DNA replication during differentiation of cortical cells in Allium roots

In the elongation zone of Allium roots a certain fraction of cortical cells undergo DNA replication during transition from newborn non-proliferative cells to mature cells. The percentage of cortical cells in the mature zone with DNA content 2C (G₀₁) and 4C (G₀₂) is roughly 60% and 40%, respectively. We propose that a replication signal whose concentration decreases the farther you go from the root tip, along the root axis, and which acts on cells after their last partitioning, could account for the observed distribution of mature cortical nuclei with different DNA contents.