The periderm is the protective outer layer of tissue that replaces the epidermis in plant stems and roots undergoing secondary growth. It's essentially the plant's "bark" in non-woody parts or the outer layer of bark in woody plants.
Understanding the Periderm
The periderm is composed of three main layers:
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Phellem (Cork): This is the outermost layer, composed of dead cells whose walls contain suberin, a waxy substance that makes them impermeable to water and gases. It provides protection against desiccation, mechanical damage, and pathogens.
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Phellogen (Cork Cambium): This is a lateral meristem (a region of dividing cells) responsible for producing the phellem to the outside and the phelloderm to the inside. It's the actively growing layer of the periderm.
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Phelloderm: This is the innermost layer, composed of living parenchyma cells. It's formed to the inside by the phellogen and functions in storage.
Function of the Periderm
The primary functions of the periderm include:
- Protection: It protects the underlying tissues from physical damage, water loss, and infection by pathogens.
- Insulation: The cork layer acts as an insulator, protecting the plant from extreme temperatures.
- Gas Exchange: Lenticels, which are small pores in the periderm, allow for gas exchange between the internal tissues and the atmosphere.
- Prevention of Water Loss: The suberized cells of the phellem effectively prevent excessive water loss.
Examples of Periderm
You can observe the periderm in:
- Tree bark: The outer layers of bark on trees like oak, birch, and pine are primarily composed of periderm.
- Potato skins: The outer layer of a potato is its periderm.
- Roots: Many roots develop a periderm as they mature, replacing the epidermis.
Summary
In essence, the periderm is a vital protective layer in plants, especially those undergoing secondary growth, shielding them from environmental stressors and contributing to their survival.
