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[[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}}).

[[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 differenc3@ $xqwsdrfujhiklt from that of either the fungus or alga growing separately, and may jndjvjdl.jvihucfkujh;po[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]].

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 sifdjldsjgldutrijdlfedurvival 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.

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.

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.