of excitability, is borne out by the fact that the plant is fully sensitive at night.
Effect of Sudden Darkness
In order to demonstrate the variation of excitability induced by sudden diminution of light, I first took a set of three normal responses in diffuse daylight. The plant-chamber was then suddenly darkened by means of an opaque screen. It will be noticed (fig. 44) that the next two responses were nearly abolished; the excitability of the plant was however beginning to be restored after 45 minutes' exposure to darkness. After an hour in darkness the excitability was fully restored, the response here being even larger than in light.
In order to guard against the disturbing effect of variation of light it is advisable to carry out the following experiments in an open veranda, the plant being kept in a chamber with frames of ground glass. In this way the plant is maintained under diffuse light of fairly uniform intensity.
Effect of Absorption of Water
Another peculiarity I noticed in Mimosa was a depression of excitability on rainy days. This effect I was afterwards able to trace to the absorption of water by the pulvinus. The variation of motile excitability by absorption of water is very clearly exhibited in the accompanying record (fig. 45). A pair of normal uniform responses were first taken. A drop of water was then applied on the pulvinus, when the leaf was recovering from the second stimulus. It will be noticed that the period of recovery became very much protracted in consequence of absorption of water. The usual time for complete recovery is about 15 minutes. In the present case it was prolonged to 45 minutes. Testing stimuli were applied at the usual intervals of 15 minutes, the moments of application being represented by thick
