The transpiration of cut leaves (bean, tomato, Hyoscyamus ) in potometers was studied as affected by light intensity, leaf temperature and air humidity, with special attention to stomatal light response. Fick's diffusion law could be applied to evaporation of water and to transpiration of leaves. Evaporation was found to be proportional to Vα(V = wind velocity),αbeing 0.76 in turbulent air. Transpiration rate increased rectilinearly with light intensity below 5 x 10 4erg sec. -1cm -2, mainly through increase in leaf temperature by radiation with incandescent light, containing much infrared radiation; with fluorescent tubes, sodium lamps, or high- pressure mercury vapour lamps, transpiration increase was mainly due to increase in stomatal opening. Stomatal resistance to diffusion could be exactly determined by measurements on transpiration, leaf temperature and air humidity. Transpiration increases at higher light intensities (saturation) were entirely due to increase in leaf temperature.Regulation of transpiration by small changes in stomatal resistance to diffusion was most pronounced at small stomatal apertures. In normal air, the stomatal response to light depended on photosynthesis of the guard cell chloroplasts (use of CO 2 ) and the response of the guard cells to CO 2 (stomatal feed back, causing fluctuations in the degree of stomatal opening).
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