Periderm

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Periderm

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

Periderm

 

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

periderm

[′per·ə‚dərm]
(botany)
A group of secondary tissues forming a protective layer which replaces the epidermis of many plant stems, roots, and other parts; composed of cork cambium, phelloderm, and cork.
(embryology)
The superficial transient layer of epithelial cells of the embryonic epidermis.
References in periodicals archive ?
The whole bark fractions retained on the 35- and 80-mesh sieves, which were rich in periderm tissue were ground further in small batches (25 g) using an ultra-centrifugal grinding mill (Retsch, Inc.
Accordingly, in addition to the partitioning at the cellular level, these results show an added benefit of ash removal from the periderm tissue targeted for further grinding to obtain a suitable plywood adhesive mix filler.
This further demonstrated that the periderm particles are resistant to disintegration by the rotating blade of the laboratory blender.
Thus, these data suggest that an operation focused on only outer bark provides only a slight benefit in the yield of the desired bark fractions rich in periderm tissue.
It is especially interesting to note that the values for percent wood failure for bark filler C were intermediate to those obtained for the fillers prepared from the bark fractions rich in either periderm (filler B) or obliterated phloem (filler A) tissues.