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
