Changes in the structure of plant populations.
Barley and perennial ryegrass were grown at various densities in boxes and Italian ryegrass and timothy 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 distribution of these parameters at a number of dates were analysed. 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 of individual tillers.