A review of the literature indicates that the stomates of leaves must be open for smog damage to occur (3,7,8,9,11). However, there is no direct relationship between degree of opening and extent of injury, nor has it been proven that injury is prevented when the stomates are closed. Moreover, most of the earlier work dealt with the reaction product of ozone and gasoline or ozone and an olefin (8,10,16 Because subsequent assays of plant leaf damage from the oxidant were desired, the rate of apparent photosynthesis was expressed on a nine-plant basis rather than on the leaf area or dry weight basis.