Fig. 8. Model for the lineage-dependent differentiation of the extra cell layers produced by aberrant periclinal divisions. In wild-type leaf primordia, an acropetal gradient of cell division does not overlap with the basipetal cell differentiation gradient, allowing cells to perceive positional information for proper differentiation. In the Xcl1 mutant, the gradient of cell division competence is shifted up (possibly through the overproduction of Xcl1 gene product), allowing cell divisions to occur after differentiation signals have been perceived.