Secondary growth occurs in the cortex of dicot stem is called extra stelar secondary growth. It takes place by the formation a ring of meristamatic tissue, called cork cambium or phellogen. It is formed by the dedifferentiation of collenchyma present in the cortex. It is It undergoes periclinal divisions, produces cells towards the outer side and innerside. The cells towards the outerside form into cork or phellem and the cells towards the innerside form into secondary cortex or phelloderm. Cork, cork cambium and secondary cortex together called Periderm.