Previous investigations in southern England on twenty-two herbaceous species have demonstrated that for widely spaced plants the diurnal solar radiation limits the net assimilation rate of all species and restricts the relative growth rate of many. In examining how far these limitations apply to other environments it is now shown that in the subtropics and tropics the levels of net assimilation rate and relative growth rate can greatly exceed those so far recorded for cool temperate regions, and these differences are attributed to the higher insolation and temperatures.From a variety of evidence it is concluded that as the distance between plants is reduced 8O the net assimilation rate is progressively diminished even in regions of high insolation through the enhanced mutual shading. In consequence levels of light which may be supra-optimal for relatively isolated individuals may yet limit the dry-matter production of a dense population. There is an optimal ratio of leaf area to ground surface (leaf-area index) for the maximal exploitation of the incoming radiation in carbon fixation by the population and this optimum will vary with the species and the light intensity. Where other environmental factors are favourable, light may limit dry-matter production everywhere.On an annual basis dry-matter production will be dependent on two components—the length of the ‘growing season’ and the period over which the leaf-area index remains optimal. In the tropics the highest annual rate of production so far recorded is 7⁸ tonnes/hect. produced by Saccharum officinarumand in north-east Europe 23.5 tonnes by Fagus sylvatica. Over short periods the rate of dry-matter production can attain 3⁸g./m.²/day and the utilization of solar energy can be as high as 4.2 per cent., or 9.5 per cent, for the range 4, 000–7, 000 A.Although information on the productivity of natural communities is still ex-ceedingly scanty, an attempt has been made to interpret the general pattern in terms of the length of the growing season, the level of solar radiation, the magni-tude of the leaf-area index of the whole community, and the period over which the leaf canopy remains green. It is postulated that in any region the vegetation reaches a dynamic equilibrium when there is the maximum exploitation of the incoming radiation to produce the greatest production of dry matter.