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Rh beyond this point the excitability undergoes depression. The change in excitability induced by the variation of external condition is not immediate; the induced effect, generally speaking, lags behind the inducing cause.

I will now give automatic records of responses taken once every hour for twenty-four hours. They prove conclusively the diurnal variation of excitability in Mimosa. After studying in detail the variations characteristic of particular times of the day, I will endeavour to correlate them with the effects brought on by the periodic changes of the environment.

Experiment 19.—As a typical example I will first give a record obtained in the month of February, that is, say, in spring. From this it will not be difficult to follow the variations which take place earlier in winter or later in summer.

The record given in Fig. 23 was commenced at 5 and continued to the same hour next day. The first thing noticeable is the periodic displacement of the base-line. This is due to the nyctitropic movements of the leaf. It should be remembered that the up-movement of the leaf is represented by down-curve, and vice versâ. After the maximum fall of the leaf, which in this case was attained at 9, there followed a reverse movement: the highest erection, indicative of maximum turgor, was reached at 6 The leaf then fell slowly and reached a middle position at noon. The extent of the nyctitropic movement varies in individual cases; in some it is slight, in others very large. The erectile movement began, as stated before, at about 9 ; in some cases, however, it may occur as early as 6