The root is one of the key organs that helps distinguish between a dicot and a monocot plant.
Anatomy of a dicot root:
The outermost single-layered tissue of the dicot root is called the epidermis. On the epidermis, you will also find some extended epidermal cells which are unicellular root hairs. The epidermis is followed by cortex, which is formed of several layers of thin-walled parenchymatous cells. These cells have intercellular spaces.
The innermost layer of the cortex comprises barrel-shaped cells without any intercellular spaces. This layer is known as the endodermis. The tangential and radial walls of the endodermal cells have deposits of a water-impermeable, material or suberin, which is also called casparian strips. Suberin is a waxy, waterproof substance present in the cell walls of cork tissue in plants.
Going back to the transverse section of the dicot root, after the endodermis, a few layers of thick-walled parenchymatous cells are present in the tissue. This is known as the pericyle.
Secondary growth, such as initiation of lateral roots and vascular cambium, takes place in the cells of the pericycle. The pith in the tissue of the dicot root is extremely small and is undeveloped. Typically, there are two to four xylem and phloem patches in a dicot root. In the later stages, a cambium ring gets formed between the xylem and phloem.
Some radially arranged parenchymatous cells are found between the vascular bundles. These cells form the conjunctive tissue, which is specialised for storage of water. The tissues present inside the endodermis, namely the pericycle, vascular bundles and pith are collectively known as stele.
Anatomy of monocot root:
In many aspects, it is similar to the dicot root. On comparing the two roots, we find the monocot root also consists of epidermis, cortex, endodermis, pericycle, vascular bundle and pith. However, there are some differences. The monocot root can have many xylem bundles, sometimes as many as six, whereas the dicot root does not have quite as many. Moreover, in a monocot root, the pith is well developed.
Although, dicot and monocot roots have some structural similarities, they differ in the number of xylem bundles and size of the pith and cortex. Moreover, secondary growth takes place only in the dicot root. In other words, these roots thicken over time.