Marked changes in difference of potential between the base and tip of leaves in intact plants are produced by ultra-violet and infra-red radiations. Little if any such change is produced by visible light. From various experiments outdoors it is concluded that these changes in electromotive force are due chiefly to infra-red radiations of sunlight. Indoors, marked differences of potential may be caused by exposing the leaf to gases drawn from the vicinity of quartz-mercury or other metallic arcs. When a constant electromotive force is impressed across the 2 electrodes, the values of the currents in the circuit are increased by either ultra-violet or infra-red radiation.