Analysis of a velocity field from a G. elegans root. (A) A 5-parameter version of the step-stool function lacking paremeters b1 and q (red) and the 4-parameter Richards function (blue) were fitted to the experimental velocity data (gray dots; n = 3678). (B) Derivatives of the velocity curves (elemental elongation rate profiles). The flatness, F, of the time-course of elemental elongation rate was determined for the step-stool model. (C) Standardized residuals for the step-stool model and (D) for the Richards model. The value of the primary criterion of goodness-of-fit, the sum of squared residuals (SS), is indicated in (C) and (D).