Rhytidome and Polyderm

Two specialised terms describe periderm-related tissues beyond the basic phellem-phellogen-phelloderm unit. Rhytidome is the technical term for the outer dead bark of an older tree - it is not a single periderm but an accumulation of several successive periderms (each with its own phellem, phellogen and phelloderm) together with the isolated cortical or phloem tissue trapped between them, built up over repeated rounds of secondary growth as older periderms are pushed outward and die (Quercus, oak, is a typical example). Polyderm, by contrast, is a specialised type of protective tissue found in the roots and underground stems of some plants (e.g. the Rosaceae family) - it consists of a single-cell-thick, suberised layer alternating with multi-layered, non-suberised cells within the periderm, giving th …