The hydrologic importance of transpiration control by stomata

Plant canopies are not passive evaporating surfaces. The regulation of water loss from plants is an important function of leaf stomata, and significant stomatal regulation occurs throughout the range of leaf‐pore diameters. Leaf pores do not ‘interfere’ with one another to any significant extent in the conductance of water vapor or other gases. The diffusive conductance of small pores is more sensitive to diameter changes at the larger apertures, thus permitting CO2 exchange for photosynthesis without dehydration of the leaf mesophyll. In still air the external resistance over broad leaves limits control by stomata, but light drafts usually minimize this effect in field situations. Characteristic diffusion coefficients for leaf types, based on a careful study of leaf microstructure, physiology, and seasonal variation, are a rational basis for cover‐type conversions or biochemical controls to reduce water loss.