Lichens
Lichens
Lichens are an intimate symbiosis, in which two species live together as a type of composite organism. Lichens are an obligate mutualism between a fungus mycobiont and an alga or blue-green bacterium phycobiont.
Each lichen mutualism is highly distinctive, and can be identified on the basis of its size, shape, color, and biochemistry. Even though lichens are not true “species,” lichenologists have developed systematic and taxonomic treatments of these mutualisms.
The fungal partner in the lichen mutualism gains important benefits through access to photosynthetic products of the alga or blue-green bacterium. The phycobiont profits from the availability of a relatively moist and protected habitat, and greater access to inorganic nutrients.
Lichen biology
The most common fungi in lichens are usually species of Ascomycetes, or a few Basidiomycetes. The usual algal partners are either species of green algae Chlorophyta or blue-green bacteria of the family Cyanophyceae. In general, the fungal partner cannot live without its phycobiont, but the algae is often capable of living freely in moist soil or water. The largest lichens can form a thallus up to 3 feet (1 m) long, although most lichens are smaller than a few inches or centimeters in length. Lichens can be very colorful, ranging from bright reds and oranges to yellows and greens, and white, gray, and black hues.
Most lichens grow very slowly. Lichens in which the phycobiont is a blue-green bacterium have the ability to fix nitrogen gas into ammonia. Some lichens can commonly reach ages of many centuries, especially species living in highly stressful environments, such as alpine or arctic tundra.
Lichens can grow on diverse types of substrates. Some species grow directly on rocks, some on bare soil, and others on the bark of tree trunks and branches. Lichens often grow under exposed conditions that are frequently subjected to periods of drought, and sometimes to extremes of hot and cold. Lichen species vary greatly in their tolerance of severe environmental conditions. Lichens generally respond to environmental extremes by becoming dormant, and then quickly becoming metabolically active again when they experience more benign conditions.
Lichens are customarily divided into three growth forms, although this taxonomy is one of convenience,
and not ultimately founded on systematic relationships. Crustose lichens form a thallus that is closely appressed to the surface upon which they are growing. Foliose lichens are joined to their substrate only by a portion of their thallus, and are somewhat leaf like in appearance. Fruticose lichens rise above their substrate, and are branched and bushy in appearance.
Most lichens regenerate asexually as lichen symbioses, and not by separate reproduction of their mycobiont and phycobiont. Reproduction is most commonly accomplished by small, specialized thallus fragments known as soredia, consisting of fungal tissue enclosing a small number of algal cells. The soredia generally originate within the parent thallus, then grow out through the surface of the thallus, and detach as small bits of tissue that are dispersed by the wind or rain. If the dispersing soredium is fortunate enough to lodge in a favorable microenvironment, it develops into a new thallus, genetically identical to the parent.
Uses of lichens
Because they can colonize bare rocks and other mineral substrates, lichens are important in soil formation during some ecological successions. For example, lichens are among the first organisms to colonize sites as they are released from glacial ice. In such situations lichens can be important in the initial stages of nitrogen accumulation and soil development during postglacial primary succession.
Lichens are an important forage for some species of animals. The best known example of this relationship involves the northern species of deer known as caribou or reindeer (Rangifer tarandus ) and the so-called reindeer lichens (Cladina spp. ) that are one of their most important foods, especially during winter.
Some lichens are very sensitive to air pollutants. Consequently, urban environments are often lack in lichen species. Some ecologists have developed schemes by which the intensity of air pollution can be reliably assayed or monitored using the biological responses of lichens in their communities, based on the health and productivity of these organisms in places variously stressed by toxic pollution. Alternatively, the chemical composition of lichens may be assayed, because their tissues can effectively take up and retain sulfur and metals from the atmosphere.
KEY TERMS
Mutualism —A mutually beneficial relationship between species.
Symbiosis —A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.
Thallus —A single plant body lacking distinct stem, leaves, and roots.
Some lichens are useful as a source of natural dyes. Pigments of some of the more colorful lichens, especially the orange, red, and brown ones, can be extracted by boiling; they can be used to dye wool and other fibers. Other chemicals extracted from lichens include litmus, which was a commonly used acid-base indicator prior to the invention of the pH meter.
Some reindeer lichens, especially Cladina alpestris, are shaped like miniature shrubs and trees. Consequently, these plants are sometimes collected, dried, and dyed, and are used in “landscaping” the layouts for miniature railroads and architectural models.
