Lichen-covered tree, Tresco, Isles of Scilly, UK. Tresco (Ynys Skaw is the second largest Island of the Isles of Scilly, Cornwall, UK Grey, leafy Parmotrema perlatum on upper half of trunk; yellowy-green Flavoparmelia caperata on middle and lower half and running up the extreme right side; and the fruiticose Ramalina farinacea

"Lichenes" from Ernst Haeckel's Artforms of Nature, 1904

Lichens (IPA: /ˈlaɪkən/^[1] or /lɪtʃ. Flavoparmelia caperata ( L) Hale, 1986 Synonym Parmelia caperata (L Ernst Heinrich Philipp August Haeckel ( February 16, 1834 — August 9, 1919)also written von Haeckel, was an eminent German Kunstformen der Natur ( German: Art Forms of Nature) is a book of lithographic and Autotype prints by German biologist ən/^[2]) are symbiotic associations of a fungus (the mycobiont) with a photosynthetic partner (the photobiont also known as the phycobiont) that can produce food for the lichen from sunlight. This article is about the biological phenomenon for other uses see Symbiosis (disambiguation The term symbiosis (from the Greek A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. The photobiont is usually either green alga or cyanobacterium. The green algae (singular green alga) are the large group of Algae from which the Embryophytes (higher plants emerged Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of Bacteria that obtain their energy A few lichens are known to contain yellow-green algae or, in one case, a brown alga. Yellow-green algae or xanthophytes are an important group of Heterokont algae The Phaeophyceae or brown algae, (singular alga is a large group of mostly marine multicellular Algae including many Seaweeds of colder 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 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, in almost all kinds, the algal cells are never enclosed inside the fungal cells themselves. It has been suggested that the fungus is sometimes penetrated by haustoria by the mycobiont, but with the development of electron microscopy there is little solid evidence of this, and if true, is an isolated occurrence and in any event is entirely unecessesary. Thus lichens are poikilohydric, that is, capable of surviving extremely low levels of water content. However, the re-configuration of membranes following a period of dehydration requires several minutes at least. During this period a "soup" of metabolites from both the mycobiont and phycobiont leaks into the extracellar spaces. This is readily available to both bionts to uptake essential metabolic products ensuring a perfect level of mutualism Definitive data derived from poikilohydric canopy mosses is provided by Coxson (1990)showing leaching from the canopy mosses in Guadaloupe of numerous matabolites immediately following rehydration. Not only do the two bionts profit, but also the all the other epiphytic organisms from the nutrient rich leachate. This fundamental phenomenon also points to a possible explanation of lichen evolution from its original phycobiont and mycobiont componants with its subsequent migration from an aquatic environment to dry land. Thus, during repeated periods of low levels of hydration in an alga and the resultant leakage of beneficial metabolites to an adjacent aquatic fungi, the mutalistic "marriage" slowly became constant.

In the natural environment, lichen "provides" the alga with water and minerals that the fungus absorbs from whatever the lichen is growing on, its substrate. In Biology a substrate is the surface a plant or animal lives upon As for the alga, it uses the minerals and water to make food for the fungus and itself. Algal and fungal components of some lichens have been cultured separately under laboratory conditions, but in the natural environment of a lichen, neither can grow and reproduce without a symbiotic partner. Indeed, although strains of cyanobacteria found in various cyanolichens are often closely related to one another, they differ from the most closely related free-living strains [1]. The lichen association is a close symbiosis: It extends the ecological range of both partners and is obligatory for their growth and reproduction in natural environoments. Propagules ("diaspores") typically contain cells from both partners, although the fungal components of so-called "fringe species" rely instead on algal cells dispersed by the "core species".

There has nonetheless been controversy as to whether the lichen combination should be considered an example of mutualism or commensalism or even parasitism. Mutualism is a Biological interaction between individuals of two different Species, where both individuals derive a fitness benefit for example increased In Ecology, commensalism is a kind of symbiotic relationship between two organisms where one benefits and the other is not significantly harmed or helped (like a bird Parasitism is a type of symbiotic relationship between Organisms of different Species. An observation offered in support of this is that cyanobacteria in laboratory settings can grow faster when they are alone rather than when they are part of a lichen. The same, however, might be said of isolated skin cells growing in laboratory culture, which grow more quickly than similar cells that are integrated into a functional tissue. However, from the work of Coxson (see above) mutualism would appear to best summarise our current knowledge.

