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:—

1. The maximum ex­cit­abil­ity of Mimosa is attained between 1 and 3 P.M., and remains constant for several hours. In connection with the constancy of response at this period, it should be remembered that when the response is at its maximum a slight increase of ex­cit­abil­ity cannot further enhance the amplitude of response.

2. The ex­cit­abil­ity, generally speaking, undergoes a continuous decline from evening to morning, the response being practically abolished at or about 8 A.M.

3. From 8 A.M. to 12 noon, the ex­cit­abil­ity is gradually enhanced in a staircase manner, till the maximum ex­cit­abil­ity is reached after 1 P.M.

I have obtained numerous records in support of these conclusions, some of which are reproduced in the following figures. In these cases responses to uniform stimuli at intervals of half an hour were taken at different parts of the day, the recorder employed being of the Resonant type.