A thin cross-section of the root of a dicotyledonous plant shows two zones. Extracellular region and Intracellular region.
1. Extra-steel region
The region outside the steel is called extra-steel region. Its various parts are-
(i) Epiblema or Root: The outermost layer of the root is called epiblema or root. The cortex is single-layered. Its wall is thin. It has unicellular rhizomes. It absorbs water and mineral salts.
(ii) Cortex : Cortex is multilayered and composed of parenchyma cells. Cells are round or ovoid with intercellular spaces. It is parenchyma like cells and the wall is very thin.
(iii) Endodermis: It is composed of single layered and tubular cells. The cells are densely packed and without gaps. Casparian ribbons are present in the walls of the cells. It has few passage cells. This layer stores food, protects the stele region, and allows water and solutes to enter through the passage cells.
2. Intrastellar region
The region from pericycle to medulla is called stellie or intrastellar zone. Its various parts are-
(i) Pericycle or pericycle: Below the endothelium there is a layer of pericycle. It is composed of parenchyma cells and the cell wall is very thin.
(ii) Vascular bundles: Xylem or phloem bundles are four in number. They are arranged radially and circularly. Protoxylem towards the periphery and metaxylem towards the center i.e. exarch. It transports water and food.
(iii) Medulla: The part surrounded by vascular bundles situated in the center of the root is called Medulla. It is composed of parenchyma cells. The cells are spherical, thin-walled, and capsuled.
(iv) Medullary Rays: The portion from medulla to pericycle through vascular bundles is called medullary rays. It is composed of narrow and elongated parenchyma cells. It stores food.