Periderm

Abstract

The periderm is a cylindrical tissue that covers the surfaces of stems and roots of perennial plants during early secondary growth; therefore it is not found in monocots and is confined to those gymnosperms and eudicots that show secondary growth. The periderm is derived from the phellogen, a meristematic region that arises via the dedifferentiation of parenchyma cells in the epidermis, cortex, phloem, or pericycle. The phellogen generates phellem (aka cork) to the outside and phelloderm to the inside (in some but not all plants). Phellem cells are dead at maturity and form a tight seal around the plant organ. Phelloderm cells, which are involved in storage and further differentiation, are typically alive at maturity. Lenticels are spongy openings in the periderm that allow for gas diffusion into and out of the stem or root. The phellogen only lives for one growing season and must arise de novo each year. The previous year’s phellogen and derivatives are cut off by the new phellogen, die, and are pushed to the exterior where they accumulate to form the rhytidome. Cork is the thick, commercially important rhytidome of the cork oak.