Fig. 21. Effect of application of more intense cold. Note sudden depression followed by abolition of excitability, also persistent after-effect.
Effect of high temperature: Experiment 18.—It has been shown that the moto-excitability is enhanced by rising temperature; there is, however, an optimum temperature above which the excitability undergoes a depression. This is seen in the following record (Fig. 22), where the normal response at 32°C. was depressed on raising the temperature to 42°C.; the excitability was, however, gradually restored when the plant was allowed to regain the former temperature.
Fig. 22. Effect of temperature above optimum. Note depression of excitability induced by high temperature, and gradual restoration on return to normal.
I may now briefly recapitulate some of the important results: darkness depresses and light exalts the moto-excitability. Excessive turgor depresses motility. Still more marked is the effect of temperature. Lowering of temperature depresses and finally abolishes the moto-excitability: rise of temperature enhances it up to an optimum temperature, but beyond this point the excitability undergoes depression. The change in excitability induced by the variation of external condition is not immediate; the induced effect, generally speaking, lags behind the inducing cause.
DIURNAL VARIATION OF EXCITABILITY.
I will now give automatic records of responses taken once every hour for twenty-four hours. They prove conclusively the diurnal variation of excitability in Mimosa. After studying in detail the variations characteristic of particular times of the day, I will endeavour to correlate them with the effects brought on by the periodic changes of the environment.
Experiment 19.—As a typical example I will first give a record obtained in the month of February, that is, say, in spring. From this it will not be difficult to follow the variations which take place earlier in winter or later in summer.