Sample profiles demonstrating the flexibility of the step-stool model. Growth zone models of increasing sigmoidal character (strongly bi-linear, A, to strongly sigmoidal, D), and two asymmetric examples with pronounced right (E) and left (F) skew. From left to right, columns show profiles of velocity, elemental elongation rates (first derivatives of the velocity profiles), second derivatives of the velocity profiles, and time-courses of elemental elongation rate, with time = 0 taken at the position where the rate reaches 50% of its maximal value. Grey shading highlights the period during which the organ element grows at >90% of its maximal elemental elongation rate (parameter F; see text for details). All models: b1 = 0.1, b2 = 1.84, c1 = 0.3, and c2 = 0.8. A: d1 = d2 = 50; B: d1 = d2 = 15; C: d1 = d2 = 9; D: d1 = d2 = 5; E: d1 = 5, d2 = 30; F: d1 = 30, d2 = 5.