Fig. 1. Vascular development in the Arabidopsis first leaf. (A-C) Lateral view. (D-K) Abaxial view. Colors in A-J: procambium, blue, xylem, brown. Colors in K illustrate our application of the classification of vein orders by Hickey (Hickey 1973): 1st, yellow; 2nd, red; 3rd, green; 4th, black; 5th, purple. Veins of the 3rd and higher order can be formally classified as either branching from a next lower-order vein or connecting only lower-order veins. Only examples of these veins are shown in H-K. Numbering of 2nd order vein loops and of the enclosed intercostal areas (IAs) from distal to proximal. lp, leaf primordium, SAM, shoot apical meristem. DAG, days after germination.