Lichen

From Wikinfo

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 (Mycophycophyta), but this ignores the fact that the components belong in separate lineages.

Lichens live on various surfaces: forest soil, trees, rocks, and walls. They are often the first to settle in uninhabited places, constituting the sole vegetation in some extreme environments such as at high mountain elevations 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 (foliose lichens); others cover the substratum like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are 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 have some potential as pollution indicators.

Classification

Lichens are classified by growth form informally into:

• foliose (leafy), e.g., Hypogymnia phsodes
• crustose (paint-like, flat), e.g., Caloplaca Heppiana
• squamulose (mixed foliose and crustose), e.g., Cladonia floerkeana
• filamentous (hair-like), e.g.,
• leprose (powdery), e.g.,
• fruiticose (branched), e.g., Cladina evensii, C. subtenuis, and Usnea australis

Image:Lichen squamulose reduced.jpg	Image:Lichen fruiticose reduced.jpg
Squamulose (mostly foliose) lichen (enlarge)	Fruiticose lichen (enlarge) Usnea australis

References

• Adapted from the Wikipedia article, "Lichen" http://en.wikipedia.org/wiki/Lichen, used under the GNU Free Documentation License