The extra stelar region grows when a cork cambium (phellogen) is formed in the cortex of epidermis. The phellogen form cells on the outre side which get suberised to form the cork (phellem). The cells on the inner side remain thin walled and form the secondary cortex (phelloderm). All the three layers i.e. phellem, phellogen and phelloderm constitute the periderm.