Sachs observed a positive phototropic curvature in the stems of Tropæolum majus in autumn; but this was reversed into negative in summer; similarly in the hypocotyl of Ivy, the positive curvature in autumn is converted into negative curvature in summer.
Certain organs are apparently insensitive to the action of light. Thus no phototropic response is found in the tendril of Passiflora even under the action of strong light. The tendrils of Vitis and Ampelopsis exhibit, according to Wiesner, positive phototropism under feeble, and negative phototropism under strong light.
The anomalies referred to above may be explained by taking into consideration the modifying influence of temperature on the excitability, and the conductivity of the organ.
EFFECT OF TEMPERATURE ON EXCITABILITY.
The excitability of an organ is abolished at a low temperature; it is enhanced by a rise of temperature up to an optimum. The temperature minimum and optimum varies in different tissues. The following table shows the enhancement of excitability of Mimosa at different temperatures, the testing stimulus being the same.
TABLE XXXII—SHOWING VARIATION OF EXCITABILITY OF PULVINUS OF Mimosa AT DIFFERENT TEMPERATURES.
| Temperature. | Amplitude of response. |
| 22°C. | 2 divisions. |
| 27°C. | 16 " |
| 32°C. | 36 " |
Below 20°C. the excitability of the pulvinus of Mimosa is practically abolished. The excitability increases till an optimum temperature is reached, above which it undergoes a decline.
Though rise of temperature enhances excitability up to an optimum, there is an antagonistic reaction induced by it which opposes the excitatory contraction. The physiological reaction of a rise of temperature, within normal range, is expansion and this must oppose the contraction induced by stimulus. Hence the effect of rise of temperature is complex; it enhances the excitability which favours contraction, while tending to oppose this contraction by the induced physiological expansion. As a result of these opposite reactions there will be a critical temperature, below which the contractile effect will relatively be greater than expansion; above the critical point, expansion will be the predominant effect. The critical temperature will obviously be different in different organs. The positive curvature may thus be increased by a slight rise, while it may be neutralised, or even reversed by a greater rise of temperature.
The induced variation of excitability due to change of temperature is not the only factor in modifying tropic curvature, for variation of conductivity also exerts a marked effect.