Lichens are named based on the fungal component, which plays the primary role in determining the lichens form. The fungus typically comprises the majority of a lichen's bulk, though in filamentous and gelatinous lichens this is not always 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. Ascomycota is a Division / Phylum of Fungi, and subkingdom Dikarya, whose members are commonly known as the Sac Fungi. Basidiomycota is one of two large phyla that together with the Ascomycota, comprise the subkingdom Dikarya, which were in general what were called Basidiolichens are Lichenized members of the Basidiomycota, a much smaller group of lichens than the far more common ascolichens in the Ascomycota. 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 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^[3]. Even more unusual than basidiolichens is the fungus Geosiphon pyriforme, a member of the Glomeromycota that is unique in that it encloses a cyanobacterial symbiont inside its cells. Geosiphon is a Genus of the Phylum Glomeromycota, Order Archaeosporales, Family Geosiphonaceae Glomeromycota (informally glomeromycetes) is one of seven currently recognized phyla within the kingdom Fungi, with approximately 200 described species Geosiphon is not usually considered to be a lichen, and its peculiar symbiosis was not recognized for many years. The genus is more closely allied to endomycorrhizal genera. A mycorrhiza (Greek for fungus roots coined by Frank 1885 typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally

The algal or cyanobacterial cells are photosynthetic, and as in higher plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy. Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state 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 substrate. In Biology a substrate is the surface a plant or animal lives upon If a cyanobacterium is present, as a primary partner or another symbiont in addition to green alga as in certain tripartite lichens, they can fix atmospheric nitrogen, complementing the activities of the green alga. Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of Bacteria that obtain their energy Nitrogen fixation is the process by which Nitrogen is taken from its natural relatively inert molecular form (N2 in the atmosphere and converted into

Morphology and structure

Crustose and foliose lichens on a wall

Lichens are often the 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. Ecological succession, a fundamental concept in Ecology, refers to more-or-less predictable and orderly changes in the composition or structure of an ecological community 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. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member ^[4] Some lichens have the aspect of leaves (foliose lichens); others cover the substrate like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (illustration, right). In Biology a substrate is the surface a plant or animal lives upon ^[5]

Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known When grown in the laboratory in the absence of its photobiont, a lichen fungus develops as an undifferentiated mass of hyphae. A hypha (plural hyphae) is a long branching filamentous cell of a Fungus, and also of unrelated Actinobacteria. If combined with its photobiont under appropriate conditions, its characteristic form emerges, in the process called morphogenesis (Brodo, Sharnoff & Sharnoff, 2001). MOrphogenesis is an EP by Industrial Black metal band.And Oceans. 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 parasitic rather than mutualistic (Ahmadjian 1993). Parasitism is a type of symbiotic relationship between Organisms of different Species. Mutualism is a Biological interaction between individuals of two different Species, where both individuals derive a fitness benefit for example increased However, this now needs to be re-examined in light of Coxons work. 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. A nutrient is food or chemicals that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment 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 reveals four layers of interlaced fungal filaments. For the chronologer/historian occasionally mentioned in the works of early Christian writers see Thallus (historian. The uppermost layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex, which can reach several hundred μm in thickness. In Botany, the cortex is the outer of the stem or Root of a plant bounded on the outside by the epidermis and on the inside by the Endodermis ^[6] This cortex may be further topped by an epicortex 0. 6-1μm thick in some Parmeliaceae, which may be with or without pores, and is secreted by cells - it is not itself cellular. ^[6] 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. In botany a haustorium (plural haustoria) is the Hyphal tip of a parasitic Fungus or of the root of a Parasitic plant (such 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 metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex. Metabolomics is the "systematic study of the unique chemical fingerprints that specific cellular processes leave behind" - specifically the study of their small-molecule metabolite Polysaccharides are relatively complex Carbohydrates They are Polymers made up of many Monosaccharides joined together by Glycosidic bonds 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

Thalli and apothecia on a foliose lichen

Many lichens reproduce asexually, either by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells. Vegetative reproduction is a type of Asexual reproduction found in plants and is also called vegetative propagation or vegetative multiplication. In Biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions 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. Soredia are common reproductive structures of Lichens Lichens reproduce asexually by employing simple fragmentation and production of soredia and insidia 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. In Biology or life science meiosis (pronounced my-oh-sis is a process of reductional division in which the number of chromosomes per cell is cut in half 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, spores are produced in spore-producing bodies, the three most common spore body types are the apothecia, perithecia and the pycnidia. In Biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions [2]

For reproduction, lichen possess insidia, soredia, and undergo simple fragmentation. These structures are also composed of a fungal hyphae wrapped around cyanobacteria. (Eichorn, Evert, and Raven, 2005) While the reproductive structures are all composed of the same components(Mycobiont and Photobiont) they are each unique in other ways. Insidia are small outgrowths on the exterior of the lichen. Soredia are powdery propagules that are released from the top of the thallus⁽¹⁾. In order to establish the lichen, the soredia propagules must contain both the photobiont and the mycobiont⁽²⁾. ^[7]

