In order to provide increase in girth and prevent rupturing of the outer ground tissues due to the formation of secondary vascular tissues, dicot stems produce a cork cambium or phellogen in the outer cortical cells. The cells of this phellogen divide peridinally to produce phellem or cork on the outer side and secondary cortex or phelloderm on the inner side. Phellem, phellogen and phelloderm collectively constitute periderm.