Page:Life Movements in Plants.djvu/100

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.
70
LIFE MOVEMENTS IN PLANTS

An inspection of the record given in Fig. 29 shows that the amplitude of response was enhanced after 4 p.m. The temperature up to that time was unusually high (38°C.),


Fig. 29. Diurnal variation of excitability exhibited by summer specimen.

and there was in consequence a depression of excitability. After that hour there was a mitigation of heat, the temperature returning towards the optimum. Hence we find that the maximum excitability was attained between the hours 4 and 6 p.m. The minimum temperature at night was higher in the present case than that of the experiment carried out in February; in the former the minimum was 25.5°C., while in the latter it was 19.5°C. On account of this difference the night record in summer shows a fall of excitability which is far more gradual than that obtained in spring. The excitability is here not totally abolished in the morning, but reaches a minimum