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Rhizocarpon Geographicum 3876
A lichen is two organisms functioning as one stable unit. Lichens comprise a fungus on the surface, living in a symbiotic relationship with algae or cyanobacterium (or both in some instances). They are very sensitive to air pollution. The green/yellow lichen in this image is a crust lichen called Rhizocarpon Geographicum, commonly known as map lichen.