In addition, lichens add significantly to the aesthetics of the ecosystems in which they occur. The lovely orange and yellow colors of Caloplaca and Xanthoria lichens add much to the ambience of rocky seashores and tundras. And the intricate webs of filamentous Usnea lichens hanging in profusion from tree branches give a mysterious aspect to humid forests. These and other less-charismatic lichens are integral components of their natural ecosystems. They are intrinsically important for this reason, as well as for the relatively minor benefits that they provide to humans.
See also Indicator species.
Resources
BOOKS
Ahmadjian, V. The Lichen Symbiosis. New York:Wiley,1993.
Richardson, D.H.S. Pollution Monitoring with Lichens. United Kingdom: Richmond, 1992.
OTHER
Lichen.com. “Lichens of North America” <http://www.lichen.com/> (accessed December 2, 2006).
University of California Museum of Paleontology. “An Introduction to Lichens: An Alliance between Two Kingdoms” <http://www.ucmp.berkeley.edu/fungi/lichens/lichens.html>(accessed December 2, 2006).
Bill Freedman
Lichens
Lichens
Lichens are the "dynamic duo" of the plant world. They consist of a fungus and a photosynthetic partner (green algae or cyanobacteria , or sometimes both) that live and grow so intimately interconnected that they appear to be a single organism. The fungus surrounds its green partner and shares in the sugars and other carbohydrates that the alga or cyanobacterium produces by photosynthesis. At the same time the fungus provides a protected environment for its food-producing partner and expands its potential habitats. Lichen fungi have a range of nutritional relationships with their associated algae or cyanobacteria from almost pure parasitism to a very benign association called symbiosis, or, more specifically, mutualistic symbiosis, wherein both partners benefit equally from the partnership. Lichens are an extremely successful life form, with thousands of species throughout the world. Some are extremely tiny and inconspicuous, little more than a black or gray smudge, but others can form broad, brightly colored patches or grow to be up to 3 meters long.
Fungal and Algal Components of Lichens
The fungi that form lichens mainly belong to the sac fungi or Ascomycetes, although a few are mushroom-forming fungi, the Basidiomycetes. Each recognizable lichen (with a few interesting exceptions) represents a separate species of fungus; about fourteen thousand are known. The name we give to each lichen is actually the name of its fungal component. There are, however, only a few hundred species of photosynthetic symbionts (photobionts for short) that are involved in lichen partnerships. Lichen fungi are very choosy about their photobionts, and so each recognizable lichen generally contains a specific photobiont. Any given photobiont may, however, be found in many different lichens. A number of lichens associate with a green alga as their main photosynthetic partner but also produce small warts or gall-like bumps containing cyanobacteria, which contribute to the lichen's nutrition and survival.
Lichen Types and Reproduction
Lichens come in many shapes and sizes. They can be roughly grouped into four growth types: crustose, foliose, squamulose, and fruticose. Crustose lichens form a thin or thick crust so tightly attached to the material on which it grows (the substrate) that one has to remove the substrate together with the lichen to make a collection. A foliose lichen is leaflike; it is flat and has a clearly distinguishable upper and lower surface. Foliose lichens are attached to the substrate directly by the lower surface or by means of tiny hairlike structures called rhizines. Squamulose lichens are scalelike with flat lobes as in foliose lichens but more like crustose lichens in size and stature. Fruticose lichens are clearly three dimensional, growing vertically as stalks or shrubby cushions or hanging down from branches or rock faces with hair- or strap-shaped branches.
The arrangement of tissues within most lichens follows the same basic plan. In a typical foliose lichen, a relatively tough upper cortex functions as a protective layer. Below the cortex is a green layer formed by the photo-biont, then comes a cottony medulla , and, finally, on the lower surface, there is usually a protective lower cortex. The rhizines develop from the lower cortex.
Lichen reproduction is rather complex because at least two organisms are involved. The lichen fungus can produce sexual fruiting bodies and spores, but the photobionts reproduce only by cell division within the lichen. When a fungal spore is dispersed by wind or water, it can germinate almost anywhere, but it will form a new lichen only if it encounters the right kind of photobiont. This is a chancy business, and the vast majority of spores perish without forming new lichens.
There is, however, a less perilous way for lichens to reproduce. Any fragment of a lichen containing both the fungus and photobiont has the potential of developing into a new lichen. Many lichens have, in fact, evolved special, easily dispersed fragments in the form of powdery particles (soredia) or spherical to elongated granules or outgrowths (isidia).
