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. _Mid-day record: Experiment 20._--The record of daily periodicity previously given shows that the excitability reaches its maximum after 12 noon, and that it remains constant at the maximum value for several hours. This fact is fully borne out in the following record obtained with a different specimen (Fig. 24). The responses were taken here from noon to 3 P.M., once every half-hour. [Illustration: FIG. 24. Mid-day record from noon to 3 P.M. exhibiting uniform excitability. Responses taken once every half-hour.] _Evening record: Experiment 21._--The record given in Fig. 23 shows that the amplitude of response falls continuously after 6 P.M. It might be thought that the diminished amplitude in the first part may be due to the natural nyctitropic fall of the leaf. The range of the pulvinar movement being limited, it is clear that the extent of the responsive fall must become smaller on account of the natural fall of the leaf during the first part of the night. That this is not the whole explanation of the decline of response in the evening will be clear from certain facts which I will presently adduce. It was stated that the leaf of _Mimosa_ exhibits nyctitropic fall from 6 to 9 P.M., after which there is a reverse movement of erection. In certain specimens, however, the erectile movement commenced as early as 6 P.M. It is obvious that in these latter cases diminution of amplitude of response cannot be due to the reduction of the range of movement of the leaf. In Fig. 25 is given a series of records from 6 to 10 P.M. obtained with a leaf in which erectile movement had commenced early in the evening. Though the full range of responsive movement was in this case available, yet the amplitude of successive responses is seen to undergo continuous diminution. [Illustration: FIG. 25. Evening record from 6 to 10 P.M., showing gradual depression of excitability.]