lichen

Note: This page may contain terms or definitions that are offensive or inappropriate for some readers.

lichen

(lī`kən), usually slow-growing organism of simple structure, composed of fungi (see FungiFungi
, kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte).
..... Click the link for more information. ) and photosynthetic green algaealgae
[plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms. These organisms were previously classified as a primitive subkingdom of the plant kingdom, the thallophytes (plants that lack true roots, stems, leaves, and flowers).
..... Click the link for more information. or cyanobacteriacyanobacteria
or blue-green algae,
photosynthetic bacteria that contain chlorophyll. For many years they were classified in the plant kingdom along with algae, but discoveries made possible by the electron microscope and new biochemical techniques have shown them to be
..... Click the link for more information. living together in a symbiotic relationship and resulting in a structure that resembles neither constituent. There are about 25,000 species, most comprised of a sac fungus (Ascomycete) and a green alga of the genus Trebouxia or Trentepohlia or a cyanobacterium of the genus Nostoc; some lichens include multiple species of fungi, and more recently scientists discovered that basidiomycete yeastsyeast,
name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes.
..... Click the link for more information. are present in many lichens in addition.

Lichens commonly grow on rocks, trees, fence posts, and similar objects. The body (thallus) of the lichen is made up of the filaments, or hyphae, of the fungus. Its typical greenish gray color is due to the combination of the chlorophyll from the photosynthetic organism with the colorless fungi, although sometimes the thallus may be red, orange, or brown. Lichens require no food source other than light, air, and minerals. They depend heavily on rainwater for their minerals and are sensitive to rain-borne pollutants. The fungal component of lichens produces acids that disintegrate rock, giving the lichen a better hold and aiding weathering processes, which eventually turn rock into soil. Lichens usually reproduce by the breaking off of a segment that contains both components.

Lichens can withstand great extremes of temperature and are found in arctic, antarctic, and tropical regions. They are often the pioneer forms of life—as in parts of Iceland and Greenland, where they are the predominant vegetation. Reindeer moss (Cladonia rangiferina) and Iceland moss (Cetraria islandica), both low, branching forms, provide food for large mammals and other animals in northern regions. Old-man's-beard (Usnea barbata) is a temperate species that hangs like Spanish moss from coniferous trees.

Before the discovery of aniline dyes, lichens were much used for silk and wool dyes. The blue and purple dyes litmuslitmus,
organic dye usually used in the laboratory as an indicator of acidity or alkalinity (see acids and bases). Naturally pink in color, it turns blue in alkali solutions and red in acids. Commonly, paper is treated with the coloring matter to form so-called litmus paper.
..... Click the link for more information. and archilarchil
or orchil
, blue, red, or purple dye extracted from several species of lichen, also called orchella weeds, found in various parts of the world. Commercial archil is either a powder (called cudbear), a pasty mass (called archil), or a drier paste (called persis).
..... Click the link for more information. are still obtained from species of lichens. Others have been used in perfume manufacturing and brewing. The "manna" of the Bible is thought by some to have been a lichen found in Old World deserts and easily carried along by wind.

Bibliography

See V. Alimadjian, The Lichen Symbiosis (1967); M. E. Hale, Jr., The Biology of Lichens (1970); I. M. Brodo et al., Lichens of North America (2001).

Lichen

(in Russian, lishai), a term used to designate many diseases of the skin, of varied external manifestations, courses, and causes.

The term “leichen” was first used by ancient Greek physicians to refer to all rashes accompanied by itching and desquamation. In modern medicine, the term “lishai” is used in Russian as a second name, defining more precisely the other medical designations of the diseases. For example, eczema is known as moknushchii lishai (exudative lichen); psoriasis, as cheshuichatyi lishai (scaly lichen); and trichophytosis, as strigushchii lishai (shearing lichen).

lichen

[′lī·kən] (botany) The common name for members of the Lichenes.

lichen

1. an organism that is formed by the symbiotic association of a fungus and an alga or cyanobacterium and occurs as crusty patches or bushy growths on tree trunks, bare ground, etc. Lichens are now classified as a phylum of fungi (Mycophycophyta) 2. Pathol any of various eruptive disorders of the skin

lichen

Note: This page may contain terms or definitions that are offensive or inappropriate for some readers.