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. Desiccation is the state of extreme dryness or the process of extreme drying Like some mosses, liverworts, ferns, and a few "resurrection plants", 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. Mosses are small soft Plants that are typically 1–10  cm (0 The Marchantiophyta are a division of Bryophyte Plants commonly referred to as hepatics or liverworts. A fern is any one of a group of about 20000 Species of Plants classified in the phylum or division Pteridophyta, also known as Filicophyta Cryptobiosis is an ametabolic state of life entered by an animal's Organisms in response to adverse environmental conditions such as Desiccation, Freezing 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 epiphytes (epi- on the surface, phyte- plant) on other plants, particularly on the trunks and branches of trees. An epiphyte is an organism that grows upon or attaches to a living plant When growing on other plants, lichens are not parasites; they do not consume any part of the plant nor poison it. Parasitism is a type of symbiotic relationship between Organisms of different Species. Some ground-dwelling lichens, such as members of the subgenus Cladina (reindeer lichens), however, produce chemicals which leach into the soil and inhibit the germination of plant seeds and growth of young plants. Cladoniaceae is a family of Lichens in the order Lecanorales. Stability (that is, longevity) of their substrate is a major factor of lichen habitats. In Biology a substrate is the surface a plant or animal lives upon 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 (higher) plant seeds cannot become established except in places where lichen crusts stabilize the sand and help retain water. Vascular plants (also known as tracheophytes or higher plants) are those Plants that have lignified tissues for conducting

Pine forest with lichen ground-cover

Lichens may be eaten by some animals, such as reindeer, living in arctic regions. The Arctic is the Region around the Earth 's North Pole, opposite the Antarctic region around the South Pole. The larvae of a surprising number of Lepidoptera species feed exclusively on lichens. A larva ( Latin; plural larvae) is a juvenile form of Animal with indirect development, undergoing Metamorphosis (for example Lepidoptera is an order of Insect that includes Moths and butterflies. These include Common Footman and Marbled Beauty. The Common Footman ( Eilema lurideola) is a Moth of the family Arctiidae. The Marbled Beauty ( Cryphia domestica) is a Moth of the family Noctuidae. 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. The Northern flying squirrel ( Glaucomys sabrinus) is one of two species of the genus Glaucomys, the only Flying squirrels found in North

Although lichens typically grow in naturally harsh environments, most lichens, especially epiphytic fruticose species and those containing cyanobacteria, are sensitive to manufactured pollutants. Pollution is the introduction of contaminants into an environment that causes instability disorder harm or discomfort to the physical systems or living organisms they are in Hence, they have been widely used as pollution indicator organisms. When growing on mineral surfaces, some lichens slowly decompose their substrate by chemically degrading and physically disrupting the minerals, contributing to the process of weathering by which rocks are gradually turned into soil. Weathering is the decomposition of earth rocks, Soils and their Minerals through direct contact with the planet's Atmosphere. 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. Mount Rushmore National Memorial, near Keystone, South Dakota, is a monumental Granite sculpture by Gutzon Borglum (1867–1941 located

Many lichens produce secondary compounds, including pigments that reduce harmful amounts of sunlight and powerful toxins that reduce herbivory or kill bacteria. Herbivory is a form of Predation in which an Organism, known as a herbivore, consumes principally Autotrophs ref name=Campbell>Campbell These compounds are very useful for lichen identification, and have had economic importance as dyes or primitive antibiotics. A dye can generally be described as a Colored substance that has an affinity to the substrate to which it is being applied In modern usage an antibiotic is a Chemotherapeutic agent with activity against Microorganisms such as Bacteria, fungi or Protozoa Extracts from many Usnea [3] species were used to treat wounds in Russia in the mid-twentieth century. Usnea is the generic and scientific name for several species of Lichen in the family Parmeliaceae, that generally grow hanging from tree branches Orcein and other lichen dyes have largely been replaced by synthetic versions [4]. Orcein, also archil, orchil, lacmus, Litmus, Citrus Red 2, and C

The European Space Agency has discovered that lichens can survive unprotected in space. The European Space Agency ( ESA) established in 1975 is an intergovernmental organisation dedicated to the exploration of space, currently with 17 member 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. The map lichen is a Species of Lichen, Rhizocarpon geographicum, which grows on rocks in mountainous areas of low pollution Events 1279 BC - Rameses II (The Great (19th dynasty becomes pharaoh of Ancient Egypt. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. 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. [5]

Growth form

Lichens are informally classified by growth form into:

• crustose (paint-like, flat), e. crustose - having the form of a crust as in the crust on a bread g. , Caloplaca flavescens
• filamentous (hair-like), e. g. , Ephebe lanata
• foliose (leafy), e. g. , Hypogymnia physodes
• fruticose (branched), e. g. , Cladonia evansii, 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. Polysaccharides are relatively complex Carbohydrates They are Polymers made up of many Monosaccharides joined together by Glycosidic bonds

