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Lichens 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.
“Crusty” lichens simply form crusts over their substrates, without distinctive lower surfaces. Some are composed entirely of grains of varying sizes.
Spotted on Dec 16, 2011
Submitted on Dec 16, 2011