The increase in the leaf area ratio during early growth and the later decrease were primarily caused by an initial increase in the growth of leaves relative to stems and roots and a later rapid increase in stem growth associated with flowering. Changes in leaf thickness with time had little effect on the leaf area ratio.Increasing temperature from 10 to 25 °C increased the leaf area ratio because the growth of leaves was promoted to a greater extent than the growth of stems and roots, and because leaves were thinner. A further increase in temperature to 30 °C lowered the leaf area ratio because of an increased proportion of stem and thicker leaves.Increasing light intensity from 200 to 2500 ft-c decreased the leaf area ratio because of an increase in the relative proportion of stem and root and increased leaf thickness.These effects of the environment were mostly independent of changes in the rate of development of the plant.