lichen

Description:

Lichens ( /ˈlaɪkən/, sometimes /ˈlɪtʃən/) are composite organisms consisting of a symbiotic organism composed of a fungus (the mycobiont) with a photosynthetic partner (the photobiont or phycobiont), usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc). The morphology, physiology and biochemistry of lichens are very different from those of the isolated fungus and alga in culture. Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps. However, they are also abundant as epiphytes on leaves and branches in rain forests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces (e.g., Collema) in otherwise mesic habitats. Lichens are widespread and may be long-lived; however, many are also vulnerable to environmental disturbance, and may be useful to scientists in assessing the effects of air pollution, ozone depletion, and metal contamination. Lichens have also been used in making dyes and perfumes, as well as in traditional medicines

Habitat:

The European Space Agency has discovered that lichens can survive unprotected in space. In an experiment led by Leopoldo Sancho from the Complutense University of Madrid, two species of lichen—Rhizocarpon geographicum and Xanthoria elegans—were sealed in a capsule and launched on a Russian Soyuz rocket on 31 May 2005. Once in orbit the capsules were opened and the lichens were directly exposed to the vacuum of space with its widely fluctuating temperatures and cosmic radiation. After 15 days the lichens were brought back to earth and were found to be in full health with no discernible damage from their time in orbit.

Notes:

Lichens are informally classified by growth form into: crustose (paint-like, flat), e.g., Caloplaca flavescens filamentous (hair-like), e.g., Ephebe lanata foliose (leafy), e.g., Hypogymnia physodes fruticose (branched), e.g., Cladonia evansii, C. subtenuis, and Usnea australis leprose (powdery), e.g., Lepraria incana squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella gelatinous lichens, in which the cyanobacteria produce a polysaccharide that absorbs and retains water.