On the basis of experimental results published during the last 25 years, but more particularly during the last 5 years and including some results presented here, the hypothesis is proposed that an important portion of the water supply from major veins in leaves travels within the epidermal tissue to sites of evaporation close to the stomatal pores. These evaporation sites are inner epidermal walls especially subsidiary and guard cell walls because these are closest to air spaces with the highest water vapour deficits. Less water than is traditionally supposed evaporates from mesophyll cell walls. Low osmotic potentials of guard cells (large negative) are not required in building up high turgor pressures. However, they are required in competing for water against the process of evaporation which causes low matric potentials to develop in subsidiary and guard cell walls so that guard colls can maintain the comparatively low turgor pressures which have been shown to operate the stomatal apparatus. Traditional views about leaf water relations and methods of estimating mesophyll resistances for carbon dioxide diffusion into leaves must be modified.

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