periderm

n.
The outer layers of tissue of woody roots and stems, consisting of the cork cambium and the tissues produced by it, such as cork.

peridermal per'i·der'mal or per'i·der'mic adj.

A group of tissues which replaces the epidermis in the plant body. Its main function is to protect the underlying tissues from desiccation, freezing, heat injury, mechanical destruction, and disease. Although periderm may develop in leaves and fruits, its main function is to protect stems and roots. The fundamental tissues which compose the periderm are the phellogen, phelloderm, and phellem.

The phellogen is the meristematic portion of the periderm and consists of one layer of initials. These exhibit little variation in form, appearing rectangular and somewhat flat in cross and radial sections, and polygonal in tangential sections.

The phelloderm cells are phellogen derivatives formed inward. The number of phelloderm layers varies with species, season, and age of the periderm. In some species, the periderm lacks the phelloderm altogether. The phelloderm consists of living cells with photosynthesizing chloroplasts and cellulosic walls.

The phellem, or cork, cells are phellogen derivatives formed outward. These cells are arranged in tiers with almost no intercellular spaces except in the lenticel regions. After completion of their differentiation, the phellem cells die and their protoplasts disintegrate. The cell lumens remain empty, excluding a few species in which various crystals can be found. The remarkable impermeability of the suberized cell walls is largely due to their impregnation with waxes, tannins, cerin, friedelin, and phellonic and phellogenic acids.

Lenticels are loose-structured openings that develop usually beneath the stomata and that facilitate gas transport through the otherwise impermeable layers of phellem. See also Bark; Sclerenchyma.

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Periderm replaces the epidermis as the protective covering in stems and roots of plants with secondary growth. The periderm consists of three structures: 1) the cork or phellem; 2) the cork cambium, or phellogen; and 3) the phelloderm. The cork or phellem is nonliving and is the protective tissue formed to the outside by the cork cambium. The cork cambium or phellogen is the meristem that produces the periderm. The phelloderm is a living parenchyma tissue formed to the inside of the meristem.

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(pĕr'ĭ-dûrm') or per·i·der·ma (pĕr'ĭ-dûr'mə)
n.

The outermost layer of the epidermis of an embryo or a fetus up to the sixth month of gestation. Also called epitrichium.

The outer layer of the bilaminar fetal epidermis, generally disappearing before birth. Called also epitrichium.