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Perennial forage grasses can be viewed as modular organisms. The phytomer is usually considered the basic unit of the grass plant and consists of a leaf blade and sheath, the internode, the node, and the associated axillary bud below the point of sheath attachment. The tiller is a collection of phytomers differentiated from a single apical meristem. The grass plant is a group of tillers that ultimately arise from a single zygote and are of the same genotype. A grass sward is a population of individual plants, often genetically related, but usually of unique genotypes. The developmental morphology of perennial grass swards is dynamic and is a function of the spatial and temporal distribution of growth stages within the population of tillers comprising the sward. Management practices are applied to forages at the sward level. Systems for quantifying developmental morphology of perennial grasses that are to be used for management purposes must be applicable at the sward, or population, level. The phyllochron concept, which is widely used in studies of cereals, has been employed in only a limited number of studies on forage management. Numerical indices have been used successfully to quantify grass morphology, but have some inherent limitations when used for predictive purposes. The principles of population ecology, particularly plant demography, provide a framework for studying the developmental morphology of perennial grass populations and can be used in conjunction with numerical indices to describe tiller populations.