Primary root growth results from a continuous production of new cells, sustained by cell division within the root apical meristem. Aer several rounds of cell division, root meristematic cells progressively exit the cell cycle, elongate, and enter into dierentiation. ese successive stages confer a longitudinal organization to the root (Figure 12.1). Close to the root tip, the stem cell niche is formed by the quiescent center (QC) and a small group of stem cells also named initials as they give rise to each root cell le. At the tip of the root, stem cell daughters rapidly dierentiate into columella cells containing statoliths that are involved in gravity sensing. More laterally, stem cells divide and give rise to the lateral root cap. Other stem cells give rise to epidermis, cortex, endodermis, pericycle, and stele cells. e number of cells of each type varies by plant species, but the radial organization into concentric circles is conserved in all land plants. In the model plant Arabidopsis, only one layer of endodermal and cortical cells is present, and the stele includes two opposite poles of protophloem and two poles of protoxylem. e radial structure of the primary root is inherited from embryogenesis; cell specication and patterning are maintained postembryonically, thanks to the stem cell niche and the involvement of hormonal and peptidic regulators (Willemsen and Scheres 2004; De Smet et al. 2008; Matsuzaki et al. 2010; see also Chapter 3).