The roots grow in length with the help of apical meristem. It is called primary growth. Apart from primary growth, roots grow in width i.e., they increase in girth. This increase is called secondary growth. It is found only in dicot roots.
Secondary growth in dicot roots occurs, following an increase in the diameter of the stem by secondary growth. Secondary growth in dicot roots is initiated by certain cells that become meristematic by dedifferentiation as there is no cambium.
Certain cells of the conjunctive tissue on the inner edge of phloem and certain cells of the pericycle lying opposite to the xylem groups, become meristematic. These strips of meristematic cells join to form a cambial ring.
The cambial ring is wavy in outline initially, but later it attains a circular outline. The cells of the cambial ring undergo mitotic divisions and produce new cells both on its outer side and inner side.
The cells on the outerside differentiate to form secondary phloem while the cells produced on the inner side differentiate into secondary xylem. Secondary tissue formed outer to the protoxylem bundle differentiates into prominent primary medullary ray thus, protoxylem does not get crushed.
Later, cork cambium (Phellogen) also differentiates in the pericycle. The cork cambium divides and gives rise to cork (Phellem) towards outside and secondary cortex (Phelloderm) towards inside. All the three i.e. Phellogen, Phellem and Phelloderm together form the Periderm of the root and has protective function.
Secondary protective tissues are also formed to replace the epidermis that becomes broken, with the increase in the girth/diameter of the root.