Ecology of Lichens
Although lichens as a whole can be found growing on a wide variety of surfaces including rock, bark, wood, leaves, peat, and soil, individual species are more or less confined to specific substrates. Lichens are most conspicuous where other forms of vegetation are sparse, such as the bark of roadside trees or the surface of granitic boulders. They are usually the first organisms to invade entirely bare rock, contributing to the first particles of soil on the rock surface. Lichens carpet the ground in the vast boreal forests of the north, drape the trees and shrubs of foggy coastal regions and tropical cloud forests, and cover the exposed rocks on mountaintops and in the Arctic. They occur from the tropics to the polar regions and from lake edges and seashores to the desert. In general, however, lichens do best where there is much light, moist air, and cool temperatures. Lichens are notoriously sensitive to even small amounts of air pollution, especially the sulfur dioxide so common in cities and near factories, and large cities often have no lichens at all. Their disappearance from an area is an early sign of deteriorating air quality.
Importance and Economic Uses of Lichens
The importance of lichens to the natural world and to humans is not well appreciated except, perhaps, for their role in soil formation. Lichens containing cyanobacteria are important sources of nitrogen in certain forest and desert ecosystems . The ground-dwelling boreal lichens preserve the ground's moisture. Lichens growing in the dry soils of the interior prairies and foothills prevent erosion.
Although lichens have usually been used as human food only in times of emergency (they are unpalatable and have very little nutritional value), a few lichen delicacies are enjoyed by native people of western North America and by the Japanese. Reindeer lichens in the boreal forest, however, are essential as winter forage for caribou herds, which are, in turn, basic to the survival and culture of northern native people. Some lichens yield a chemical called usnic acid, which is an effective antibiotic against certain types of bacteria. Other chemicals produced only by lichens have been used as a source of rusty red, yellow, and purple dyes for coloring wool and silk. Extracts of oakmoss lichens have been used for generations in the perfume industry. The litmus used to determine the acidity of solutions comes from a lichen. The most important use of lichens today, however, is for detecting and monitoring air pollution.
see also Algae; Boreal Forest; Fungi; Plant Community Processes.
Irwin M. Brodo
Bibliography
Casselman, Karen L. Craft of the Dyer: Colour from Plants and Lichens, 2nd ed. New York: Dover Publications Inc., 1993.
Hale, Mason E. How to Know the Lichens, 2nd ed. Dubuque, IA: W. C. Brown Co., 1979.
McCune, Bruce, Linda Geiser, Alexander Mikulin, and Sylvia D. Sharnoff. Macrolichens of the Pacific Northwest. Corvallis, OR: Oregon State University Press, 1997.
Nash, Thomas H., III, ed. Lichen Biology. Cambridge: Cambridge University Press, 1996.
Richardson, D. H. S. "Pollution Monitoring with Lichens." Naturalists' Handbook 19. Slough, England: Richmond Publishing Co., 1992.
Sharnoff, Sylvia D., and Stephen Sharnoff. "Lichens of North America Project" [On-line] 1997. Available at http://www.lichen.com.
Lichens
Lichens
Lichens are an intimate symbiosis , in which two species live together as a type of composite organism . Lichens are an obligate mutualism between a fungus mycobiont and an alga or blue-green bacterium phycobiont.
Each lichen mutualism is highly distinctive, and can be identified on the basis of its size, shape, color , and biochemistry . Even though lichens are not true "species" in the conventional meaning of the word, lichenologists have developed systematic and taxonomic treatments of these mutualisms.
The fungal partner in the lichen mutualism gains important benefits through access to photosynthetic products of the alga or blue-green bacterium. The phycobiont profits from the availability of a relatively moist and protected habitat , and greater access to inorganic nutrients .
Lichen biology
The most common fungi in lichens are usually species of Ascomycetes, or a few Basidiomycetes. The usual algal partners are either species of green algae Chlorophyta or blue-green bacteria of the family Cyanophyceae. In general, the fungal partner cannot live without its phycobiont, but the algae is often capable of living freely in moist soil or water . The largest lichens can form a thallus up to 3 ft (1 m) long, although most lichens are smaller than a few inches or centimeters in length. Lichens can be very colorful, ranging from bright reds and oranges, to yellows and greens, and white, gray, and black hues.
Most lichens grow very slowly. Lichens in which the phycobiont is a blue-green bacterium have the ability to fix nitrogen gas into ammonia . Some lichens can commonly reach ages of many centuries, especially
species living in highly stressful environments, such as alpine or arctic tundra .