Paleontology

The extreme habitats that lichens inhabit are not ordinarily conducive to producing fossils. ^[8] Though lichens may have been among the first photosynthesizers to colonize land, the oldest fossil lichens in which both symbiotic partners have been recovered date to the Early Devonian Rhynie chert, about 400 million years old. The Devonian is a geologic period and system of the Paleozoic era spanning from to  million years ago. The Rhynie chert is an Early Devonian Lagerstätte found near the village of Rhynie, Aberdeenshire, Scotland, containing exceptionally ^[9] The slightly older fossil Spongiophyton has also been interpreted as a lichen on morphological^[10] and isotopic^[11] grounds, although the isotopic basis is decidedly shaky. Spongiophyton was a thallose fossil of the early to mid Devonian, which is notoriously difficult to classify ^[12] It has been suggested - although not yet proven - that the even older fossil Nematothallus was a lichen. Nematothallus is a form Genus comprising cuticle-like Fossils It was first described by Lang in 1933 and Edwards and Rose (1984 both provided thorough ^[13]

It has also been claimed that Ediacaran fossils were lichens;^[14] although this claim was met with scepticism and has since been retracted by its author. The Ediacara (ˌiːdɪˈækərə formerly Vendian) biota are ancient lifeforms of the Ediacaran Period which represent the earliest known complex ^[13] A lichen-like symbiosis, however, has been observed in marine fossils from the Ediacaran, 600 million years ago. The Ediacaran Period (ˌiːdiˈækərən named after the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic ^[15]

Lichen examples

• Iceland moss
• Oakmoss
• Parmelia (lichen)
• Reindeer moss
• Rock tripe

Gallery

Xanthoparmelia cf. Iceland moss ( Cetraria islandica) is a Lichen whose erect or ascending foliaceous habit gives it something of the appearance of a Moss, whence probably Oakmoss, also known as Evernia prunastri, is a type of Lichen used extensively in modern Perfumery. This article is about the genus of lichen Parmelia for other uses see Parmelia (disambiguation. Cladonia rangifera undersideJPG|thumb|The underside of C rangiferina ]] Cladonia rangiferina, also known as Reindeer lichen (c Rock tripe is a Lichen of the Genus Umbilicaria that grows on rocks lavicola, a foliose lichen, on basalt.	Usnea australis, a fruticose form, growing on a tree branch	Map lichen (Rhizocarpon geographicum) on rock	The cyanobacterium Hyella caespitosa with fungal hyphae in the lichen Pyrenocollema halodytes
Physcia millegrana (a foliose lichen), with an unlichenized polypore fungus (bottom right), on a fallen log. The map lichen is a Species of Lichen, Rhizocarpon geographicum, which grows on rocks in mountainous areas of low pollution	Reindeer moss (Cladonia rangiferina)	Hypogymnia cf. Cladonia rangifera undersideJPG|thumb|The underside of C rangiferina ]] Cladonia rangiferina, also known as Reindeer lichen (c tubulosa with Bryoria sp. and Tuckermannopsis sp. in the Canadian Rockies	Crustose lichens on limestone in Alta Murgia-Southern Italy
Cladonia cf. cristatella, a lichen commonly referred to as 'British Soldiers'. Notice the red tips.	Foliose lichens on rock growing outward and dying in the center. These lichens are at least several decades old.	Letharia sp. with Bryoria sp. on pine branches near Blackpine Lake, Washington	Lichen growing on a tree branch
Reddish-colored lichen on volcanic rock in Craters of the Moon National Monument (Idaho, USA)

See also

• Ethnolichenology
• Lichenometry
• Lichenology

Notes

Those interested in lichens should see Banfield et al. , 1999, "Biological impact on mineral dissolution: Application of the lichen model to understanding mineral weathering in the rhizosphere. " Proc. Natl. Acad. Sci. 96:3404-3411.

References

• Ahmadjian, V. 1993. The Lichen Symbiosis. New York: John Wiley & Sons.
• Brodo, I. M. , S. D. Sharnoff, and S. Sharnoff, 2001. Lichens of North America. Yale University Press, New Haven.
• http://www.newscientistspace.com/article/dn8297 Hardy lichen shown to survive in space
• 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.
• Kershaw, K. A. "Physiological Ecology of Lichens", 1985. Cambridge University Press Cambridge.
• 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 81 - 32.

External links

• Lichens of Russia - South Ural
• University of Sydney lichen biology
• Lichens of Belgium, Luxembourg and northern France
• Crustose species of lichen order Caliciales in Norway
• ESA article on lichen survivability in low earth orbit
• diploma thesis about physiology of lichens - (in German)
• The British Lichen Society
• Lichen Information and Pictures at blackturtle.us
• Pacific Northwest Fungi Online Journal, includes articles on lichens
• International Association for Lichenology*
• French Association for Lichenology* Warning : The site seems unavailable anymore
• Fungi that discovered agriculture
• Lichens of Chile