94,129
edits
Changes
From Gardenology.org - Plant Encyclopedia and Gardening Wiki
Jump to navigationJump to searchno edit summary
[[Image:Haeckel Lichenes.jpg|thumb|230px|"Lichenes" from [[Ernst Haeckel]]'s ''Artforms of Nature'', 1904]]'''Lichens''' are [[symbiosis|symbiotic]] associations of a [[fungus]] (the mycobiont) with a [[Photosynthesis|photosynthetic]] partner (the photobiont also known as the phycobiont) that can produce food for the lichen from sunlight. The photobiont is usually either [[green algae]] or [[cyanobacteria]]. A few lichens are known to contain yellow-green algae or, in one case, a [[brown alga]]. Some lichens contain both green algae and cyanobacteria as photobionts; in these cases, the cyanobacteria symbiont component may specialize in fixing atmospheric nitrogen for metabolic use. The word is pronounced as though it is spelled "liken" ({{IPA2|laɪkən}}).
The body (thallus) of most lichens is quite different from that of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth (Sanders 2001). The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations; however, the algal cells are never enclosed inside the fungal cells themselves. (The fungus ''[[Geosiphon|Geosiphon pyriforme]]'' is unique in that it encloses a cyanobacterial symbiont inside its cells, but this exceptional association is not usually considered to be a lichen.) The fungus may or may not penetrate into the algal cells with fine [[Hyphae|hyphal protrusions]].
In general, the symbiosis is considered obligatory for successful growth and reproduction of the fungus; however, the significance for the algal symbiont is less clear. For some algae, the symbiosis may be obligatory for survival in a particular habitat; in other cases, the symbiosis might not be advantageous for the alga. Thus, there is some controversy as to whether the lichen symbiosis should be considered an example of [[mutualism]] or [[parasitism]] or [[commensalism]]. Nonetheless, the lichen is typically a highly stable association which probably extends the ecological range of both partners.
There is evidence that lichens might involve a controlled form of parasitism of the algal cells. In laboratory settings, cyanobacteria grow faster when they are alone rather than when they are part of a lichen. But there is also a mutualistic side to the relationship: the fungus part of the lichen provides the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its [[Substrate (biology)|substrate]]. As for the alga, it uses the minerals and water to make food for the fungus and itself.
Lichens take the external shape of the fungal partner and hence are named based on the fungus. The fungus most commonly forms the majority of a lichen's bulk, though in filamentous and gelatinous lichens this may not always be the case. The lichen fungus is typically a member of the [[Ascomycota]]—rarely a member of the [[Basidiomycota]], and then termed '''basidiolichens''' to differentiate them from the more common '''ascolichens'''. Formerly, some lichen taxonomists placed lichens in their own division, the '''Mycophycophyta''', but this practice is no longer accepted because the components belong to separate [[Scientific classification|lineages]]. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum<ref name=Lutzoni_2004>{{cite journal | author=Lutzoni et al | title= Assembling the fungal tree of life: progress, classification, and evolution of subcellular traits | journal=Amer J Bot | year=2004 | pages=1446-1480 | volume=91 }}</ref>.
The algal or cyanobacterial cells are [[photosynthesis|photosynthetic]], and as in higher plants they [[redox|reduce]] atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the [[substratum]]. If a [[Cyanobacteria|cyanobacterium]] is present, as a primary partner or another symbiont in addition to green alga. as in certain tripartite lichens, they can [[nitrogen fixation|fix atmospheric nitrogen]], complementing the activities of the green alga.
==Morphology and structure==
[[image:N2 Lichen.jpg|thumb|230 px|right|Crustose and foliose lichens on a wall]]
Lichens are often the [[ecological succession|first to settle]] in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the arctic regions. Recent [[ESA]] research shows that lichen can even endure extended exposure to space. Some lichens have the aspect of leaves (foliose lichens); others cover the substratum like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (''illustration, right'').
