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Internal leaf anatomy of 56 species in 21 families from alpine, forest, and desert habitats was examined for relationships between anatomical characteristics that might be linked functionally to photosynthesis. Leaf mesophyll architecture, particularly the amount of cell surface area exposed to intercellular spaces (A(mes)), expressed per unit leaf area (A), or A(mes)/A, has previously been associated with increased photosynthesis per unit leaf area. The numerical value of A(mes)/A is influenced by leaf anatomical characteristics that include leaf thickness; the amount of intercellular air space; and mesophyll cell size, shape, and density. The A(mes)/A values measured using digital image analysis of oblique and transverse sections of leaves ranged from 2 to 34. The most important single variable predicting A(mes)/A was not leaf thickness or cell dimensions but the percent of mesophyll volume occupied by intercellular air space, %IAS. In addition, the extent of contact between individual mesophyll cells appeared to be much greater than previously reported, particularly in the palisade layer and in xeromorphic species, in which up to 95% of a cell’s surface area contacted other cells. This high degree of cell-to-cell contact substantially reduced the total mesophyll cell area in contact with air space. Measured values of A(mes)/A were compared with computed estimates based on geometric conversions that assumed mesophyll cells were shaped as spheres, cylinders, or ellipsoids. These assumptions of uniform geometry led to overestimation of A(mes)/A by up to 20-fold. Differences in mesophyll architecture such as leaf porosity, rather than leaf thickness alone, are important for evaluating the impact of internal cell area or chloroplast area available for CO2 absorption during photosynthesis.