Barley [Hardeum sativum] and perennial ryegrass [Lolium perenne] were grown at various densities in boxes and Italian ryegrass [L. multiflorum] and timothy [Phleum pratense] at various densities in the field. The initial seed weight, tiller number, height and dry weight were determined separately for each plant, and the frequency distributions of these parameters at a number of times was analyzed. The frequency distributions of the characters were not normal at any time. The direction and degree of departure from normality for any characters of a population changed with time. Populations sown at widely different densities often differed only slightly in their departure from normality. Large differences were often observed between populations sown at the same density but in different spatial arrangements. Thinning in perennial ryegrass populations occurred by mortality of plants rather than individual tillers.