Periderm

Whenever a stem or root thickens through secondary growth, its original epidermis (and, often, the primary cortex beneath it too) is no longer able to keep stretching to cover the widening surface, and is progressively replaced by a new protective tissue of secondary origin called the periderm. The periderm is not a single tissue but a three-layered unit, built from a second lateral meristem distinct from the vascular cambium: phellogen (the cork cambium itself, the actively dividing layer), phellem (cork, cut off outward by the phellogen) and phelloderm (secondary cortex, cut off inward by the phellogen) …