Lichens can grow on diverse types of substrates. Some species grow directly on rocks , some on bare soil, and others on the bark of tree trunks and branches. Lichens often grow under exposed conditions that are frequently subjected to periods of drought , and sometimes to extremes of hot and cold. Lichen species vary greatly in their tolerance of severe environmental conditions. Lichens generally respond to environmental extremes by becoming dormant, and then quickly becoming metabolically active again when they experience more benign conditions.
Lichens are customarily divided into three growth forms, although this taxonomy is one of convenience, and is not ultimately founded on systematic relationships. Crustose lichens form a thallus that is closely appressed to the surface upon which they are growing. Foliose lichens are only joined to their substrate by a portion of their thallus, and they are somewhat leaf-like in appearance. Fruticose lichens rise above their substrate, and are much branched and bushy in appearance.
Most lichens regenerate asexually as lichen symbioses, and not by separate reproduction of their mycobiont and phycobiont. Reproduction is most commonly accomplished by small, specialized fragments of thallus known as soredia, consisting of fungal tissue enclosing a small number of algal cells. The soredia generally originate within the parent thallus, then grow out through the surface of the thallus, and detach as small bits of tissue that are dispersed by the wind or rain. If the dispersing soredium is fortunate enough to lodge in a favorable microenvironment, it develops into a new thallus, genetically identical to the parent.
Uses of lichens
Because they are capable of colonizing bare rocks and other mineral substrates, lichens are important in soil formation during some ecological successions. For example, lichens are among the first organisms to colonize sites as they are released from glacial ice . In such situations lichens can be important in the initial stages of nitrogen accumulation and soil development during post-glacial primary succession.
Lichens are an important forage for some species of animals. The best known example of this relationship involves the northern species of deer known as caribou or reindeer (Rangifer tarandus) and the so-called reindeer lichens (Cladina spp.) that are one of their most important foods, especially during winter.
Some species of lichens are very sensitive to air pollutants. Consequently, urban environments are often highly impoverished in lichen species. Some ecologists have developed schemes by which the intensity of air pollution can be reliably assayed or monitored using the biological responses of lichens in their communities. Monitoring of air quality using lichens can be based on the health and productivity of these organisms in places variously stressed by toxic pollution . Alternatively, the chemical composition of lichens may be assayed, because their tissues can effectively take up and retain sulfur and metals from the atmosphere.
Some lichens are useful as a source of natural dyes. Pigments of some of the more colorful lichens, especially the orange, red, and brown ones, can be extracted by boiling and used to dye wool and other fibers. Other chemicals extracted from lichens include litmus, which was a commonly used acid-base indicator prior to the invention of the pH meter.
Some of the reindeer lichens, especially Cladina alpestris, are shaped like miniature shrubs and trees. Consequently, these plants are sometimes collected, dried, and dyed, and are used in "landscaping" the layouts for miniature railroads and architectural models.
In addition, lichens add significantly to the aesthetics of the ecosystems in which they occur. The lovely orange and yellow colors of Caloplaca and Xanthoria lichens add much to the ambience of rocky seashores and tundras. And the intricate webs of filamentous Usnea lichens hanging in profusion from tree branches give a mysterious aspect to humid forests . These and other, less charismatic lichens are integral components of their natural ecosystems. These lichens are intrinsically important for this reason, as well as for the relatively minor benefits that they provide to humans.
See also Indicator species.
Resources
books
Ahmadjian, V. The Lichen Symbiosis. New York: Wiley, 1993.
Richardson, D.H.S. Pollution Monitoring With Lichens. United Kingdom: Richmond, 1992.
Bill Freedman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Mutualism
—A mutually beneficial relationship between species.
- Symbiosis
—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.
- Thallus
—A single plant body lacking distinct stem, leaves, and roots.
Lichens
Lichens
Lichens are an intimate symbiosis, in which two species live together as a type of composite organism. Lichens are an obligate mutualism between a fungus mycobiont and an alga or blue-green bacterium phycobiont.
Each lichen mutualism is highly distinctive, and can be identified on the basis of its size, shape, color, and biochemistry . Even though lichens are not true "species" in the conventional meaning of the word, lichenologists have developed systematic and taxonomic treatments of these mutualisms.
The fungal partner in the lichen mutualism gains important benefits through access to photosynthetic products of the alga or blue-green bacterium. The phycobiont profits from the availability of a relatively moist and protected habitat, and greater access to inorganic nutrients.