Although the form of a lichen is determined by the [[DNA|genetic material]] of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of [[hypha]]e. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called [[morphogenesis]] (Brodo, Sharnoff & Sharnoff, 2001). In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.
There is evidence to suggest that the lichen symbiosis is [[parasitism|parasitic]] rather than [[mutualism|mutualistic]] (Ahmadjian 1993). The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of [[nutrient]] exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (ibid.)
Under magnification, a section through a typical foliose lichen [[Thallus (tissue)|thallus]] reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the [[cortex]]. In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called [[cephalodia/cephalodium]]. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an [[haustorium]]. Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the [[medulla]]. Beneath the medulla, the bottom surface resembles the upper surface and is called the [[lower cortex]], again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as [[rhizines]], which serve to attach the thallus to the substrate on which it grows. Lichens also sometimes contain structures made from fungal [[metabolite]]s, for example crustose lichens sometimes have a [[polysaccharide]] layer in the cortex. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.
==Reproduction==
[[Image:Lichen reproduction.jpg|thumb|230px|Thalli and apothecia on a foliose lichen]]
Many lic
hens reproduce asexually, either by [[vegetative reproduction]] or through the dispersal of [[diaspore]]s containing algal and fungal cells. ''Soredia'' (singular [[soredium]]) are small groups of algal cells surrounded by fungal filaments that form in structures called ''soralia'', from which the soredia can be dispersed by wind. Another form of diaspore are ''isidia'', elongated outgrowths from the thallus that break off for mechanical dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.
Many lichen fungi appear to reproduce sexually in a manner typical of fungi, producing spores that are presumably the result of sexual fusion and [[meiosis]]. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form. This may be a common form of reproduction in basidiolichens, which form fruitbodies resembling their nonlichenized relatives. Among the ascolichens, [[spore]]s are produced in spore-producing bodies, the three most common spore body types are the '''apothecia''', '''perithecia''' and the '''pycnidia'''.
[http://news.bbc.co.uk/2/hi/uk_news/scotland/5356368.stm]
==Ecology==
Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing.
A major ecophysiological advantage of lichens is that they are poikilohydric (''poikilo''- variable, ''hydric''- relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation. Like some [[moss|mosses]], [[Marchantiophyta|liverworts]], [[fern]]s, and a few "[[resurrection plant]]s", upon desiccation, lichens enter a metabolic suspension or stasis (known as [[cryptobiosis]]) in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.
Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs and monuments. Many lichens also grow as [[epiphyte]]s (''epi''- on the surface, ''phyte''- plant) on other plants, particularly on the trunks and branches of trees. When growing on other plants, lichens are not [[parasite]]s; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of genus [[Cladoniaceae|Cladina]] (reindeer lichens), however, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability (that is, longevity) of their substratum is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, [[vascular plant|vascular (higher) plant]] seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
[[Image:CladonioPinetum.jpg|right|thumb|230px|Pine forest with lichen ground-cover]]
Lichens may be eaten by some animals, such as [[reindeer]], living in [[arctic]] regions. The [[larva|larvae]] of a surprising number of [[Lepidoptera]] species feed exclusively on lichens. These include [[Common Footman]] and [[Marbled Beauty]]. However, lichens are very low in protein and high in carbohydrates, making them unsuitable for some animals. Lichens are also used by the [[Northern Flying Squirrel]] for nesting, food, and a water source during winter.
Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured [[pollution|pollutants]]. Hence, they have been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly degrade their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of [[weathering]] by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on [[Mount Rushmore National Memorial]] that requires the employment of mountain-climbing conservators to clean the monument.
Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce [[herbivory]] or kill bacteria. These compounds are very useful for lichen identification, and have (or had) economic importance as [[dye]]s or primitive [[antibiotics]]. Extracts from many ''[[Usnea]]'' [http://www.tcbmed.com/Newsletters/Volume4-Issue4-Usnea.html|//www.tcbmed.com/Newsletters/Volume4-Issue4-Usnea.html] species were used to treat wounds in Russia in the mid-twentieth century. [[Orcein]] and other lichen dyes have largely been replaced by synthetic versions [http://waynesword.palomar.edu/ecoph5.htm].
