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.