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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 taxonomists place lichens in their own division (thallus reveals four layers of interlaced filaments (fungus). The upper layer is formed by densely agglutinated fungal hyphae building a protective outer layer. It is called cortex. Beneath the upper cortex you will see the algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. The algae contain chlorophyll that permits the plant to live in a purely mineral environment. Mostly the fungus protects the alga against drought. Beneath the algal there is a layer of loosely interwoven fungal hyphae without algal cells. this layer is called medulla. The lower surface resembles the upper, it is the lower cortex consisting of densely packed fungal hyphae. The lower cortex often bears structures serving the attachment of the thallus to the substrate on which it grows, such as rhizinae or a tomentum. 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 have some potential as pollution indicators.
Lichens are classified by growth form informally into:
Squamulose (mostly foliose)
lichen (enlarge) |
Fruticose lichen (enlarge)
Usnea australis |
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