A cross section taken through the zone of cell maturation shows a series of concentric tissue layers, starting with the epidermis on the outside, followed by the root cortex, and then the endodermis (Fig. 31.16). Recall that the endodermis plays a key role in the selective uptake of nutrients (Chapter 29). The Casparian strip, a waxy band surrounding each endodermal cell, forces all materials that will enter the xylem to cross the plasma membranes of these cells. At the center of the root is a single vascular bundle in which the xylem is arranged as a fluted cylinder, with phloem nestled in each groove. In cross section, the xylem is arranged in a star shape; the number of points of the star differs in different plant species.

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As in stems, the hormone auxin plays a central role in specifying which cells differentiate into vascular tissues. Auxin produced in shoot meristems is transported downward through the mature stem and into the roots by the phloem. At the end of the fully formed phloem, auxin moves by polar transport from cell to cell until it reaches the root apical meristem and the root cap. As auxin moves through the elongation zone, it triggers the formation of procambial cells that subsequently become phloem and xylem.

In this way, the production of auxin by developing leaves and the polar transport of auxin from cell to cell ensures that the vascular system is continuous between roots and leaves. Phloem extends closer to the root tip than do fully lignified xylem conduits. The root apical meristem does not need direct access to the xylem because it can obtain water and nutrients directly from the soil but depends entirely on the phloem for carbohydrates needed for growth.

So far, we have described only the primary growth of roots. However, roots can undergo secondary growth as well. Through secondary growth, roots increase in diameter and add new xylem and phloem. As in stems, the vascular cambium forms at the interface between the xylem and the phloem. Indeed, the vascular cambium must be continuous from the shoot to the roots to ensure that the secondary xylem and phloem provide an unbroken connection between the roots and the leaves. In older roots with extensive secondary growth, a cork cambium is also formed. As in stems, the cork cambium renews the outer layers of cells that protect the living tissues from damage and predation.