creased gradually ; this was secured by means of a poten- tiometer' slide. In my experiment a current having an intensity of Vi micro-ampere was found to be effective. Responses at intervals of 10 min., as we hav3 seen, exhibit marked fatigue. Two responses were recorded on a fast- moving plate, N before, and C after, the application of the current. It will be seen (Fig. 39) how the application of current has, by hastening the recovery, enhanced the amplitude of response and brought about a diminution of fatigue. In connection with this, I may state that the tonic condition is, in general, improved as an after-effect of the passage of current. This is seen in some cases by a slight increase in excitability ; in others, where the res- ponses had been irregular, the previous passage of a cur- rent tends to make the responses more uniform.
ACTION OF LIGHT AND DARKNESS ON EXCITABILITY. In taking continuous records of responses I was struck by the marked change of excitability exhibited by the intact plant under variation of light. Thus the appearance of a cloud was quickly followed by an induced depression, and its disappearance by an equally quick restoration of excit- ability. This may b^ explained on the theory that certain explosive chemical compounds are built up by the photo- synthetic processes in green leaves, and that the intensity of response depends on the presence of these compounds. But the building up of a chemical compound must neces- sarily be a slow process, and it is difficult on the above hypothesis to connect the rapid variation of excitability with the production of a chemical compound, or its ces- sation, concomitant with changes in the incident light.
Experiment 82. — In order to find out whether photo-syn- thesis had any effect on excitability, I placed an intact plant in a dark room and obtained from it a long series of responses under uniform test-stimulus. While this was
