The effect of sudden darkness was found to induce a transient depression, followed by revival of excitability. The effect of persistent darkness was to induce a depression.
Exposure to light from darkness caused a transient depression, followed by an enhancement of excitability.
Excessive turgor induced a diminished response.
Lowering of temperature induced a depression of excitability, culminating in an abolition of response. The after-effect of excessive cold was a prolonged depression of excitability.
Excitability was enhanced by rising temperature up to an optimum; above this point a depression was induced.
Owing to physiological inertia the change of excitability induced by variation of external condition lags behind the inducing cause.
The diurnal variation of excitability is primarily due to diurnal variation of temperature. The effect is modified in a minor degree by variation of light.