The [[European Space Agency]] has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen – ''[[Rhizocarpon geographicum]]'' and ''[[Xanthoria elegans]]'' – were sealed in a capsule and launched on a Russian Soyuz rocket on [[31 May]] [[2005]]. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit. [http://www.esa.int/esaHS/SEMUJM638FE_index_0.html]
==Growth form==
Lichens are informally classified by growth form into:
* crustose (paint-like, flat), e.g., ''[[Caloplaca flavescens]]''
* [[filamentose]] (hair-like), e.g., ''[[Ephebe lanata]]''
* foliose (leafy), e.g., ''[[Hypogymnia physodes]]''
* fruticose (branched), e.g., ''[[Cladina evensii]]'', ''[[C. subtenuis]]'', and ''[[Usnea australis]]''
* [[leprose]] (powdery), e.g., ''[[Lepraria incana]]''
* [[squamulose]] (consisting of small scale-like structures, lacking a lower cortex), e.g., ''[[Normandina pulchella]]''
* gelatinous lichens, in which the cyanobacteria produce a [[polysaccharide]] that absorbs and retains water.
==See also==
[[Ethnolichenology]]
==Images of Lichens==
<gallery>
Image:Lichen_squamulose.jpg|A foliose lichen on basalt.
Image:Usnea australis.jpg|''Usnea australis'', a fruticose form, growing on a tree branch
Image:Rhizocarpon geographicum01.jpg|[[Map lichen]] (''Rhizocarpon geographicum'') on rock
Image:Hyella caespitosa hypae.jpg|The cyanobacterium ''Hyella caespitosa'' with fungal hyphae in the lichen ''Pyrenocollema halodytes''
Image:LogLichen.jpg|Foliose lichen grows on a fallen log.
Image:Kananakislichen.jpg|Lichen found in the Canadian Rockies
Image:Lichenlimestone.JPG|Lichens on Limestones of Alta Murgia-Southern Italy
Image:Plants flowers ice rocks lichens 209.jpg| A lichen commonly referred to as 'British Soldiers'. Notice the red tips.
Image:Plants flowers ice rocks lichens 230.jpg|One type of growth habit.
</gallery>
== Linked References ==
{{Reflist|2}}
==References==
* Ahmadjian, V. 1993. ''The Lichen Symbiosis''. New York: John Wiley & Sons.
* Brodo, I.M., S.D. Sharnoff, and S. Sharnoff, 2001. <cite>Lichens of North America</cite>. Yale University Press, New Haven.
http://www.newscientistspace.com/article/dn8297 <cite>Hardy lichen shown to survive in space</cite>
http://www.lichen.com
* Gilbert, O. 2004. ''The Lichen Hunters.'' The Book Guild Ltd. England.
* Hawksworth, D.L. and Seaward, M.R.D. 1977. ''Lichenology in the British Isles 1568 - 1975.'' The Richmond Publishing Co. Ltd., Richomd, 1977.
* Knowles, M.C. 1929. "The lichens of Ireland." ''Proceedings of the Royal Irish Academy'' '''38''':1 - 32.
* Purvis, O.W., Coppins, B.J., Hawksworth, D.L., James, P.W. and Moore, D.M. (Editors) 1992. ''The Lichen Flora of Great Britain and Ireland.'' Natural History Museum, London.
* Sanders, W.B. 2001. "Lichens: interface between mycology and plant morphology." ''Bioscience'' '''51''': 1025-1035.
* Seaward, M.R.D. 1984. "Census Catalogue of Irish Lichens." ''Glasra'' '''8'''1 - 32.
