boundary layer

The layer of liquid or gas next to a solid surface, which flows more slowly than that further away from the surface. A boundary layer of air exists at a leaf surface, its width decreasing with increasing wind speed. It is one of the factors that controls the rate of diffusion of water vapour from a leaf. Transpiration is thus greater from a leaf margin, where the boundary layer is narrower and offers less resistance, than from the centre of the lamina. See also essential oil.