The root apical meristem produces all the cells that contribute to the growth and development of the root (Figure 33.10A). Some of the daughter cells from the apical (tip) end of the root apical meristem contribute to a root cap, which protects the delicate growing region of the root as it pushes through the soil. The root cap secretes a mucopolysaccharide (slime) that acts as a lubricant. Even so, the cells of the root cap are often damaged or scraped away and must therefore be replaced constantly. The root cap is also the structure that detects the pull of gravity and thus controls the downward growth of roots.

In the middle of the root apical meristem is a quiescent center, in which cell divisions are rare. The quiescent center cells may be in G1 phase of the cell cycle for 10 days and prevent the surrounding meristem cells from differentiating. The daughter cells produced above the quiescent center (that is, away from the root cap) become the three primary meristems.

The apical and primary meristems constitute the zone of cell division, the source of all the cells of the root’s primary tissues. Just above this zone is the zone of cell elongation, where the newly formed cells are elongating and thus pushing the root farther into the soil. Above that zone is the zone of maturation, where the cells are differentiating, taking on specialized forms and functions. These three zones grade imperceptibly into one another; there is no abrupt border separating them.