(d) : Dermatogen is the region or histogen of single layer of outermost cells formed from the apical meristem. Dermatogen gives rise to epidermis of stem and other aerial parts. Periblem is the middle histogen which forms the cortex of stem and roots. Plerome is the central histogen which forms stele or part of stem and root inner to endodermis. Part of plerome that forms vascular tissues is called procambium. The pericycle layer converts into a secondary meristem called cork cambium or phellogen which divides to form secondary cortex or phelloderm.