Fig. 29. Diurnal variation of excitability exhibited by summer specimen.
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.), 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 after 8 A.M.; the sensitiveness is then gradually enhanced in a staircase manner.
TABLE III—SHOWING THE RELATION BETWEEN HOURS OF THE DAY, TEMPERATURE, AND EXCITABILITY. (SUMMER SPECIMEN.)
| Hours of day. | Temperature. | Amplitude of Response. | Hours of day. | Temperature. | Amplitude of Response. | ||||||
| 1 | p.m. | 38°.0 | C. | 22.0 | mm | 1 | a.m. | 26°.0 | C. | 21.5 | mm. |
| 2 | " | 38°.0 | " | 23.0 | " | 2 | " | 26°.0 | " | 20.0 | " |
| 3 | " | 38°.0 | " | 24.5 | " | 3 | " | 25.5° | " | 18.5 | " |
| 4 | " | 37°.0 | " | 28.0 | " | 4 | " | 25.5° | " | 17.0 | " |
| 5 | " | 35.5° | " | 29.0 | " | 5 | " | 25.5° | " | 16.0 | " |
| 6 | " | 33°.0 | " | 27.0 | " | 6 | " | 26°.0 | " | 15.0 | " |
| 7 | " | 31°.0 | " | 26.0 | " | 7 | " | 27°.0 | " | 14.0 | " |
| 8 | " | 30°.0 | " | 26.0 | " | 8 | " | 29°.0 | " | 13.0 | " |
| 9 | " | 29°.0 | " | 25.0 | " | 9 | " | 30.5° | " | 11.0 | " |
| 10 | " | 27°.0 | " | 24.5 | " | 10 | " | 33°.0 | " | 16.0 | " |
| 11 | " | 27°.0 | " | 24.0 | " | 11 | " | 35°.0 | " | 17.0 | " |
| 12 | " | 26.5° | " | 22.5 | " | 12 | " | 37°.0 | " | 21.0 | " |
SUMMARY.
The moto-excitability of Mimosa was gauged every hour of the day and night, by the amplitude of the response to a testing stimulus. This is effected by means of automatic devices which excite the plant periodically by an absolutely constant stimulus, and record the corresponding mechanical response.
From the record thus obtained, it was found that the excitability of the plant is not the same throughout the day, but undergoes a variation characteristically different at different times of the day. In a typical case in spring the excitability attained its maximum value after 1 P.M. and remained constant for several hours. There was then a continuous fall of excitability, the minimum being reached at about eight in the morning. The plant at this time was practically insensitive. The moto-excitability was then gradually enhanced in a staircase manner till it again reached a maximum next afternoon.