The growth, root fraction and nitrogen content of Dactylis glomerata L. grown hydroponically in constant environmental conditions, were measured in the vegetative phase. The variation of these parameters denotes a progressive change in the chemical composition of the plant, which can be attributed to changing proportions of structural material, which is immediately available for growth, and storage material. On the basis of a kinetic model of plant growth, a definition of the two components was proposaed, which led us to derive a relationship between relative growth rate and nitrogen content. This relationship was found to be compatible with experimental data.