Periderm – Structure, Development and Function

What is Periderm?

• Periderm is a secondary dermal tissue that arises within the ground tissue of stems, specifically near the surface. It serves as a protective barrier, replacing the epidermis as the plant undergoes secondary growth. Like the epidermis, the periderm is a complex tissue, composed of three primary layers: the phellem (commonly known as cork), the phellogen (cork cambium), and the phelloderm.
• The outermost layer, the phellem, consists of dead cells that have thick secondary walls impregnated with suberin, a waxy substance that makes this layer highly impermeable to water and gases. Phellem is the most prominent and thickest part of the periderm, acting as the primary protective layer.
• Beneath the phellem is the phellogen, a lateral meristem that functions similarly to vascular cambium but produces tissues that contribute to the plant’s outer protection rather than its internal structure. The phellogen generates phellem cells toward the exterior and phelloderm cells toward the interior. The phelloderm is a thin layer of living cells that typically plays a minor role in the periderm’s overall structure and function.
• As the plant ages, the phellogen may form deeper within the stem, sometimes within the phloem layer, separating portions of the phloem from the vascular tissues. The combination of tissues, including the phellem, phellogen, phelloderm, and outer layers of phloem, along with any remaining epidermis, collectively forms what is commonly referred to as bark.
• Periderm formation is mainly seen in dicotyledons and gymnosperms during secondary growth, and is rare in monocotyledons. It is typically found along the stems and roots of these plants but can also develop in response to injury, forming a protective “wound periderm” at the site of damage. Additionally, periderm may form on surfaces exposed after the abscission of leaves or branches, further reinforcing the plant’s defense mechanisms.

Structure of Periderm

The periderm is a secondary tissue that replaces the epidermis during secondary growth in plants, providing a protective barrier. It consists of three main components: the phellogen (cork cambium), phellem (cork), and phelloderm. These three layers arise through a sequential process and perform distinct functions in the plant’s protective system. Each layer plays a role in maintaining the structural integrity of the plant while ensuring protection from environmental stress. Below is a detailed breakdown of each part of the periderm:

• Phellogen (Cork Cambium):
  • Phellogen is a lateral meristem, simpler in structure compared to the vascular cambium. It serves as the meristematic tissue responsible for generating both the phellem and the phelloderm.
  • It originates in various tissues, including the epidermis, hypodermis, cortex, or even the phloem, depending on the plant species.
  • Structurally, phellogen consists of rectangular, radially flattened cells with protoplasm that contains tannins and chloroplasts.
  • Unlike most plant tissues, intercellular spaces in the phellogen are generally absent, except in specialized structures like lenticels.
  • Phellogen activity increases the thickness of the plant’s axis by dividing periclinally, producing phellem cells outward and phelloderm inward.
  • The initial development of phellogen varies among plants. For example, it may arise in the epidermis in species like Nerium and Pyrus, or in the second or third cortical layers in Robinia and Aristolochia.
  • The origin of phellogen within the epidermal cells leads to changes in cell structure: central vacuoles disappear, the cytoplasm becomes granular, and the layer divides tangentially.
  • More phellem cells are produced on the outer side than phelloderm on the inner side, and in some species, phelloderm may be absent or minimal.
• Phellem (Cork Cells):
  • Phellem, also known as cork, is made up of dead cells that form the outermost protective layer of the periderm.
  • Cork cells are devoid of intercellular spaces and have walls impregnated with suberin, a waxy substance that provides water resistance. In some cells, suberin is absent, and these cells are termed phelloids.
  • The cell walls can either be colorless or contain pigments, depending on the species.
  • Cork cells have a uniform, polygonal shape when viewed tangentially, and are often radially elongated in some species. For example, in Quercus suber, commercial cork cells are radially elongated in cross-section, while in Betula and Prunus, cork cells are elongated tangentially.
  • Once fully developed, cork cells die, their protoplasts disintegrate, leaving only the tough, protective cell walls that act as a barrier.
• Phelloderm:
  • Phelloderm is the innermost layer of the periderm and consists of living cells produced by the phellogen. These cells resemble parenchyma cells found in the plant’s cortex.
  • The number of phelloderm layers varies across species and changes with age and environmental factors.
  • Phelloderm cells have cellulosic walls and contain chloroplasts, making them photosynthetically active in some species. In addition to photosynthesis, phelloderm cells are involved in starch storage.
  • Like other parenchyma cells, phelloderm cells are pitted and loosely arranged. However, they are distinct from cortical cells due to their radial arrangement, which results from their formation by the tangential division of the phellogen.
  • Occasionally, specialized cells such as sclereids may occur within the phelloderm. The term “secondary cortex” is sometimes used to describe phelloderm, though this is considered inaccurate by some.

