The parameter , calculated on the basis of incident radiation, was found equal to 2.9 pg CO2 per pmol photon (Table 1), or 0.066 mol CO2 per mol photon. This parameter is analogous to the quantum yield (although based on incident rather than absorbed radiation), for which Ehleringer and Bjijrkman (1977) gave a nearly constant average value for C3 species of 0.073 mol CO2 per mol photon. If we assume that Begonia leaf absorptance is 0.85, the quantum yield for Begonia would be equal to 0.066 mol CO, per mol photon i 0.85, i.e. 0.078 mol CO2 per mol photon, which is very near the above-mentioned average value for C3 plants.