Sample calculations of the water-loss rate and the temperature of a leaf have been made in terms of three climatic factors (i. e. net radiation intensity, ambient temperature and ambient vapour pressure) and two diffusion resistances, respectively within and outside the leaf tissue. The results indicate the variability of the ratio of the respective water losses from a leaf and a water surface. Increasing the external diffusion resistance, e. g. by lowering the wind speed, is shown to lead to either an increase or a decrease of transpiration, depending on atmospheric humidity. The cooling of a leaf resulting from transpiration is shown to increase as the ambient air becomes more dry and calm.