The constancy of K in the relation of relative light intensity I/I0 to leaf area index F I/I0 = e-KF is examined for a stand presumed to consist of foliage inclined at a constant angle α. When the relative light intensity must be considered over a range of angles of penetration β, K is not constant unless α = 0⚬, when K = 1, if transmission and reflection are neglected. The departure from linearity increases with increasing values of α. The values of K for diffuse and direct light are usually different. Some implications of these findings for prediction of photosynthetic production in plant stands is discussed. The limitations imposed by the theoretical treatment, compared with the structure of actual plant communities, are briefly reviewed.