Fig. 6. (A) Schematic summary of vein-associated temporal expression profiles. Green bars indicate the duration of maximum expression of each marker, ramped termini indicate gradual onset or decline of marker gene expression. (B) Termination of vein formation by differentiating mesophyll (distal, top; proximal, bottom; arrows connect consecutive stages of leaf development). Mesophyll differentiation (green) progressing from distal to proximal interferes with polar acquisition of preprocambial cell fates (light blue) at different stages of vein ontogeny, resulting in the formation of a freely ending vein distally and a connecting vein proximally.