The organization of tissues in the primary root is simpler than in the primary stem because no leaves are produced on the roots and, consequently, there is no need to connect the vascular system laterally to offshoots. The primary body, produced by the three primary meristems, consists of a central cylinder of vascular tissue, the stele, surrounded by large storage parenchyma cells—the cortex—on the outside of which lies a protective layer of cells—the epidermis.
The cortex, composed primarily of parenchyma cells, is the largest part of the primary root, but in most dicots (eudicots) and in gymnosperms that undergo extensive secondary growth, it is soon crushed, and its storage function assumed by other tissues. Three layers of cortex are recognized: the hypodermis (also called exodermis), the endodermis and, between them, the storage parenchyma. The outer and inner layers of the cortex, the hypodermis and endodermis, are cylinders of tightly packed cells with heavily suberized walls and no intercellular spaces. (Suberin is the fatty substance that gives cork its distinctive attributes.) In contrast, the storage parenchyma cells are thin-walled and loosely packed with many intercellular spaces among them.
Hypodermis (exodermis). Just under the epidermis forming the outermost layer of the cortex is a layer one or two cells in width called the hypodermis. Since its cell walls are heavily suberized and impermeable to water its apparent function is to keep the water and nutrients (which are absorbed in the root zone further down the root) from leaking out through the cortex. The hypodermis is especially well developed in plants of arid regions and in those with shallow root systems. It also deters the entrance of soil microorganisms.
Endodermis. The innermost layer of the cortex is the endodermis, which is readily identifiable by the presence of Casparian strips, bands of suberin present on transverse and radial walls of its cells—the walls perpendicular to the surface of the root. The endodermis regulates the passage of water and dissolved substances by forcing them to move through living plasma membranes and plasmodesmata and not simply diffuse through the porous cell walls. The absorption and translocation of materials is thus selective; not everything in the surrounding soil gets through and into the plant body. An endodermis almost always is present in roots and generally never in stems.
Storage parenchyma. The bulk of the cortex consists of thin-walled, living parenchyma cells, which store starch and other substances. The cells expand or shrink as materials move in and out of their protoplasts. The large volume of air present in the intercellular spaces of this tissue provides important aeration for roots.
The stele includes all of the tissues inside of the cortex: the pericycle, the vascular tissues—xylem and phloem—and, in some plants, a pith. Most dicot (eudicot) roots have a solid core of xylem in their center whereas most monocots have a pith composed of parenchyma.
Pericycle. The pericycle is a cylinder of parenchyma, one or at most a few cells in width, which lies in the stele immediately inside the endodermis. The cells retain their ability to divide throughout their lives, and localized divisions in the pericycle give rise to lateral (branch) roots. When secondary growth occurs in roots, the vascular cambium and usually the first cork cambium originate in the pericycle. Other cell divisions in the pericycle produce additional pericycle cells.
Vascular tissues. Most dicot (eudicot) roots differ from eudicot stems in having a lobed column of primary xylem as their core with phloem tissue occurring as strings of cells between the lobes. This arrangement is called a protostele. The primary xylem of monocots, on the other hand, forms a cylinder around a central mass of pith parenchyma, a siphonostele. The way in which the vascular tissues develop is useful in tracing ancestral relationships in the plant kingdom.