Primary Growth of a Root

The light micrograph and diagram below take us into the tip of an onion root. Mitosis is concentated in the zone of cell division, where the apical meristem and its immediate growth products, the three primary meristems, are located. The apical meristem also maintains the root cap by generating new cells that replace those that are sloughed off. Most lengthening of the root occurs in the zone of elongation. Cells become functionally mature in the zone of differentiation. The zones grade into one another without sharp boundaries.