lichen

[li´ken] 1. any of certain plants formed by the mutualistic combination of an alga and a fungus.2. any of various papular skin diseases in which the lesions are typically small, firm papules set very close together.lichen amyloido´sus a condition characterized by localized cutaneous amyloidosis.lichen fibromucinoido´sus (lichen myxedemato´sus) a condition resembling myxedema but unassociated with hypothyroidism, marked by mucinosis and a widespread eruption of asymptomatic, soft, pale red or yellowish, discrete papules.lichen ni´tidus a usually asymptomatic chronic inflammatory eruption consisting of numerous glistening, flat-topped, discrete, smooth, commonly skin-colored micropapules, located most often on the penis, lower abdomen, inner thighs, flexor aspects of the wrists and forearms, breasts, and buttocks. Widespread involvement may produce confluence of the lesions, with formation of scaly plaques.lichen pila´ris lichen spinulosus.lichen planopila´ris a variant of lichen planus characterized by formation of cone-shaped horny papules around the hair follicles, in addition to the typical lesions of ordinary lichen planus.lichen pla´nus an inflammatory skin disease with wide, flat, purplish, shiny papules in circumscribed patches; it may involve the hair follicles, nails, and buccal mucosa; called also lichen ruber planus.lichen ru´ber monilifor´mis a generalized or localized eruption with either round, dome-shaped, waxy, dark or bright red papules, or waxy, yellow, milia-like papules, often forming a moniliform (string-of-beads) pattern, sometimes arranged in keloidal bands. Some authorities consider the condition to be a variant of lichen simplex chronicus.lichen ru´ber pla´nus lichen planus.lichen sclero´sus (lichen sclero´sus et atro´phicus) a chronic atrophic skin disease marked by white papules with an erythematous halo and keratotic plugging. It sometimes affects the vulva (kraurosis vulvae) or penis (balanitis xerotica obliterans).lichen scrofuloso´rum (lichen scrofulo´sus) a form of tuberculid manifested as an eruption of clusters of lichenoid papules on the trunk of children with tuberculous disease.lichen sim´plex chro´nicus dermatosis of psychogenic origin, marked by a pruritic discrete, or more often, confluent lichenoid papular eruption, usually confined to a localized area. Mechanical trauma, such as scratching or rubbing the area, is a factor in its development. The lesions may arise from normal skin or they may occur as a complication of other forms of dermatitis. Called also circumscribed or localized neurodermatitis and lichen chronicum simplex.
Treatment consists of administration of corticosteroids applied locally as a cream or given by intralesional injection to relieve the pruritus. The area should be protected by light dressings and the patient encouraged to avoid mental stress, emotional upsets, and irritation of the affected area. The application of very hot or very cold compresses may afford temporary relief of the itching. The condition tends to become chronic with unexplained remissions and reappearance of lesions in a different part of the body.lichen spinulo´sus a condition in which there is a horn or spine in the center of each hair follicle; called also lichen pilaris.lichen stria´tus a self-limited condition characterized by a linear lichenoid eruption, usually in children.lichen urtica´tus papular urticaria.

li·chen

(lī'ken), A discrete flat papule or an aggregate of papules giving a patterned configuration resembling lichen growing on rocks. [G. leichēn, lichen; a lichenlike eruption]

lichen

(lī′kən)n.1. A composite organism made up of a fungus, usually an ascomycete, that grows symbiotically with an alga or a cyanobacterium and characteristically forms a crustlike or branching growth on rocks or tree trunks.2. Medicine Any of various skin diseases characterized by patchy eruptions of small, firm papules.tr.v. li·chened, li·chening, li·chens To cover with lichens.

li′chen·ous adj.

li·chen

(lī'kĕn) A discrete flat papule or an aggregate of papules giving a patterned configuration resembling lichens growing on rocks. [G. leichēn, lichen; a lichenlike eruption]

lichen

(li'ken) [Gr. leichen, lichen] 1. Any form of papular skin disease, esp. lichen planus.2. In botany, any of numerous plants consisting of a fungus growing symbiotically with algae. They form characteristic scaly or branching growths on rocks or barks of trees.

myxedematous lichen

Generalized eruption of asymptomatic nodules caused by mucinous deposits in the upper layers of the skin and in vessels and organs.

lichen nitidus

A rare skin condition characterized by small, chronic, asymptomatic papules that are usually pink and are usually located only on the penis, abdomen, and flexor surfaces of the elbows and palms.

lichen pilaris

Keratosis pilaris.

lichen planopilaris

A follicular papulosquamous eruption, typically found on the scalp, and often associated with lichen planus.

