Although it is generally accepted that stomatal movement is the primary mechanism by which the plant exercises control over transpiration, two non-stomatal mechanisms have also been proposed. One of these envisages the development of a high resistance to vapour movement between the actual evaporative surfaces in the internal mesophyll cell walls and the natural cell wall surfaces, and has been referred to as "incipient drying". The other envisages a substantial depression of the vapour pressure at the evaporative surfaces because of solute accumulation or steep local gradients of water potential.
In the present paper evidence is presented which suggests that neither of these mechanisms is likely to occur under conditions of normal water supply, that is at water contents high enough to prevent permanent wilting. "Incipient drying" appears improbable because of the high permeability of the water pathway through the walls of the leaf cells and also because of the small void sizes in the interfibrillar spaces of the walls themselves. Significant vapour pressure depression also seems improbable, partly because the relatively high rates of water exchange between cells tend to prevent the development of steep local gradients of leaf water potential, and partly because excessive solute accumulation at the evaporative surfaces is prevented by back diffusion towards the xylem terminals.
It is therefore concluded that non-stomatal mechanisms are unlikely to exercise significant control over transpiration under normal conditions. Only under conditions of extremely severe desiccation is direct control by such mechanisms possible, and then there is also likely to be complete stomatal closure.