1. Extensive measurements have been obtained (a) relating photosynthesis and light intensity for a large range of CO2 concentrations and (b) relating photosynthesis and CO2 at different light intensities. From these families of curves, the limiting factor relationship can be secured for any value of the photosynthesis rate.
2. In terms of previous work an equation has been derived for describing these relations between the intensity and CO2 concentration necessary to produce a definite amount of photosynthesis. This equation furnishes an exact description for all the data, except those for low rates of photosynthesis where a slightly different equation is required. The nature of the two equations suggests that a simple first order reaction determines the velocity of the light process at low photosynthesis rates, but that at high rates the mechanism is complicated by another factor.