Abstract
CO(2) exchange between air and leaf is conceived as a current along a resistor with a respiratory current, unaffected by CO(2) concentration, entering the resistor. The conclusion follows that a plant placed in an atmosphere free of CO(2) will increase the CO(2) concentration to the compensation concentration along a curve determined by the resistor and the volume of the atmosphere. This was verified. Also a photosynthesis rate calculated from the parameters of the observed curve agreed with an independent observation of photosynthesis in CO(2)-free air. The decrease in net photosynthesis caused by respiration is, according to the model, the CO(2) compensation concentration divided by the concentration in the atmosphere.