Increasing salinity led to substantially higher ratios of mesophyll surface area to leaf area (A(mes)/A) for Phaseolus vulgaris and Gossypium hirsutum and a smaller increase for Atriplex patula, a salt-tolerant species. The increase in internal surface for CO(2) absorption did not lead to higher CO(2) uptake rates, since the CO(2) resistance expressed on the basis of mesophyll cell wall area (r(cell)) increased even more with salinity. The differences among species in the sensitivity of photosynthesis to salinity in part reflect the different A(mes)/A and r(cell) responses.