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''Crustose and foliose lichens on a wall ()''
Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. Lichens take the external shape of the fungal partner and hence are named based on the fungus. The lichen fungus is typically a member of the Ascomycota, rarely a member of the Basidiomycota. Some place lichens in their own division (Mycophycophyta) but this ignores the fact that the components belong in separate lineages.

Lichens live on the soil of forests, on the surface of rocks, or on walls. They are often the first to settle in uninhabited places, constituting the sole vegetation of some extreme environments such as on high mountains and at high latitudes. Some of them live in the tough conditions of deserts, and others survive on frozen soil of the arctic regions. Some lichens have the aspect of leaves (foliaceous lichens); others cover their support like a crust (crustaceous lichens); others adopt shrubby forms (fruticose lichens); and there are the gelatinous lichens. See lichen types below.

When seen under magnification, a section through the lichen thallus reveals two layers of interlaced filaments (fungus), among which are scattered round green structures named gonidia (sing. gonidium), which are algae. The alga contains chlorophyll that permits the plant to live in a purely mineral environment. Mostly the fungus protects the alga against drought. Soredia (sing. soredium), which contain algal cells as well fungal filaments, come loose from the lichen and serve as a means for their reproduction and dispersal. Lichens are the only food available for many animals living in arctic regions, such as reindeer.

Although they can grow in harsh environments in nature, many lichens are sensistive to man-made pollutants. Hence, they could be used as pollution indicators.


Lichens are classified by growth form informally into:

Squamulose (mostly foliose)
lichen ()
Fruiticose lichen ()
Usnea australis