Acclimation of the photosynthesis of C3 plants to elevated atmospheric CO2 concentrations is ffequently observed. Some hypotheses ffequently proposed to explain this phenomenon are: (7) stomatal closure; (2) inhibition of photosynthesis by starch accumulation, and (5) reduced activity or concentration of ribulose-1,5- bisphosphate carboxylase/oxygenase. These hypotheses are compared with experimental evidence ffom the literature.