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Phellem - A cork cambium or phellogen produces the outer bark or phellem. Periderm - Outer bark or periderm is mostly dead cells lined with a fatty substance called suberin or cork. "Periderm" is made up of a phellogen, phelloderm, and phellem. Suberin is a lipid that in the outer periderm of phellem waterproofs outer bark. Suberin- impregnated phellem is called cork. The "phellogen" (bark cambium) is the outer most part of the symplast and the end of the phloem rays. See "Phellem Shedding" Woody roots also have an outer bark or periderm made up of three layers: the phellogen, phelloderm and phellem.