Stomatal resistance is most important because gas exchange between leaves and external atmosphere takes place entirely through stomatal pores. Stomatal resistance depends mainly on the size and shape of the stomatal cavity and size of stomatal aperture. Can you tell which pore size, small or large would exert greater resistance? Of course, smaller the pore greater would be the resistance for the outward movement of water vapour. On an average a standard pore measures about 20 μm in length and about 11 μm wide at its widest point when fully open.