Root Cortex and Endodermis
The cortex is the region between the pericycle and the epidermis, including the innermost layer, the endodermis. In underground roots the rhizodermis becomes worn away, and is replaced as an outer layer either by a periderm that forms in the cortex (in most woody eudicots and magnoliids) or by a suberinized or lignified exodermis (in some monocots), which is sometimes multilayered.
Apart from these specialized layers, most cortical cells are parenchymatous and often perform an important storage function. In some plants, such as Daucus carota (carrot), the tap root is a modified swollen storage organ with a wide cortex. In most roots the bulk of the cortical cells are formed by sequential periclinal divisions, the innermost cells (later the endodermis) being the last formed.
Many plants with underground stems (corms, bulbs or rhizomes), particularly bulbous or cormous monocots such as Crocus, Freesia and Hyacinthus, periodically produce contractile roots which draw the stem deeper into the soil. These roots grow downwards, and then shorten vertically and expand radially. They are recognizable by their wrinkled surface, and characteristically possess two or three clearly distinct concentric regions of cortical parenchyma, distinguishable by cell size, including a region of collapsed outer cortical cells interspersed with occasional thicker-walled cells. In some species the process of root contraction is initiated by active cell enlargement in the inner cortex, followed by collapse of outer cortical cells and subsequent surface folding. In other species the collapse of outer cortical cells results from the difference between atmospheric pressure and relatively low xylem pressure (due to transpiration), causing centripetal loss of turgidity.
The endodermis is a uniseriate cylinder of cortical cells sur-rounding the central vascular region, adjacent to the pericycle. Endodermal cells are typically characterized by deposition of a band of suberin or lignin in their primary walls, termed a Casparian strip, which forms a barrier against non-selective passage of water through the endodermis. Older endodermal cells often possess thick lamellated secondary walls, in most cases on the inner periclinal wall, so that the Casparian strip is not apparent. The secondary wall is often lignified, and therefore serves as a second effective barrier to water loss. Occasional endodermal cells (passage cells) can remain thin-walled, probably for selective passage of water between the cortex and vascular region.