The part of the plant which is seen under the soil is the roots. They develop from the radicle. The root fixes the plant firmly to the soil and absorbs water and minerals from the soil .There are two types of root-Dicot root and monocot root. Dicots root have limited number of vascular bundles which are arranged in the form of a ring. Monocot roots have large number of vascular bundles which are scattered. Both monocot and dicot root have exarch xylem.
A dicot root has three regions-Piliferous layer, Cortex, stele.
Piliferous layer (Epiblema): It is the outer most protective layer. It consists of a single layer of thin walled living cells. Some of the cells transform into unicellular tubular outgrowths called root hairs which absorb water and minerals from the soil.
Cortex: The region in-between piliferous layer and the stele. It is made up of thin walled parenchyma cells with intercellular spaces. The cells are arranged in radial rows or concentric layers. Chloroplasts are generally absent. But in aerial roots chloroplast are present. In water plants, air cavities are present in the cortex. The inner most layer of the cortex is the endodermis composed of single layer of compactly arranged barrel shaped cells. The radial and inner tangential walls are thickened due to the deposition of suberin.These thickenings are called casparian thickenings which prevent plasmolysis .The cells opposite to the protoxylem cells remains thin walled without casparian thickenings and are called passage cells.
Stele: Central portion delimited by the pericycle is the stele which consists of pericycle, vascular tissues, conjunctive tissue and pith. Pericycle is single layered and parenchymatous.
Vascular tissue consists of xylem and phloem. Xylem conducts water and minerals and phloem conducts food. Xylem consists of protoxylem and metaxylem. Protoxylem is seen towards the periphery and metaxylem towards the center called exarch. The parenchymatous region in-between xylem and phloem is the conjunctive tissue. The inner most region of the stele is called the pith.