Root apex
During the later stages of development of embryo, the cells at the root pole become arranged in a pattern characteristic of the species. This group of cells comprises the apical meristem of the primary root. The cells of this region are all relatively undifferentiated and meristematic, densely protoplasmic and with large nuclei and they all undergo active division. The tissues of the mature root are eventually derived from a number of these cells of the apical meristem, which are termed initials. In contrast to the apical meristem of the shoot, that of the root produces cells not only towards the axis but also away from it, for it initiates the root cap and because of the presence of root cap the root meristem is not terminal but sub terminal in its position, in the sense that it is located beneath the root cap. The root apex also differs from the shoot meristem in that it forms not lateral appendages comparable to the leaves, and no branches. The root branches are usually initiated beyond the region of most active growth and they arise endogenously. It also produces no nodes and internodes, and therefore, the root grows more uniformly in length than the shoot, in which the internodes elongate much more than the nodes.
In the angiosperms there are three, rarely four, group of initials. In the dicotyledons the distal group forms the cap and the dermatogen; the median group, the periblem; the innermost, the plerome. The most characteristics is the common origin of cap and dermatogen. In monocotyledons, there are three groups of initials which form four zones, but the outermost, independently, form the cap, and that next beneath, the dermatogen and periblem. The most characteristic of this type is that the origin and structure of cap is independent. Moreover the two zones that are formed by one group of initials are different from those similarly formed in the dicotyledons.
The quiescent Centre
In the apical meristem of root of plants with a regular arrangement of cells in the apical meristem, it is possible to conclude from the study of cell lineages that there is a central region of cells which divide rarely or not at all. These in active or passive cells constitute the Quiescent Centre. The cells on the periphery of this hemispherical or cup shaped region are meristematic and may be regarded as the constituents of the pro meristem. The Quiescent Centre develops during the ontogeny of the root. A Quiescent Centre is not found in the roots with a single apical cell.
The physiological and cytological properties of the cells in the Quiescent Centre have now been studied in a number of species. The cells in this region have a lower concentration of DNA, RNA and protein than any other cells in the root apex. The cells of Quiescent Centre also have fewer mitochondria, little endoplasmic reticulum, and the smallest dictyosomes, nuclei and nucleoli. They are less sensitive to radiation damage than other cells of the meristem. The function of the Quiescent Centre may be to provide a reserve block of diploid cells within the root. The Quiescent Centre may be site of hormone synthesis.