To determine on a quantitative and mathematical basis the effects of seasonal changes in the levels of daylight and temperature on vegetative growth and development in two years pot experiments in the open were carried out at successive weekly intervals between May and September. So as to minimize errors arising from ontogenetic drifts the procedure adopted was to sow at intervals of a few days throughout the season batches of pots with seed of Helianthus annuus and to select pots containing plants of a standard morphological status for the start of each weekly experiment. At the beginning and end of the week half the pots were harvested, the plants divided into root, stem, and leaf, the leaf area estimated, and the dry weights determined. The diurnal changes in air temperature were continuously recorded while the amount of daylight, excluding infra-red and ultraviolet radiation, was measured with a specially constructed integrating recorder. From the biological data for each week twelve variables were calculated, namely the relative growth rates of both the whole plant and the individual parts, the proportion by dry weight of the individual parts (root-, stem-, and leaf-weight ratios), the ratio of leaf area to total plant weight (leaf-area ratio), the rate of leaf expansion, the ratio of leaf area to leaf weight, and the net assimilation rate on the criteria of leaf area and weight. The main independent variables considered were the mean weekly temperature, the mean daily maximum minus the mean nightly minimum temperature, the total amount of light per week, and the time of year when the individual experiment was undertaken. Multiple regression analyses showed that (i) save for the stem-weight ratio the data for the two years could be pooled, (ii) the fluctuation in diurnal temperature was of little account, (iii) transformation of the light data to either logarithms or square roots did not improve the fit and (iv) for some of the dependent variables, e.g. leaf-area ratio, the ‘time of year’ effect was significant but could be eliminated if the equation was modified to predict the value at the end of the week, given the initial value and the light and temperature data. The final series of multiple regressions revealed that (i) the leaf-weight ratio is not controlled by either the amount of light or mean temperature, (ii) the relative growth rate of the root and the root-weight ratio are positively linked only with temperature, (iii) the rate of leaf growth either in area or weight together with the net assimilation rate (area basis) are positively dependent on light alone, (iv) the net assimilation rate (weight basis) and the relative growth rates of the whole plant and the stem are directly and positively correlated with both temperature and light, and (v) the leaf-area ratio, the ratio of leaf area to leaf weight and the stem-weight ratio are depressed by increasing light but augmented by rising temperature. In the individual regressions for net assimilation rate (area and weight), the relative growth rates of the whole plant, stem and leaf weight, and the ratios of stem weight and leaf area to leaf weight the percentage variation accounted for ranged from 47 to as high as 91 per cent. The implication of these findings in relation to experiments in controlled environmental chambers are discussed.