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.

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.

Fig. 25. Evening record from 6 to 10 P.M., showing gradual depression of ex­cit­abil­ity.

Record in the morning: Experiment 22.—The ex­cit­abil­ity is, as we have seen, nearly abolished about 8 A.M., after which there is a gradual restoration. This gradual enhancement of ex­cit­abil­ity to a maximum in the course of the forenoon is seen well illustrated in the record below (Fig. 26).