Results are reported for two experiments in which bean plants were grown in a range of daylengths from i hour to continuous illumination at temperatures of 15° C and 25° C. Marked differences in total leaf area and in areas of individual leaves were found, and these are related in a quadratic fashion with total radiation received per day. With high levels of radiation a decrease in leaf area was noted, and this is not explicable in terms of a simple nutritional hypothesis. There are pronounced effects of total radiation on specific leaf area but the mechanism by which these effects are brought about is not clear. The most important influence of temperature on the expansion of the leaf surface appear to be: firstly, by controlling the rate at which leaves unfold from the stem apex; and secondly, by controlling the partition of dry matter between the leaves and the rest of the plant. No effects of daylength on vegetative development were found.