lichen

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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).
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) 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).
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 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
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 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.
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 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.
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 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).
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 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