Vertical growth of young cress roots (Lepidium sativum L.) which have not yet attained their steady state is analyzed in terms of the relative elemental rate of elongation (RELEL). To this end, an empirical model function is suggested which describes, by means of six parameters, the movement of marker particles on the root surface. These parameters are interpreted as morphological and physiological quantities. Their determination by independent measurements is in good agreement with that obtained by fitting the model function to the experimental data. The RELEL is then evaluated analytically, so that numerical smoothing and differentiating algorithms with all their problems are avoided. The result demonstrates that-apart from the root cap-nearly the whole root is elongating, including the root hair region and part of the basal portion beyond it. This explains the basal curvature component that is observed during the graviresponse of cress roots.