I may now briefly recapitulate some of the important results: darkness depresses and light exalts the moto-excitability. Excessive turgor depresses motility. Still more marked is the effect of temperature. Lowering of temperature depresses and finally abolishes the moto-excitability: rise of temperature enhances it up to an optimum temperature, but 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._ DIURNAL VARIATION OF EXCITABILITY. 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. [Illustration: FIG. 23. Record for twenty-four hours, exhibiting diurnal variation of excitability (spring specimen). The displacement of base-line is due to nyctitropic movement.] The record given in Fig. 23 was commenced at 5 P.M. 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 P.M., there followed a reverse movement: the highest erection, indicative of maximum turgor, was reached at 6 A.M. 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 P.M.; in some cases, however, it may occur as early as 6 P.M. In following the characteristic variations of response occurring throughout the day, we find that while they are practically uniform between the hours of 5 and 6 P.M., a continuous decline is manifested after setting in of darkness (7 P.M.); the fall of excitability continues even after sunrise (6-30 A.M.), response being practically abolished at 8 A.M. The excitability is then gradually restored in a staircase manner, the maximum being reached after 12 noon. After attaining this, the excitability remains more or less constant till the evening. It will be noticed that the amplitude of response at 5 P.M. on the second day was the same as the corresponding response on the previous day. The results of this and numerous other records taken in spring may be summarized as:--