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. 

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