EFFECT OF TEMPERATURE ON CONDUCTION.
The conducting power of an organ is greatly enhanced with rise of temperature. Thus in Mimosa the velocity of transmission of excitation is doubled by a rise of temperature through 9°C. (p. 100). An organ which is practically non-conducting at a low temperature will become conducting at a higher temperature.
Thus at a low temperature the organ may be non-conducting, and the excitatory contraction under unilateral stimulus will remain localised at the proximal side; this will give rise to a positive curvature. But under rising temperature, the power of transverse conduction will be increased and the excitation will be conducted to the distal side. The result of this will be a neutralisation or reversal into negative curvature (p. 139). A positive curvature is thus reversed into negative by change of excitability and conductivity, induced by rise of temperature; examples of this will be given presently.
PHOTOTROPIC RESPONSE OF TENDRILS.
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.
Fig. 145.—(a) Positive curvature of tendril of Passiflora at 15°C.; (b) negative phototropic curvature at 30°C.