==External links==
{{commonscat|Lichenes}}
* [http://bugs.bio.usyd.edu.au/Mycology/Plant_Interactions/Lichen/lichenBiology.shtml University of Sydney lichen biology]
* [http://www.lichenology.info Lichens of Belgium, Luxembourg and northern France]
* [http://thavibu.com/caliciales/ Crustose species of lichen order Caliciales in Norway]
* [http://www.esa.int/esaHS/SEMUJM638FE_index_0.html ESA article on lichen survivability in low earth orbit]
* [http://home.arcor.de/stefan.wic/diplomarbeit.pdf diploma thesis about physiology of lichens]
* http://www.theBLS.org.uk
* [http://www.blackturtle.us/DV_PLANTS/LICHEN/ Lichen Information and Pictures at blackturtle.us]
* [http://pnwfungi.org/ Pacific Northwest Fungi Online Journal, includes articles on lichens]
* [http://www.lichenologie.org/ French Association for Lichenology]
* [http://www.naturlink.pt/canais/Artigo.asp?iArtigo=2977&iLingua=2 Fungi that discovered agriculture]
*[http://www.chilebosque.cl/lich_en.html Lichens of Chile]
[[Category:Cryptogams]]
[[Category:Lichens| ]]
[[Category:Mycology]]
[[Category:Symbiosis]]
[[Category:Indicator species]]
[[Category:Bioindicators]]
The body (thallus) of most lichens is quite different from that of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth (Sanders 2001). The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations; however, the algal cells are never enclosed inside the fungal cells themselves. (The fungus ''[[Geosiphon|Geosiphon pyriforme]]'' is unique in that it encloses a cyanobacterial symbiont inside its cells, but this exceptional association is not usually considered to be a lichen.) The fungus may or may not penetrate into the algal cells with fine [[Hyphae|hyphal protrusions]].
In general, the symbiosis is considered obligatory for successful growth and reproduction of the fungus; however, the significance for the algal symbiont is less clear. For some algae, the symbiosis may be obligatory for survival in a particular habitat; in other cases, the symbiosis might not be advantageous for the alga. Thus, there is some controversy as to whether the lichen symbiosis should be considered an example of [[mutualism]] or [[parasitism]] or [[commensalism]]. Nonetheless, the lichen is typically a highly stable association which probably extends the ecological range of both partners.
There is evidence that lichens might involve a controlled form of parasitism of the algal cells. In laboratory settings, cyanobacteria grow faster when they are alone rather than when they are part of a lichen. But there is also a mutualistic side to the relationship: the fungus part of the lichen provides the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its [[Substrate (biology)|substrate]]. As for the alga, it uses the minerals and water to make food for the fungus and itself.
Lichens take the external shape of the fungal partner and hence are named based on the fungus. The fungus most commonly forms the majority of a lichen's bulk, though in filamentous and gelatinous lichens this may not always be the case. The lichen fungus is typically a member of the [[Ascomycota]]—rarely a member of the [[Basidiomycota]], and then termed '''basidiolichens''' to differentiate them from the more common '''ascolichens'''. Formerly, some lichen taxonomists placed lichens in their own division, the '''Mycophycophyta''', but this practice is no longer accepted because the components belong to separate [[Scientific classification|lineages]]. Neither the ascolichens nor the basidiolichens form monophyletic lineages in their respective fungal phyla, but they do form several major solely or primarily lichen-forming groups within each phylum<ref name=Lutzoni_2004>{{cite journal | author=Lutzoni et al | title= Assembling the fungal tree of life: progress, classification, and evolution of subcellular traits | journal=Amer J Bot | year=2004 | pages=1446-1480 | volume=91 }}</ref>.
The algal or cyanobacterial cells are [[photosynthesis|photosynthetic]], and as in higher plants they [[redox|reduce]] atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, provides minerals obtained from the [[substratum]]. If a [[Cyanobacteria|cyanobacterium]] is present, as a primary partner or another symbiont in addition to green alga. as in certain tripartite lichens, they can [[nitrogen fixation|fix atmospheric nitrogen]], complementing the activities of the green alga.
