Primary Root Structure

The seedling radicle ultimately becomes the primary root (tap root), which frequently develops side branches (lateral roots). In monocots the seedling radicle commonly dies at an early stage; the stem-borne (adventitious) roots of the mature plant originate from differentiated cells (Fig. 3.4). Adventitious roots can be branched or unbranched. Although roots can originate from various organs, their basic primary structure retains a characteristic root groundplan that is different from that of the stem. Each root possesses clearly-defined concentric tissue regions: dermal tissue (epidermis), ground tissue (cortex, including the endodermis) and central vascular tissue surrounded by a pericycle (Fig. 3.3).

Root Apex

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.