epidermis(redirected from layers of skin)
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the flexible tissue (integument) enclosing the body of vertebrate animals. In humans and other mammals, the skin operates a complex organ of numerous structures (sometimes called the integumentary system) serving vital protective and metabolic functions.
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The outermost layer (occasionally several layers) of cells on the primary plant body. Its structure is variable; this article singles out five structural components of the tissue: (1) cuticle; (2) stomatal apparatus (including guard cells and subsidiary cells); (3) bulliform (motor) cells; (4) trichomes; and (5) root hairs.
Leaves, herbaceous stems, and floral organs usually retain the epidermis through life. Most woody stems retain it for one to many years, after which it is replaced. In roots it is usually short-lived. See Leaf, Periderm
Cutin is a mixture of fatty substances characteristically found in epidermal cells. It impregnates the outer cell walls and occurs as a continuous layer (cuticle) on the outer surface. The cuticle covers the surfaces of young stems, leaves, floral organs, and even apical meristems. Waxes appear as a deposit on the outside of the cuticle in many plants; the bloom on purple grapes and plums is an example. Most often the waxes are present in small quantity, but the leaves of some plants may be almost white with wax (Echeveria subrigida). The waxes of a few species are of great commercial value in the manufacture of polishes for floors, furniture, automobiles, and shoes. Other substances, such as gums, resins, and salts, usually in crystalline form, may be deposited on the outside of the cuticle.
The apertures in the epidermis which are surrounded by two specialized cells, the guard cells, are known as stomata. The singular form, stoma, is derived from the Greek word for mouth. However, some authorities prefer to include both aperture and guard cells within the concept of stoma. The apertures of stomata are contiguous with the intercellular space system of underlying tissues and thus permit gas exchange between internal cells and the external environment. The opening and closing of the stomatal aperture is caused by relative changes in turgor between the guard cells and surrounding epidermal cells.
Bulliform (motor) cells are large, highly vacuolated cells that occur on the leaves of many monocotyledons but are probably best known in grasses. They are thought to play a role in the unfolding of developing leaves and in the rolling and unrolling of mature leaves in response to alternating wet and dry periods.
Appendages derived from the protoderm are known as trichomes; the simplest are protrusions from single epidermal cells. Included in the concept, however, are such diverse structures as uniseriate hairs, multiseriate hairs (Begonia, Saxifraga), anchor hairs, stellate hairs, branched (candelabra) hairs, peltate scales, stinging hairs, and glandular hairs (see illustration). Cotton and kapok fibers are unicellular epidermal hairs.
Root hairs are thin-walled extensions of certain root epidermal cells. They develop only on growing root tips and may arise from any epidermal cell, or from specialized cells known as trichoblasts. The life of a given root hair is usually numbered in days. See Root (botany), Secretory structures (plant)
(1) The outermost layer of skin of the animal body. Epidermis develops from the external germ layer, or ectoderm. In invertebrates the excretions of single-layer epidermis that harden in the air may be skeletal (for example, the shells of mollusks) or protective (for example, the cuticles of worms and arthropods) elements.
In man and other vertebrates five layers of epidermis are distinguished (from within outward): the stratum basale (basal cell layer), the stratum spinosum (layer of Malpighi), the stratum granulosum (layer of Langerhans), the stratum lucidum (layer of Oehl), and the stratum corneum (horny layer). The cells of the epidermal layers close to the connective-tissue layer of skin are cylindrical or cubical. They flatten increasingly toward the surface and undergo cornification, forming a constantly renewed horny layer because of the underlying layers. The toughness of the epidermis is due to the presence in the cells of proteins that form tonofibrils. Epidermis regenerates mainly by cell division in the layer of Malpighi.
(2) In plants, the primary surface layer of tissue that develops on all the young organs of a shoot, flower, fruit, and seed. Epidermal cells are flat and lack interstitial spaces. The surface of the thick external wall is covered by a cuticle; often there is also a waxy coating. Live or atrophied hairs function as a screen to reflect some of the sunlight. In addition to ordinary epidermal cells, highly specialized structures are present. These include guard cells, various types of trichomes (glandular, sensory, clothing), hydathodes, and motor cells. Water and nutrients can penetrate the pores and pectin strands in the outer walls of the epidermal cells.
The epidermis protects the internal tissues of plants from desiccation, mechanical injury, and infection. Its system of stomata regulates gas exchange and transpiration. Glycosides, tanning substances, and alkaloids with phytoncidal properties accumulate in epidermal cells; the glandular hairs elaborate essential oils, resins, and mucus. The clothing hairs synthesize the hormones and enzymes needed for the normal activity of plants.
REFERENCEMiroslavov, E. A. Struktura i funktsiia epidermisa lista pokrytose-mennykh rastenii. Leningrad, 1974.
I. S. MIKHAILOVSKAIA