The influences of seasonal changes in light radiation and temperature on the vegetative growth of Helianthus annuus and Vicia faba have been investigated in the east of Scotland by pot experiments, carried out in the open at weekly intervals between June and September in 1956 and May and October in 1957. To minimize the effects of ontogenetic drift pots containing plants of a similar morphological status were selected from batches sown every few days. At the beginning and end of each experiment replicated and paired pots were harvested and the dry weights of the leaves, stems, and roots together with the leaf areas determined. From these data weekly values for net assimilation rate, leaf-area ratio (ratio of leaf area to plant weight), and relative growth rate were calculated. Simultaneously, records were kept of the diurnal changes in air temperature and of light energy by means of an integrating photometer. Multiple regressions linking light and temperature with net assimilation rate, leaf-area ratio, and relative growth rate were calculated separately for each year. A significant ‘time-of-season’ trend was largely eliminated by including an additional variable, the initial leaf-area ratio. In the individual regressions the variance accounted for was very high, ranging from 75 to 97 per cent. The results demonstrated that for both species the net assimilation rate and relative growth rate were positively dependent on light and temperature. The leaf-area ratio of both species was negatively affected by light, but only for V. faba was there a positive relationship between the leaf-area ratio and temperature. H. annuus grew faster than V. faba during the major part of the season, largely because of its higher leaf-area ratio. The results are compared with prior investigations in England and elsewhere.