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The relationship between stomatal aperture (a) and guard cell pressure (P g ) was measured directly in four different species (Vicia faba, Tradescantia virginiana, Ginkgo biloba and Nephrolepis exaltata) using a special cell pressure probe technique. The effect of epidermal turgor (P ep ) on this relationship was also measured in T. virginiana. The relationship was sigmoidal for V. faba and T. virginiana, but entirely convex for G. biloba and N. exaltata. Epidermal turgor was found to have a pronounced closing effect on stomata of T. virginiana. Maximum aperture with full epidermal turgor (0·92 MPa) was about half that with zero epidermal turgor. Also, with full epidermal turgor stomata of T. virginiana did not begin to open until P g was more than 1·25 MPa. These characteristics were used to develop an expression for a as a function of P g and P ep . Results for the different species are compared and discussed in terms of possible advantages and limitations of water economy.