Root apices possess a terminal protective root cap and a proximal root apical meristem (Fig. 3.1). The quiescent centre is a group of relatively inactive cells at the very centre and tip of the root apical meristem. The cells of the quiescent centre divide infrequently; their role is obscure, but they maintain initial cells in an undifferentiated state. These cells, together with the root cap initials, are derived from the uppermost cell of the suspensor (hypophysis) in the embryo (Fig. 6.7). Cell division activity occurs in the cells surrounding the quiescent centre.
In Arabidopsis thaliana the initial cells lie in clearly defined regions relative to the quiescent centre, the pericycle and vascular initials proximal to it (on the shoot side), the root cap and epidermis initials distal to it (on the root cap side) and the cortical and endodermal initials radial to it. However, in other species (e.g. Vicia faba) there is an undifferentiated initiating region common to all root tissues. The active region is termed the promeristem.
The junction between the root cap and the root apical meristem is either clearly defined by a distinct cell boundary (termed closed organization, as in Zea mays and Arabidopsis thaliana), or ill-defined (termed open structure, as in Vicia faba: Fig. 3.2), though inter-mediates exist (e.g. in Daucus carota). In open meristems the boundary between the cap and the rest of the root is unstable.