MAXIMUM POSITIVE CURVATURE UNDER CONTINUED ACTION OF LIGHT. The positive curvature is, as we have seen, due to the contraction of the proximal side and expansion of the distal side. The curvature will increase with growing contraction of the proximal side; a maximum curvature is however reached since: (1) the contraction of the cells must have a limit, (2) the bending organ offers increasing resistance to curvature, and (3) the induced curvature tends to place the organ parallel to the direction of light when the tropic effect is reduced to a minimum. The pulvinus of _Erythrina_ exemplifies the type of reaction in which the positive curvature reaches a maximum, (see below Fig. 132) beyond which there is no further change. This is due to absence of transverse conductivity in the organ. The modifying effect of transverse conductivity on response will be dealt with in the next chapter. SUMMARY. The positive phototropic curvature is brought about by the joint effects of the directly stimulated proximal, and indirectly stimulated distal side. The phototropically curved organ undergoes recovery after brief stimulation. The recovery after moderate stimulation is hastened by the previously stimulated side exhibiting an acceleration of the rate of growth above the normal. The after-effects of photic and mechanical stimulation are similar. The latent period of photic reaction is shortened with the increasing intensity of light. The seedling of wheat responds to a flash of light from an electric spark, the duration of which is about a hundred thousandth part of a second. Tissues in which the power of transverse conduction is negligible, the positive phototropic curvature under continued action of light attains a maximum without subsequent neutralisation or reversal. XXX.--DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM _By_ SIR J. C. BOSE, _Assisted by_ GURUPRASANNA DAS. I have explained how under the action of unilateral light the positive curvature attains a maximum. There are, however, cases where under the continued action of strong light the tropic movement undergoes a reversal. Thus to quote Jost: "Each organism may be found in one of the three different conditions determined by the light intensity, _viz._ (1) a condition of positive heliotropism, (2) a condition of indifference, (3) a condition of negative heliotropism"[12]. No explanation has however been offered as to why the same organ should exhibit at different times, a positive, a neutral, and a negative irritability. These changing effects exhibited by an identical organ is thus incompatible with the theory of specific sensibility, assumed in explanation of characteristic differences in phototropic response. [12] Jost--_Ibid_--p. 462.