I shall here adduce considerations which will show that the apparent anomalies regarding the response of tendrils to light is due to the variation of transverse conductivity of the organ. In a semi-conducting tissue, while the excitatory effect of feeble stimulus remains localised at the proximal side, the effect of stronger stimulus is conducted to the distal side. This explains the positive phototropic curvature of tendrils of _Vitis_ and _Ampelopsis_ under feeble light, and its reversal into negative curvature under intense light. As the conducting power is increased with rise of temperature it is evident that at a certain temperature the tropic effect will be exactly neutralised by transverse conduction. Lowering of temperature, by reducing the transmission of excitation to the distal side, will restore the positive curvature. Enhancement of conduction under rise of temperature will, on the other hand, increase the antagonistic reaction of the distal side and give rise to a negative curvature. I shall in verification of the above, describe experiments which I have carried out on the phototropic response of the tendril of _Passiflora_, supposed to be insensitive to the action of light. _Phototropic response of the tendril of_ Passiflora: _Experiment 145._--The tendril was cooled by keeping it for a long time in a cold chamber, maintained at 15°C. The effect of unilateral light on the cooled specimen was found to be positive; the tendril was next allowed to assume the temperature of the room which was 30°C. The response was now found to have undergone a change into negative. The positive and negative phototropic curvatures of an identical organ at different temperatures is seen in the two records given in figure 145. Neutralisation takes place at an intermediate temperature, and the organ thus appears insensitive to light. SEASONAL VARIATION OF PHOTOTROPIC CURVATURE. [Illustration: FIG. 145.--(_a_) Positive curvature of tendril of _Passiflora_ at 15°C.; (_b_) negative phototropic curvature at 30°C.] Reference has been made of the phototropic curvature of _Tropæolum_ and of Ivy undergoing a change from positive in autumn to negative in summer. The experiment described above shows that rise of temperature, by enhancing transverse conductivity, transforms the positive into negative heliotropic curvature. The reversal of the phototropic curvature of _Tropæolum_ and Ivy, from positive in autumn to negative in summer, finds a probable explanation in the higher temperature condition of the latter season. This inference finds independent support from the fact previously described (p. 100) that while the velocity of conduction of excitation in the petiole of _Mimosa_ is as high as 30 mm. per second in summer, it is reduced to about 4 mm. in late autumn and early winter. ANTAGONISTIC EFFECTS OF LIGHT AND OF RISE OF TEMPERATURE.