|III Rhythmic |Considerable absorption |Initiation of multiple | | tissue. | of energy, immediate | response in _Desmodium | | | or prior. | gyrans_ previously at | | | | standstill; multiple | | | | response under continuous | | | | action of light in | | | | _Biophytum_. | +--------------------------------------------------------------------+ SUMMARY. There is no line of demarcation between tropic and nastic movements. In a differentially excitable organ the effect of strong unilateral stimulus becomes internally diffused, and causes greater contraction of the more excitable side of the organ. In the absence of transverse conduction, the positive curvature reaches a maximum without neutralisation or reversal. The leaflets of _Erythrina indica_ and of _Clitoria ternatea_ thus fold upwards, the apices of the leaflets pointing towards the sun. Internally diffused excitation under strong light induces greater contraction of the more excitable half of the pulvinule, causing upward folding of _Mimosa_ leaflet, and downward folding of the leaflets of _Biophytum_ and _Averrhoa_. XXXV.--EFFECT OF TEMPERATURE ON PHOTOTROPIC CURVATURE _By_ SIR J. C. BOSE, _Assisted by_ GURUPRASANNA DAS. I shall in this chapter deal with certain anomalies in phototropic curvature, brought about by variation of temperature and by seasonal change; certain organs again are apparently erratic in their phototropic response. SEASONAL CHANGE OF PHOTOTROPIC ACTION. 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.