Fig. 3. Leaves are initiated from a pair of epidermal cells. (A-D) Schematic of full-length leaf sectors that traversed all three Ad-Ab leaf layers and support a two-cell (A), four-cell (B), six-cell (C) and eight-cell (D) mediolaterally extended strip of cells that are recruited from the meristem to make a leaf. (E) Schematic of sectors occupying two of the three leaf layers (i.e. those that are formed when the Ad-Ab axis comprises two cell layers). Sectors are a maximum of a one-sixth leaf width indicating that the Ad-Ab axis is not established until at least six cells have been recruited in the M-L axis. Notably, some sectors traverse the full length of the leaf indicating that Ad-Ab axis formation precedes P-D axis formation. (F) Schematic of sectors occupying just one cell layer (i.e. those that are formed when the Ad-Ab axis comprises three cell layers). No sectors traverse the full length of the leaf showing that the P-D axis is established before divisions in the Ad-Ab axis are completed. (G) Summary of earliest cell division patterns in the leaf as inferred from 1362 sectors. At the six-cell stage, the next cell division can either be in the M-L axis to form an eight-cell strip, or in the Ad-Ab axis to produce 12 founder cells.