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Periderm - Bark is made up of an outer periderm and an inner phloem. 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". The phellogen (bark cambium) is the outer most part of the symplast and the end of the phloem rays. Current growth has a thin covering of epidermis. The one-year-old twigs have a thin covering called a primary periderm. Green, cortex-like tissues are common under the periderm in young trees of many species. Woody roots also have an outer bark or periderm made up of three layers: the phellogen, phelloderm and phellem.