Anatomy of the Dicotyledonous and Monocotyledonous Root

The internal (transverse-section) structure of a root reflects its two chief jobs: absorbing water and minerals from the soil, and anchoring the plant firmly in place. Working from the outside in, a young dicot root shows the following layers.

The outermost layer is the epiblema (sometimes called the epidermis of the root), a single layer of thin-walled, compactly arranged cells. Unlike the epidermis of a stem or leaf, the epiblema has no cuticle -- a cuticle would block water absorption -- and many of its cells extend outward as unicellular root hairs, which greatly increase the surface area available for absorbing soil water and dissolved minerals.

Beneath the epiblema lies a wide cortex, made up of several layers of thin-walled, loosely arranged parenchyma cells with clear intercellular spaces, used chiefly for the storage of food and for the lateral movement of absorbed water toward the centre of the root. The innermost layer of the cortex is structurally distinct enough to be given its own name: the endodermis, a single, compact layer of barrel-shaped cells whose radial and transverse walls carry a band of suberin (a waterproofing substance) known as the casparian strip. Because the casparian strip blocks water from simply diffusing through the endodermal cell walls, water and dissolved minerals crossing the endodermis are forced to pass through the living cytoplasm of each endodermal cell, letting the cell control exactly what enters the vascular tissue.

Just inside the endodermis lies the pericycle, usually a single layer of thin-walled parenchyma cells. The pericycle is developmentally important: it is the layer from which lateral (branch) roots originate, and in a dicot root it is also the layer that gives rise to part of the vascular cambium when secondary growth begins.

At the very centre lies the vascular cylinder (stele). In a dicot root this typically consists of a limited number of xylem patches -- usually between two and six, so the root is described as diarch to hexarch -- arranged like the spokes of a wheel, radiating out from the centre, with phloem patches occupying the spaces between them (a radial vascular bundle, as introduced in the previous section). The xylem is exarch: its earliest-formed part, the protoxylem (narrow-diameter cells, formed first), lies toward the periphery, while its later-formed part, the metaxylem (wider-diameter cells), lies toward the centre. Because the vascular cylinder in a dicot root is compact, the pith at the very centre is usually very small or entirely absent, its space instead taken up by the innermost, largest metaxylem elements. Thin-walled conjunctive tissue (parenchyma) fills the gaps between the radiating xylem and phloem patches. …