Lichen

Lichens are symbiotic organisms made up by the association of microscopic green algae (sometimes cyanobacterium) 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 in the soil of forests, on the surface of rocks or on walls. They are the first to settle in uninhabited places, often constituting the sole vegetation of high mountains. Some of them live in the rough conditions of the deserts, and others survive in the 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.

When seen by the microscope, the crosscut of lichens unfolds two layers of interlaced filaments (fungus), among which there are round green cells named gonidia (sing. gonidium), which are algae.

The thallus of a lichen results from the association of an alga and a fungus (mushroom). The alga contains the chlorophyll, which permits the plant to live in a purely mineral environment. Mostly the fungus is an ascomycete[?] that protects the alga against drought. Soredia (sing. soredium), which contain alga cells as well fungus filaments, get loose from the lichen and serve as a means for their reproduction. Lichens are most of the time 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-caused pollution. Hence, they can be used as pollution indicators.

Classification

Lichens are classified informally into:

• Foliose (leafy), e.g., Hypogymnia phsodes
• Crustose (crusty-flat), e.g., Caloplaca Heppiana
• Squamulose (mixed), e.g., Cladonia floerkeana
• Filamentous (hairy), e.g., '
• Leprose (powdery), e.g., '
• Fruiticose (branched), e.g., Cladina evensii and C. subtenuis