Continuous stimulation converts the abnormal _positive_ to normal _negative_. In sub-tonic growing organs stimulus likewise induces a _positive_ response, by expansion, increase of turgor and acceleration of the rate of growth. Continuous stimulation converts the abnormal acceleration to normal retardation.