BLACK1 showed how in genetically uniform stands of subterranean clover, small differences in seedling size could, owing to interplant competition, lead to progressively greater differences among the plants as growth proceeded. In contrast, observed stands of two F1 maize hybrids which were disappointingly irregular in the early stages of growth became progressively more uniform in appearance. At the flowering stage the plants were strikingly even to the eye, and experimental errors were low in samples taken at maturity to estimate dry weight of tops and grain (coefficients of variation, 7–8 per cent).