==Morphology and structure==
[[image:N2 Lichen.jpg|thumb|230 px|right|Crustose and foliose lichens on a wall]]
Lichens are often the [[ecological succession|first to settle]] in places lacking soil, constituting the sole vegetation in some extreme environments such as those found at high mountain elevations and at high latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the arctic regions. Recent [[ESA]] research shows that lichen can even endure extended exposure to space. Some lichens have the aspect of leaves (foliose lichens); others cover the substratum like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (''illustration, right'').
Although the form of a lichen is determined by the [[DNA|genetic material]] of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of [[hypha]]e. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called [[morphogenesis]] (Brodo, Sharnoff & Sharnoff, 2001). In a few remarkable cases, a single lichen fungus can develop into two very different lichen forms when associating with either a green algal or a cyanobacterial symbiont. Quite naturally, these alternative forms were at first considered to be different species, until they were first found growing in a conjoined manner.
There is evidence to suggest that the lichen symbiosis is [[parasitism|parasitic]] rather than [[mutualism|mutualistic]] (Ahmadjian 1993). The photosynthetic partner can exist in nature independently of the fungal partner, but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of [[nutrient]] exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (ibid.)
Under magnification, a section through a typical foliose lichen [[Thallus (tissue)|thallus]] reveals four layers of interlaced fungal filaments. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the [[cortex]]. In lichens that include both green algal and cyanobacterial symbionts, the cyanobacteria may be held on the upper or lower surface in small pustules called [[cephalodia/cephalodium]]. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae, and in some cases penetrated by an [[haustorium]]. Beneath this algal layer is a third layer of loosely interwoven fungal hyphae without algal cells. This layer is called the [[medulla]]. Beneath the medulla, the bottom surface resembles the upper surface and is called the [[lower cortex]], again consisting of densely packed fungal hyphae. The lower cortex often bears rootlike fungal structures known as [[rhizines]], which serve to attach the thallus to the substrate on which it grows. Lichens also sometimes contain structures made from fungal [[metabolite]]s, for example crustose lichens sometimes have a [[polysaccharide]] layer in the cortex. Although each lichen thallus generally appears homogeneous, some evidence seems to suggest that the fungal component may consist of more than one genetic individual of that species. This seems to also be true of the photobiont species involved.
==Reproduction==
[[Image:Lichen reproduction.jpg|thumb|230px|Thalli and apothecia on a foliose lichen]]
Many lic
hens reproduce asexually, either by [[vegetative reproduction]] or through the dispersal of [[diaspore]]s containing algal and fungal cells. ''Soredia'' (singular [[soredium]]) are small groups of algal cells surrounded by fungal filaments that form in structures called ''soralia'', from which the soredia can be dispersed by wind. Another form of diaspore are ''isidia'', elongated outgrowths from the thallus that break off for mechanical dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves by wind action, to resume growth when moisture returns.
Many lichen fungi appear to reproduce sexually in a manner typical of fungi, producing spores that are presumably the result of sexual fusion and [[meiosis]]. Following dispersal, such fungal spores must meet with a compatible algal partner before a functional lichen can form. This may be a common form of reproduction in basidiolichens, which form fruitbodies resembling their nonlichenized relatives. Among the ascolichens, [[spore]]s are produced in spore-producing bodies, the three most common spore body types are the '''apothecia''', '''perithecia''' and the '''pycnidia'''.
[http://news.bbc.co.uk/2/hi/uk_news/scotland/5356368.stm]
==Ecology==
Lichens must compete with plants for access to sunlight, but because of their small size and slow growth, they thrive in places where higher plants have difficulty growing.
