A new mathematical treatment is given to the concept of relative elemental growth rate, the limiting value at a point of the relative rate of tissue growth. The new point of view requires consideration of a coordinate system in which each point on the root surface is defined by its initial position on the root axis. The resultant technique of plotting growth parameters provides the ability to distinguish between waves of growth propagating toward the root tip away from cells and waves which tend to propagate along with the cells. Thus it is possible to model the plant growth-regulatory process in terms of auxin and other regulators, considering both cell-associated maxima of regulator concentration and maxima which propagate through the tissue.