[Illustration: FIG. 147.--After-effect of rise of temperature, persistent negative curvature: (_a_) response one hour after rise of temperature; (_b_) series of negative responses after 4 hours (successive stimuli applied at vertical lines).] SUMMARY. Rise of temperature, within limits, enhances the general excitability of the organ. This has the effect of increasing positive phototropic curvature. But the physiological expansion induced by rise of temperature exerts an antagonistic effect. The transverse conductivity is increased with the rise of temperature; this favours neutralisation and reversal of phototropic curvature. Tendrils of _Passiflora_, supposed to be phototropically insensitive, exhibit positive curvature at low, and negative curvature at a moderately high temperature. The change of phototropic curvature exhibited by _Tropæolum majus_ and Ivy, from positive in autumn to negative in summer, is probably due to the effect of temperature. Higher temperature with enhanced transverse conductivity in summer, may thus convert positive into negative curvature. The physiological effects of rise of temperature and the stimulus of light are antagonistic to each other. Rise of temperature tends to neutralise or reverse the positive phototropic curvature. The after-effect of temperature is often very persistent. XXXVI.--ON PHOTOTROPIC TORSION _By_ SIR J. C. BOSE, _Assisted by_ SURENDRA CHANDRA DAS. In addition to positive or negative curvatures light induces a responsive torsion. With regard to this Jost says:-- "The mechanics of the torsions have not as yet been fully explained. It has long been believed that these torsions were occasioned only by the action of a series of external factors, such as light, gravity, weight of the organ which individually led to curvatures, but in combination induced torsions; but later investigations have shown that torsions might appear when light only was the functional external factor.... If the torsions cannot generally be regarded as due to the combination of two curvatures, we are completely in the dark as to the mechanics of their production."[23] [23] Jost--_Ibid_--p. 465. A leaf when struck laterally by light undergoes a twist, so that the upper surface is placed, more or less, at right angles to the incident rays; as no explanation was available for this movement, the suggestion has been made that the particular reaction is for the advantage of the plant. I shall presently show, that it is possible to reverse this normal torsion and thus make the upper surface of the leaf move away from light. The experiments which I shall presently describe will, it is hoped, throw light on the obscure phenomenon. I shall be able to show: (1) that the torsional response is not dependent on the combination of two curvatures, (2) that it is also independent of the effect of weight,