A major ecophysiological advantage of lichens is that they are poikilohydric (''poikilo''- variable, ''hydric''- relating to water), meaning that though they have little control over the status of their hydration, they can tolerate irregular and extended periods of severe desiccation. Like some [[moss|mosses]], [[Marchantiophyta|liverworts]], [[fern]]s, and a few "[[resurrection plant]]s", upon desiccation, lichens enter a metabolic suspension or stasis (known as [[cryptobiosis]]) in which the cells of the lichen symbionts are dehydrated to a degree that halts most biochemical activity. In this cryptobiotic state, lichens can survive wider extremes of temperature, radiation and drought in the harsh environments they often inhabit.
Lichens do not have roots and do not need to tap continuous reservoirs of water like most higher plants, thus they can grow in locations impossible for most plants, such as bare rock, sterile soil or sand, and various artificial structures such as walls, roofs and monuments. Many lichens also grow as [[epiphyte]]s (''epi''- on the surface, ''phyte''- plant) on other plants, particularly on the trunks and branches of trees. When growing on other plants, lichens are not [[parasite]]s; they do not consume any part of the plant nor poison it. Some ground-dwelling lichens, such as members of genus [[Cladoniaceae|Cladina]] (reindeer lichens), however, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Stability (that is, longevity) of their substratum is a major factor of lichen habitats. Most lichens grow on stable rock surfaces or the bark of old trees, but many others grow on soil and sand. In these latter cases, lichens are often an important part of soil stabilization; indeed, in some desert ecosystems, [[vascular plant|vascular (higher) plant]] seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water.
[[Image:CladonioPinetum.jpg|right|thumb|230px|Pine forest with lichen ground-cover]]
Lichens may be eaten by some animals, such as [[reindeer]], living in [[arctic]] regions. The [[larva|larvae]] of a surprising number of [[Lepidoptera]] species feed exclusively on lichens. These include [[Common Footman]] and [[Marbled Beauty]]. However, lichens are very low in protein and high in carbohydrates, making them unsuitable for some animals. Lichens are also used by the [[Northern Flying Squirrel]] for nesting, food, and a water source during winter.
Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured [[pollution|pollutants]]. Hence, they have been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly degrade their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of [[weathering]] by which rocks are gradually turned into soil. While this contribution to weathering is usually benign, it can cause problems for artificial stone structures. For example, there is an ongoing lichen growth problem on [[Mount Rushmore National Memorial]] that requires the employment of mountain-climbing conservators to clean the monument.
Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce [[herbivory]] or kill bacteria. These compounds are very useful for lichen identification, and have (or had) economic importance as [[dye]]s or primitive [[antibiotics]]. Extracts from many ''[[Usnea]]'' [http://www.tcbmed.com/Newsletters/Volume4-Issue4-Usnea.html|//www.tcbmed.com/Newsletters/Volume4-Issue4-Usnea.html] species were used to treat wounds in Russia in the mid-twentieth century. [[Orcein]] and other lichen dyes have largely been replaced by synthetic versions [http://waynesword.palomar.edu/ecoph5.htm].
The [[European Space Agency]] has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen – ''[[Rhizocarpon geographicum]]'' and ''[[Xanthoria elegans]]'' – were sealed in a capsule and launched on a Russian Soyuz rocket on [[31 May]] [[2005]]. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit. [http://www.esa.int/esaHS/SEMUJM638FE_index_0.html]
==Growth form==
Lichens are informally classified by growth form into:
* crustose (paint-like, flat), e.g., ''[[Caloplaca flavescens]]''
* [[filamentose]] (hair-like), e.g., ''[[Ephebe lanata]]''
* foliose (leafy), e.g., ''[[Hypogymnia physodes]]''
* fruticose (branched), e.g., ''[[Cladina evensii]]'', ''[[C. subtenuis]]'', and ''[[Usnea australis]]''
* [[leprose]] (powdery), e.g., ''[[Lepraria incana]]''
* [[squamulose]] (consisting of small scale-like structures, lacking a lower cortex), e.g., ''[[Normandina pulchella]]''
* gelatinous lichens, in which the cyanobacteria produce a [[polysaccharide]] that absorbs and retains water.