Function of Periderm

The periderm plays a crucial role in protecting plants, especially as they undergo secondary growth and replace the epidermis. This secondary protective layer provides a variety of vital functions that help maintain the plant’s overall health and survival.

• Protection from Desiccation:
  • The periderm prevents excessive water loss from the plant’s internal tissues. The outer phellem (cork) layer, filled with suberin, acts as a waterproof barrier, limiting water evaporation and protecting the plant from dehydration, especially in dry environments.
• Defense Against Freezing:
  • By forming a protective barrier, the periderm helps insulate the plant from extreme cold. The suberin-filled cork cells act as a thermal buffer, reducing the risk of freezing damage to the inner, more sensitive tissues during cold seasons.
• Shielding from Heat Injury:
  • The periderm also defends against heat damage by reducing the effects of high temperatures on the internal tissues. The cork layer helps in maintaining a more stable internal temperature, minimizing potential harm caused by intense sunlight or high external temperatures.
• Protection from Mechanical Damage:
  • The tough, resilient nature of the phellem provides a layer of defense against physical injury. Whether from environmental forces like wind or from external threats such as animal grazing, the cork layer absorbs mechanical stress, preventing damage to underlying tissues.
• Barrier Against Pathogens and Pests:
  • The periderm serves as a boundary, restricting the entry of pathogens and insects. The cork layer’s impermeable nature helps block the invasion of harmful microorganisms and insect pests, creating a line of defense that minimizes infection and disease.
• Prevention of Disease:
  • By sealing off the internal tissues from external factors, the periderm significantly reduces the plant’s vulnerability to diseases. It acts as a physical barrier, preventing pathogens from entering and spreading throughout the plant.
• Compensation for Loss of Epidermis:
  • The periderm develops after the epidermis is shed during secondary growth. Its primary function is to replace the lost epidermis, continuing to protect the plant against various environmental stresses.
• Facilitating Gaseous Exchange through Lenticels:
  • Although the periderm restricts water and pathogen entry, it allows for gas exchange. This is facilitated by lenticels, small openings in the periderm that enable oxygen and carbon dioxide to pass through, ensuring the plant’s tissues receive the gases necessary for respiration.

Organization of Periderm

The periderm, a protective tissue formed during secondary growth in plants, develops in a structured manner to ensure the plant’s defense and survival. Its organization is both sequential and layered, adapting to the needs of the plant as it grows. Below is an in-depth look at how the periderm is organized:

• Initial Formation:
  • The first periderm typically forms during the first year of growth in both stems and roots. In stems, it commonly originates in the sub-epidermal layer. However, in certain species such as Berberis and Vitis, the first periderm appears deeper, within the primary phloem.
• Periderm Formation in Roots:
  • In roots, the initial periderm develops in the pericycle. However, in some cases where the root cortex serves as a storage area for food, the periderm can form closer to the surface.
• Development of Subsequent Periderms:
  • After the formation of the first periderm, subsequent periderms may arise either during the same year or later. These layers develop progressively deeper beneath the first periderm, typically originating from the parenchyma of the phloem, including the ray cells.
• Uniform Initiation of the First Phellogen:
  • The first phellogen (cork cambium) generally initiates uniformly around the circumference of the axis. In some cases, particularly in roots, it may begin in localized areas, but these regions will continue growing to form a more complete layer.
• Subsequent Periderms:
  • Over time, new periderms develop in discontinuous, but often overlapping layers. While initially discontinuous, these layers may eventually appear as continuous or partly continuous layers around the axis, depending on the species and environmental conditions.
• Origin of Phellogen:
  • The phellogen arises from living cells that are meristematic, meaning they have the potential to divide and form new cells. These cells often contain chloroplasts, starch, and tannins when periderm formation begins, but these substances gradually disappear as the phellogen develops.
• Cell Division and Layer Formation:
  • The phellogen divides periclinally (parallel to the surface), producing phellem (cork cells) on the outer side and phelloderm on the inner side. The phellem forms more abundantly, while the phelloderm often remains a very thin layer, sometimes consisting of only a few cells.
• Types of Bark Formation:
  • Depending on the organization of the periderm layers, two types of bark can be distinguished:
    • Scale Bark: This type forms when successive periderms develop in restricted, overlapping layers. Each layer cuts out small scales of tissue. Examples of plants with scale bark include Pinus and Pyrus.
    • Ring Bark: This occurs when successive periderms form in concentric rings around the axis, producing continuous sheets of bark. Plants such as Vitis and Lonicera exhibit ring bark formation.