Page:Life Movements in Plants.djvu/102

72 the excitability attained its maximum value after 1 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.

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.