Page:Life Movements in Plants.djvu/98

68 predominant influence of temperature on diurnal variation of excitability.

It has been shown (page 59) that owing to physiological inertia, the change of excitability, generally speaking, lags behind the inducing cause. This fact finds striking illustration in the lag exhibited by the curve of excitability in reference to the temperature curve. The minimum temperature was attained at about 4, but the excitability was not reduced to a minimum till four hours later and again there is a marked fall of temperature after 5 , but the excitability did not become depressed till two hours later.

There is again the factor of variation of light, the effect of which is not so great as that of temperature. The periods of maximum of light and temperature are, however, not coincident.

We may now discuss in greater detail the diurnal variation of excitability in Mimosa, taking the typical case, the record of which is given in Fig. 23. The temperature here is seen to remain almost constant, and at an optimum, from 1 to 5, the condition of light is also favourable. Hence the excitability is found to be constant, and at its maximum between these hours. The temperature begins to fall after 6, and there is, in addition, the depressing action of gathering darkness. Owing to the time-lag, the fall of excitability does not commence immediately at 6, but an hour afterwards, and continues till the next morning. During this period we have the cumulative effect of twelve hours' darkness and the