périderme

Périderme is the protective tissue of plants formed during secondary growth in stems and roots, replacing the epidermis as the plant increases in girth. It forms the outermost protective layer known as bark.

Structure: The périderme comprises three tissues: phellogen (cork cambium), phellem (cork), and phelloderm. The cork cambium is a meristem that tangentially divides to produce phellem outward and phelloderm inward. Phellem consists of cork cells that become suberized, dead at maturity, forming a waterproof layer. Phelloderm are living parenchyma cells just inside the cork cambium.

Origin and growth: In many woody plants, the epidermis is destroyed and replaced by periderm produced by the cork cambium. Over time, new phellogen forms successively as the stem thickens, producing additional layers of cork and phelloderm and contributing to the thickened bark. In some species, lenticels appear in the périderme to permit gas exchange.

Function: The périderme protects against mechanical injury and water loss, provides a barrier against pathogens, and helps reduce transpiration. Suberin in cork cells gives waterproofing. Lenticels enable gas exchange between internal tissues and the atmosphere.

Notes: The term is commonly used in French botanical literature; English equivalent terms include periderm or bark. The periderm is distinct from the epidermis and is crucial in woody plant anatomy and the development of bark.