==See also==
[[Ethnolichenology]]
==Images of Lichens==
<gallery>
Image:Lichen_squamulose.jpg|A foliose lichen on basalt.
Image:Usnea australis.jpg|''Usnea australis'', a fruticose form, growing on a tree branch
Image:Rhizocarpon geographicum01.jpg|[[Map lichen]] (''Rhizocarpon geographicum'') on rock
Image:Hyella caespitosa hypae.jpg|The cyanobacterium ''Hyella caespitosa'' with fungal hyphae in the lichen ''Pyrenocollema halodytes''
Image:LogLichen.jpg|Foliose lichen grows on a fallen log.
Image:Kananakislichen.jpg|Lichen found in the Canadian Rockies
Image:Lichenlimestone.JPG|Lichens on Limestones of Alta Murgia-Southern Italy
Image:Plants flowers ice rocks lichens 209.jpg| A lichen commonly referred to as 'British Soldiers'. Notice the red tips.
Image:Plants flowers ice rocks lichens 230.jpg|One type of growth habit.
</gallery>
== Linked References ==
{{Reflist|2}}
==References==
* Ahmadjian, V. 1993. ''The Lichen Symbiosis''. New York: John Wiley & Sons.
* Brodo, I.M., S.D. Sharnoff, and S. Sharnoff, 2001. <cite>Lichens of North America</cite>. Yale University Press, New Haven.
http://www.newscientistspace.com/article/dn8297 <cite>Hardy lichen shown to survive in space</cite>
http://www.lichen.com
* Gilbert, O. 2004. ''The Lichen Hunters.'' The Book Guild Ltd. England.
* Hawksworth, D.L. and Seaward, M.R.D. 1977. ''Lichenology in the British Isles 1568 - 1975.'' The Richmond Publishing Co. Ltd., Richomd, 1977.
* Knowles, M.C. 1929. "The lichens of Ireland." ''Proceedings of the Royal Irish Academy'' '''38''':1 - 32.
* Purvis, O.W., Coppins, B.J., Hawksworth, D.L., James, P.W. and Moore, D.M. (Editors) 1992. ''The Lichen Flora of Great Britain and Ireland.'' Natural History Museum, London.
* Sanders, W.B. 2001. "Lichens: interface between mycology and plant morphology." ''Bioscience'' '''51''': 1025-1035.
* Seaward, M.R.D. 1984. "Census Catalogue of Irish Lichens." ''Glasra'' '''8'''1 - 32.
==External links==
{{commonscat|Lichenes}}
* [http://bugs.bio.usyd.edu.au/Mycology/Plant_Interactions/Lichen/lichenBiology.shtml University of Sydney lichen biology]
* [http://www.lichenology.info Lichens of Belgium, Luxembourg and northern France]
* [http://thavibu.com/caliciales/ Crustose species of lichen order Caliciales in Norway]
* [http://www.esa.int/esaHS/SEMUJM638FE_index_0.html ESA article on lichen survivability in low earth orbit]
* [http://home.arcor.de/stefan.wic/diplomarbeit.pdf diploma thesis about physiology of lichens]
* http://www.theBLS.org.uk
* [http://www.blackturtle.us/DV_PLANTS/LICHEN/ Lichen Information and Pictures at blackturtle.us]
* [http://pnwfungi.org/ Pacific Northwest Fungi Online Journal, includes articles on lichens]
* [http://www.lichenologie.org/ French Association for Lichenology]
* [http://www.naturlink.pt/canais/Artigo.asp?iArtigo=2977&iLingua=2 Fungi that discovered agriculture]
*[http://www.chilebosque.cl/lich_en.html Lichens of Chile]
[[Category:Cryptogams]]
[[Category:Lichens| ]]
[[Category:Mycology]]
[[Category:Symbiosis]]
[[Category:Indicator species]]
[[Category:Bioindicators]]
