Resistances to the Diffusion of Gas and Vapor in Leaves

Considerable error may be incurred when the component leaf resistances to gases are calculated from the rates of photosynthesis and transpiration measured for the entire leaf. This is due to the fact that there are two parallel pathways for diffusion — through the upper and the lower leaf surfaces — which in most leaves are asymmetrical. True values for the resistances, and predictions of the results of their modification, have been obtained from separate measurements of the photosynthesis and transpiration via the two leaf surfaces.