The most common fungi in lichens are usually species of Ascomycetes, or a few Basidiomycetes. The usual algal partners are either species of green algae Chlorophyta or blue-green bacteria of the family Cyanophyceae. In general, the fungal partner cannot live without its phycobiont, but the algae is often capable of living freely in moist soil or water. The largest lichens can form a thallus up to 3 ft (1 m) long, although most lichens are smaller than a few inches or centimeters in length. Lichens can be very colorful, ranging from bright reds and oranges, to yellows and greens, and white, gray, and black hues.
Most lichens grow very slowly. Lichens in which the phycobiont is a blue-green bacterium have the ability to fix nitrogen gas into ammonia. Some lichens can commonly reach ages of many centuries, especially species living in highly stressful environments, such as alpine or arctic tundra.
Lichens can grow on diverse types of substrates. Some species grow directly on rocks, some on bare soil, and others on the bark of tree trunks and branches. Lichens often grow under exposed conditions that are frequently subjected to periods of drought, and sometimes to extremes of hot and cold. Lichen species vary greatly in their tolerance of severe environmental conditions. Lichens generally respond to environmental extremes by becoming dormant, and then quickly becoming metabolically active again when they experience more benign conditions.
Lichens are customarily divided into three growth forms, although this taxonomy is one of convenience, and is not ultimately founded on systematic relationships. Crustose lichens form a thallus that is closely appressed to the surface upon which they are growing. Foliose lichens are only joined to their substrate by a portion of their thallus, and they are somewhat leaf-like in appearance. Fruticose lichens rise above their substrate, and are much branched and bushy in appearance.
Most lichens regenerate asexually as lichen symbioses, and not by separate reproduction of their mycobiont and phycobiont. Reproduction is most commonly accomplished by small, specialized fragments of thallus known as soredia, consisting of fungal tissue enclosing a small number of algal cells. The soredia generally originate within the parent thallus, then grow out through the surface of the thallus, and detach as small bits of tissue that are dispersed by the wind or rain. If the dispersing soredium is fortunate enough to lodge in a favorable microenvironment, it develops into a new thallus, genetically identical to the parent.
Because they are capable of colonizing bare rocks and other mineral substrates, lichens are important in soil formation during some ecological successions. For example, lichens are among the first organisms to colonize sites as they are released from glacial ice. In such situations, lichens can be important in the initial stages of nitrogen accumulation and soil development during post-glacial primary succession.
Lichens are important forage for some species of animals. The best known example of this relationship involves the northern species of deer known as caribou or reindeer (Rangifer tarandus ) and the so-called reindeer lichens (Cladina spp. ) that are one of their most important foods, especially during winter.
Some species of lichens are very sensitive to air pollutants. Consequently, urban environments are often highly impoverished in lichen species. Some ecologists have developed schemes by which the intensity of air pollution can be reliably assayed or monitored using the biological responses of lichens in their communities. Monitoring of air quality using lichens can be based on the health and productivity of these organisms in places variously stressed by toxic pollution. Alternatively, the chemical composition of lichens may be assayed, because their tissues can effectively take up and retain sulfur and metals from the atmosphere.
Some lichens are useful as a source of natural dyes. Pigments of some of the more colorful lichens, especially the orange, red, and brown ones, can be extracted by boiling and used to dye wool and other fibers. Other chemicals extracted from lichens include litmus, which was a commonly used acid-base indicator prior to the invention of the pH meter.
Some of the reindeer lichens, especially Cladina alpestris, are shaped like miniature shrubs and trees. Consequently, these plants are sometimes collected, dried, and dyed, and are used in "landscaping" the layouts for miniature railroads and architectural models.
In addition, lichens add significantly to the aesthetics of the ecosystems in which they occur. The lovely orange and yellow colors of Caloplaca and Xanthoria lichens add much to the ambience of rocky seashores and tundras. The intricate webs of filamentous Usnea lichens hanging in profusion from tree branches give a mysterious aspect to humid forests. These and other, less charismatic lichens are integral components of their natural ecosystems. These lichens are intrinsically important for this reason, as well as for the relatively minor benefits that they provide to humans.
Lichens
Lichens
Lichens are composed of fungi and algae. Varying in color from pale whitish green to brilliant red and orange, lichens usually grow attached to rocks and tree trunks and appear as thin, crusty coatings, as networks of small, branched strands, or as flattened, leaf-like forms. Some common lichens are reindeer moss and the red "British soldiers." There are approximately 20,000 known lichen species . Because they often grow under cold, dry, inhospitable conditions, they are usually the first plants to colonize barren rock surfaces.
