The periderm is the outer protective layer that forms on the stems and roots of plants undergoing secondary growth. This secondary tissue replaces the epidermis, the initial protective layer, as the plant increases in girth and its outer surface is stretched and damaged. Its formation is a defining characteristic of a woody habit.
The Three Structural Components
The periderm is a complex of three distinct, concentrically arranged tissues. At the center is the phellogen, or cork cambium, which is the meristematic layer responsible for cell division. As a lateral meristem, the phellogen divides parallel to the stem surface, facilitating the increase in the plant’s diameter.
The phellogen produces new tissues in two directions. Cells produced outward differentiate into the phellem, or cork layer, which is the outermost component. Cells produced inward form the phelloderm, the innermost layer. The phellogen is thus sandwiched between its two derivatives.
The phellem layer consists of dead cells at maturity, tightly packed without intercellular spaces. These cells deposit suberin, a waxy substance, into their cell walls, making the phellem highly impervious to water and gases. Conversely, the phelloderm consists of living cells that often function in the storage of starches and nutrients beneath the cork cambium.
Essential Functions in Plant Protection
The periderm provides protection for the plant’s inner tissues. The thick, rigid phellem offers mechanical protection, absorbing physical damage from animals or debris. This covering prevents structural breaches that could compromise the stem or root integrity.
The suberin in the phellem cells forms a waterproof barrier that minimizes water loss from the plant’s surface. This prevents desiccation, especially after the epidermis is shed. By sealing the surface, the periderm reduces uncontrolled evaporation.
The impermeable nature of the periderm also defends against pathogens like bacteria and fungi. Microorganisms are blocked by the suberized cell walls and the compact structure of the cork layer, isolating the living tissues beneath.
Development and Replacement of the Epidermis
Periderm formation is a direct consequence of secondary growth, which increases the plant’s diameter. As the stem expands, the epidermis is stretched, ruptures, and loses function. This mechanical stress triggers the development of the phellogen.
The phellogen typically originates from living parenchyma cells located beneath the epidermis, often within the cortex. These cells regain the ability to divide and form the cork cambium. The initiation location varies between species, sometimes forming directly under the epidermis or deeper within the cortex or pericycle.
Once established, the phellogen begins bifacial division. Cells cut off outward become the water-resistant phellem, and those cut off inward become the living phelloderm. This continuous activity pushes the old epidermal remnants outward, leading to their shedding and the replacement of the initial protective tissue.
Lenticels: Specialized Structures for Respiration
Since the phellem is largely impermeable due to its suberin content, specialized structures are required for gas exchange for the living cells beneath. These structures are lenticels, which appear as raised, lens-shaped pores on the periderm surface. Lenticels are functionally analogous to stomata, allowing entry for oxygen and exit for carbon dioxide.
Lenticel formation begins when the phellogen produces a mass of loosely arranged, unsuberized cells called complementary tissue, instead of compact phellem. The pressure from this tissue eventually ruptures the outermost periderm layers, creating an open pore. This porous tissue, with its numerous intercellular spaces, facilitates the diffusion of gases between the internal tissues and the atmosphere.