The primary objectives have been to evaluate for different climatic conditions the role played by solar radiation in determining the accumulation of dry matter by terrestrial species and the manner in which the resulting photosynthetic products are utilized for the physiological and metabolic processes concerned with growth and development. The interacting effects of different levels of radiation and temperature on the components of growth of whole plants have been examined for a range of species from both tropical and temperate environments. These studies have been extended to include the interrelationships of density and the leaf area index on the productivity of populations of selected species. In some of these investigations the concepts of growth analysis have been combined with techniques which now permit the CO<latex>$_2$</latex> fluxes within the canopy to be continuously recorded over long periods and so allow the diurnal changes in the levels of photosynthesis and respiration in the field to be directly assessed. This approach, coupled with others where similar measurements are made in a closed system, has provided much new information on the balance between gross and net photosynthetic activity. Differences in photosynthetic potential between species have been further interpreted in terms of (i) the pattern of light interception by canopies of differing structure, (ii) the component processes of photosynthesis at varying stages of development and (iii) the factors which are involved in the distribution and use of assimilates within the plant or population.