The root apical meristem is the region near a growing root tip that maintains dividing cells and supplies descendants for root development. It lies behind the root cap and differs from the root elongation zone, where much of the increase in cell length occurs. Producing new cells and enlarging existing cells are related but distinct contributions to growth.[ref]
Cell position can help determine what those descendants become. In a 1995 experiment, selected Arabidopsis thaliana root-meristem cells were removed by laser ablation. The resulting patterns supported positional control of cell fate despite the normally regular relationships among descendants. The authors inferred directional signals from more mature cells toward initial cells.[ref]
Cell fate in the Arabidopsis root meristem determined by directional signalling by C. van den Berg and others. Abstract: ablation, clonal relationships and directional signaling
The experimental contrast is illuminating. A strict lineage-only model predicts identity from a cell's ancestry; a positional model also depends on its neighbors and location. Altering the local arrangement can distinguish these explanations more directly than merely recording the regular pattern of an undisturbed root. The results concern the tested developmental system, rather than making every plant cell equally capable of every fate.
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