The fungus and the alga form a symbiotic relationship within the lichen. The fungus forms the body of the lichen, called the thallus. The thallus attaches itself to the surface of a rock or tree trunk, and the fungal cells take up water and nutrients from the environment . The algal cells grow inside the fungal cells and perform photosynthesis , as do other plant cells, to form carbohydrates.
Lichens are essential in providing food for other organisms, breaking down rocks, and initiating soil building. They are also important indicators and monitors of air pollution effects. Since lichens grow attached to rock and tree surfaces, they are fully exposed to airborne pollutants, and chemical analysis of lichen tissues can be used to measure the quantity of pollutants in a particular area. For example, sulfur dioxide , a common emission from power plants , is a major air pollutant. Many studies show that as the concentrations of sulfur dioxide in the air increase, the number of lichen species decreases. The disappearance of lichens from an area may be indicative of other, widespread biological impacts.
Sometimes, lichens are the first organisms to transfer contaminants to the food chain. Lichens are abundant through vast regions of the arctic tundra and form the main food source for caribou (Rangifer tarandus ) in winter. The caribou are hunted and eaten by northern Alaskan Eskimos in spring and early summer. When the effects of radioactive fallout from weapons-testing in the arctic tundra were studied, it was discovered that lichens absorbed virtually all of the radionuclides that were deposited on them. Strontium-90 and cesium-137 were two of the major radionuclide contaminants. As caribou grazed on the lichens, these radionuclides were absorbed into the caribous' tissues. At the end of the winter, caribou flesh contained three to six times as much cesium-137 as it did in the fall. When the caribou flesh was consumed by the Eskimos, the radionuclides were transferred to them as well.
See also Indicator organism; Symbiosis
[Usha Vedagiri ]
RESOURCES
BOOKS
Connell, D. W., and G. J. Miller. Chemistry and Ecotoxicology of Pollution. New York: Wiley, 1984.
Smith, R. L., and T. M. Smith Ecology and Field Biology. 6th ed. Upper Saddle River, NJ: Prentice Hall, 2002.
Weier, T. E., et al. Botany: An Introduction to Plant Biology. New York: Wiley, 1982.
Lichen
Lichen
A lichen is a compound organism built of a fungus intimately entwined about cyanobacteria or cells of an alga. From a distance, a lichen is a brightly colored coat on a tree, a low, bushlike structure, or greenish growths hanging from branches. Lichens are found in diverse places, from tropical rain forests to dry grasslands, shrinking where water is scarce and growing lushly where water is plentiful. They are particularly plentiful in the tundra, where they feed reindeer and are known as "reindeer moss." Some lichens even grow in association with a third organism, such as on the cuticle of an insect.
Each partner of a lichen contributes different synthetic capabilities. The cyanobacterium or algal cell, which comprises less than 10 percent of the mass of the dual organism, is vital to its survival because it can photosynthesize, capturing solar energy. The fungus secretes acids that release minerals and water from rocks. The fungus seems to benefit more from this living partnership, for it grows more slowly alone than when part of a lichen, but the situation is the opposite for the alga or cyanobacterium. Lichens may reproduce with knoblike structures that house sex cells from both components. These reach new sites carried by rain, wind, or animals.
Lichens play key roles in ecosystems . They can survive extremes of altitude and temperature that either component alone cannot. By growing within rock crevices, they contribute to soil formation, the first event as life comes to an area. Despite their hardiness, lichens are exquisitely sensitive to pollution because they cannot detoxify and excrete harmful chemicals.
Humans have used lichens in various ways. As a food, it might have been the biblical "manna from heaven." Various cultures have used lichens to create and dye fabrics, to tan leather, to poison arrows, and to treat infections. About 13,500 types of lichen are recognized.
see also Algae; Forest, Boreal; Fungi; Plant
Ricki Lewis
Bibliography
Milius, Susan. "Yikes! The Lichens Went Flying." Science News 158, no. 9 (26 August 2000): 140.
Purvis, William. Lichens. Washington, DC: Smithsonian Institution Press, 2000.
lichens
lichen
li·chen / ˈlīkən/ • n. 1. a simple slow-growing plant that typically forms a low crustlike, leaflike, or branching growth on rocks, walls, and trees.2. [usu. with adj.] a skin disease in which small pimples or bumps occur close together.DERIVATIVES: li·chened adj. (in sense 1)li·chen·ol·o·gy / ˌlīkəˈnäləjē/ n. (in sense 1) li·chen·ous / -nəs/ adj. (in sense 2).