Periderms

Periderm is a protective tissue that replaces the epidermis in many plants undergoing secondary growth. It consists of cork cambium (phellogen), cork (phellem), and phelloderm. The cork cambium is a meristem that forms when a ring of cells in the cortex or pericycle becomes meristematic. It divides outward to produce cork cells and inward to produce phelloderm. Cork cells become suberized, die as they mature, forming a tough, water-impermeable layer. Phelloderm remains living and may be parenchymatous. Together these layers constitute the periderm and, along with underlying secondary phloem, constitute the bark.

Periderm replaces the epidermis in regions of secondary growth and serves to protect the inner tissues, limit water loss, and reduce pathogen entry. The suberized cork provides insulation and a barrier to desiccation. Gas exchange is facilitated by lenticels, which are specialized zones or breaks in the periderm.

Origin and variation: Cork cambium may arise from residual cortical cells, the endodermis, or the pericycle, depending on species and organ; its activity results in a multilayered protective covering as the plant increases in girth. In many woody plants, periderm development is continuous, with periodic formation of new cork layers as the plant expands. In some species, periderm discontinuities or lenticels allow localized gas exchange. The term bark broadly refers to all tissues outside the vascular cambium, including periderm and remnants of older tissues.