Lichens

Lichens sometimes are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner usually either a green alga or cyanobacterium. 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 in otherwise mesic habitats. Lichens are widespread and may be long-lived; however, many species are also vulnerable to environmental disturbance, and may be useful to scientists in assessing the effects of air pollution, ozone depletion, and metal contamination.

The body of most lichens is quite different from those of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth.

Source: en.wikipedia.org/wiki/Lichen - 25.06.2010