(c) : In hypodermis or outer cortical cells, a layer becomes meristematic which is known as cork cambium or phellogen. This phellogen also cuts off cells both on its outer side and inner side. The cells cut off on outer side are phellem or cork cells and cells cut off on inner side are phelloderm or secondary cortex. Phellem, phellogen and phelloderm collectively constitute periderm. Phellem (cork) Phellogen (cork cambium) Phelloderm (secondary cortex)Structure of periderm (showing $3$ layers)The function of periderm is protective (because at maturity epidermis ruptures and hence the function is performed by periderm).