Periderm is a protective tissue that forms on the surface of plant stems and roots, particularly in woody plants. It replaces the epidermis, which is the initial outer layer, as the plant grows and undergoes secondary thickening. Periderm helps to shield the underlying tissues from various environmental challenges, contributing to the plant’s overall well-being and longevity.
The Plant’s Protective Outer Layer
It is composed of three distinct layers: the cork cambium, also known as phellogen; the cork, or phellem; and the phelloderm. The phellogen is a meristematic layer of actively dividing cells.
The phellem, or cork, forms the outermost part of the periderm and is made up of dead cells. These cells are typically polygonal in shape and are densely packed without intercellular spaces. Their cell walls are impregnated with suberin, a waxy substance that makes them water-resistant. The innermost layer, the phelloderm, consists of living parenchyma cells that may resemble the cortical tissue and can sometimes contain chloroplasts, contributing to photosynthesis.
How Periderm Develops
Periderm formation begins with the cork cambium, or phellogen. The phellogen consists of radially flattened, rectangular cells that divide to produce new cells.
The cork cambium produces phellem cells towards the outside of the plant and phelloderm cells towards the inside. As phellem cells mature, they accumulate suberin in their cell walls, a hydrophobic waxy substance that renders them impermeable to water and gases. This suberization process, along with the death of the phellem cells at maturity, creates a robust, protective barrier. The continuous production of these layers contributes to the increasing girth of the stem or root.
Vital Functions of Periderm
The periderm serves multiple protective functions for the plant. Its primary role is to shield internal tissues from physical damage, such as abrasions or impacts. The suberin-impregnated cork cells also prevent excessive water loss, or desiccation. This barrier also helps to deter the entry of pathogens like fungi and bacteria.
Periderm also offers insulation against extreme temperatures. Although the cork layer is largely impermeable, specialized structures called lenticels facilitate gas exchange between the plant’s internal tissues and the atmosphere. These small, often raised, porous areas are composed of loosely packed cells, allowing oxygen to enter for respiration and carbon dioxide to exit, especially in older stems where stomata are no longer present.
Clarifying Periderm and Bark
The terms “periderm” and “bark” are often used interchangeably, but they refer to different components of a plant’s outer covering. Periderm is a specific tissue within the bark, formed by the cork cambium and its derivatives.
Bark, in a broader sense, encompasses all tissues located outside the vascular cambium. This includes the periderm, as well as the secondary phloem (which transports sugars), and any remaining primary tissues like the epidermis or cortex that have not yet been shed. Periderm is a component of the bark, which represents the entire complex of outer tissues on a woody stem or root.
Periderm’s Role in Everyday Life
The periderm of certain trees has significant practical applications, notably the cork harvested from the cork oak tree, Quercus suber. This cork is primarily the phellem layer, which is unusually thick and regenerates after harvesting. The unique properties of cork make it a valuable material for various industries.
Cork is lightweight, highly impermeable to liquids and gases, and possesses elasticity, making it ideal for products like wine stoppers. Its insulating and damping capacities also contribute to its use in flooring, wall coverings, and insulation materials in construction and even aeronautics. The sustainable harvesting of cork supports a significant global industry.