In three experiments the effects of light intensity and nutrient supply (nitrate or phosphate) and their combined effects on the growth and morphogenesis of two shade-tolerant plant species and a non-tolerant species were studied. Nutrient supply was limited by placing the plants on a standard nutrient solution for a limited period each day and placing them on a nitrogen-free or phosphate-free solution for the rest of the day. The effects of light intensity and nitrate supply on growth and morphogenesis showed a marked interaction: low nitrate supply caused a much greater decrease in the relative growth rate under high light intensity, because of much larger changes in the dry matter ditribution; the net assimilation rate was only slightly affected by nitrate supply. The effects of light intensity and phosphate supply on the dry matter distribution and the net assimilation rate both showed interaction, but the effects on the relative growth rate were independent. Low phosphate supply caused greater changes in the dry matter distribution under high light intensity and a greater decrease in the net assimilation rate under low light intensity; the relative growth rate decreased to the same extent under both high and low light intensities. The experimental data were compared with the balanced quantitative model for root/shoot ratios proposed by Thornley (1972). The results were very satisfactory, but it was concluded that the model must be used in its exact form and that the use of approximations cannot be allowed.