The net assimilation rate (E) of a plant population (the mean rate of increase in total dry weight per unit leaf area, measured over a period of 1 or 2 weeks), represents the excess of the rate of photosynthesis of the leaves (P) over the rate of respiration of the whole plants (R), both expressed per unit leaf area (E=P—R). To determine P, photosynthesis was prevented on some days during the experimental period by shading plants, and the effect on E was measured. The plants were held in a controlled environment to ensure that external conditions affecting photosynthesis were the same on all days and effects on change with age in the rate of photosynthesis were avoided by appropriate distribution of the shading days. With these precautions, E was linearly related to n, when n (the number of days when photosynthesis was permitted in an experimental period of 15 days) ranged from 15 to 9. The regression coefficient b of E on n measures the contribution of 1 day's photosynthesis to E, so the photosynthetic component P of EN,the net assimilation rate when photosynthesis is permitted every day in an experimental period of N days, is given by Nb. The respiratory component, R,is estimated by the difference EN—P, or graphically by extrapolating the regression line to n= 0.

This method showed that young sugar-beet plants had a greater net assimilation rate than barley plants solely because they had a greater rate of photosynthesis of the leaves; the values of the rate of respiration of whole plants for the two species were identical.