Figure 3: Synthetic model for leaf expansion and vein development.
From: Developmentally based scaling of leaf venation architecture explains global ecological patterns
This model was developed from data for Arabidopsis leaves21,22,23,24,25,26,27,28 and 27 other dicotyledonous species. (a) The primordium, formed by apical and marginal meristems, undergoes a 'slow' limited expansion phase, principally because of cell proliferation, and a 'rapid' dramatic expansion phase, mainly because of cell expansion, though cell divisions continue. (b) Vein orders form in overlapping sequence, beginning with vascular identity gene expression and signal transduction in ground meristem, followed by the differentiation of procambium, and finally vein maturation. The 1° and 2° veins are formed during the slow phase, the 3° veins next, and the minor veins principally during the rapid phase. The procambial strands initiated for each vein order are limited by the need to maintain a critical cell number or distance between new strands and the duration that the ground meristem can perceive vascular-forming signals. (c) Vein densities of each order peak as procambium forms, then decline as leaf expansion pushes veins apart, but the minor vein density stabilises as vein initiation is maintained during leaf expansion. (d) The 1° and 2° veins have prolonged diameter growth, whereas 3° veins and minor veins rapidly attain maximum diameter. Although a few additional 2° veins form later in development, this affects only a minority of the mature vein system.