LICHEN PLANUS

LICHEN PLANUS

lichen planus

An inflammatory rash marked by the presence of itchy, red to violet, polygon-shaped papules, which typically appear on the scalp, in the oral cavity, or on the limbs. The papules may merge into plaques crisscrossed by Wickham striae. Typically, the rash persists for 1 to 2 years and then spontaneously improves although about one in five patients will suffer a recurrence. Synonym: lichen ruber planus See: illustration

Etiology

The cause of the rash is unknown, but it is occasionally associated with the use of chemicals (such as photoprocessing compounds, gold) or medications (such as beta blockers, diuretics, nonsteroidal anti-inflammatory drugs).

Treatment

Corticosteroids, applied topically, taken orally, or injected into the lesions, are often effective.

illustration

lichen ruber moniliformis

Large verrucous lesions of lichen planus arranged like beads in a necklace.

lichen ruber planus

Lichen planus.

LICHEN SCLEROSUS ET ATROPHICUS

lichen sclerosus et atrophicus

A chronic atrophic skin disorder marked by the appearance of discrete, flat-topped, white papules, which may coalesce and degenerate. The skin affected by the rash, which occurs most often on the vulva, is often thin, shiny, and scarred. Although this condition is not considered precancerous, squamous cell carcinomas arise in 1% to 5% of cases. See: illustration

Symptoms

Itching of the vulva, which may be intractable, is the most common complaint.

Treatment

Potent topical corticosteroids produce remission, but not cure, in the great majority of patients.

Synonym: vulvar dystrophy; vulvar pruritus

lichen scrofulosus

An eruption of tiny punctate reddish-brown papules arranged in circles or groups in young people with tuberculosis. The lesions are caused by the spread of the tubercle bacilli through the blood to the skin.

LICHEN SIMPLEX CHRONICUS

lichen simplex chronicus

An itching papular eruption that is circumscribed and located on skin that has become thickened and pigmented as a result of scratching. Synonym: circumscribed neurodermatitis See: neurodermatitis for illus. illustration

lichen spinulosus

Keratosis pilaris.

lichen striatus

A papular eruption usually seen on one extremity of a child. It is arranged in linear groups and consists of pink papules. The disease, though self-limiting, may last for a year or longer.

tropical lichen

Prickly heat.

lichen tropicus

Prickly heat.

lichen

Any skin eruption.

lichen

a composite organism formed by the symbiotic association (see SYMBIOSIS of a green alga or a CYANOBACTERIUM and a fungus, usually from the ASCOMYCOTA or BASIDIOMYCOTA. The fungus makes up 80–90% of the association. The fungus gains oxygen and carbohydrates from the alga or cyanobacterium. The alga or cyanobacterium gains water, CO₂ and mineral salts from the fungus and is protected from desiccation. Lichens reproduce vegetatively by means of soredia (hyphal cells enclosing algal or cyanobacterial cells) and sexually by means of the fungal apothecia or PERITHECIA. However, where no algae or cyanobacteria of the usual association occur, the germinating fungal spore dies. Lichens are very common on trees and rocks in unpolluted areas and can be used as an INDICATOR SPECIES.

lichen

li·chen

lichen

li•chen

li·chen

lichen

lichen

lichen

Bibliography

Lichen

lichen

lichen

lichen

lichen

li·chen

lichen

li·chen

lichen

myxedematous lichen

lichen nitidus

lichen pilaris

lichen planopilaris

lichen planus

Etiology

Treatment

lichen ruber moniliformis

lichen ruber planus

lichen sclerosus et atrophicus

Symptoms

Treatment

lichen scrofulosus

lichen simplex chronicus

lichen spinulosus

lichen striatus

tropical lichen

lichen tropicus

lichen

lichen

LICHEN

lichen

Words related to lichen

noun any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks

Related Words

noun any thallophytic plant of the division Lichenes

Related Words