+-----------------------------------+ |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. 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.