The effect of light on the transpiration of leaves

Abstract

The method employed is essentially that described in my paper “On a Method of Studying Transpiration,” where it was applied to the investigation of the relation between the relative humidity of the air and the loss of water by leaves. The stomata of the plants used were closed by vaseline or cocoa-fat rubbed in, and the leaves were then incised to allow of transpiration. No attempt was made to subject the plants to light of known intensity. My object was to compare the transpiration occurring in a dark room with that in a north light at a laboratory window. The rates of transpiration were estimated either by weighing or by means of a potometer, and the general plan was to subject the plant to alternate light and dark periods of something like an hour. The psychrometric condition of the laboratory air and that of the dark room was estimated by the wet and dry bulb thermometer, and the transpiration rates corrected for any differences, in the manner described in the paper above referred to. The first experiment was made by a plan which has some merits, but was afterwards replaced by the simpler method of moving the apparatus from the window to dark room and back again to the light.