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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 Sclerenchyma
the protective tissue on the stems, roots, tubers, and rhizomes of perennial and, less frequently, annual plants; it consists of cork (phellem), phelloderm, and phellogen (cork cambium). The periderm is tissue of secondary origin. Its middle part, the phellogen, arises from the epidermis (apple and willow stems), the subepidermal layer (birch, linden, and elder stems), the deeper layers of the primary cortex (barberry and pine stems), the pericycle (raspberry, currant, and spirea stems; the roots of the majority of plants), or the phloem (grape stems).
As a result of division of phellogen cells, a multilayer cork— the protective tissue proper—is formed on the outside, while one or more layers of phelloderm cells are formed beneath. The latter consists of living cells that differ from the parenchymatous cells of the cortex in their radial distribution. Sometimes the phelloderm is absent (raspberry, bittersweet).
Cork cells are dead and are impervious to air and water. The cavities of the dead cells fill up with air, thus intensifying the heat-insulating properties of cork tissue. (Gas exchange and evaporation in plants are accomplished through lenticels in the periderm.)
Several periderms usually develop in plant organs; each successive one occurs beneath the preceding layer. The formation of only one periderm is relatively rare (aspen, alder, perennial herbs). In time, the outer periderms and the tissues enclosed between them die, forming bark on the surface of the organ.
M. A. GULENKOVA