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Lichens. Yellow lichens growing on the trunk of a conifer tree in a forest. Lichens comprises an alga and a fungus living in partnership. The alga is confined to a layer of single cells in the body of the fungus. It produces nutrients for the lichen by photosynthesis. Lichens grow on stones and trees from the Arctic to tropical rainforests. Photographed in Mariposa Grove, Yosemite National Park, California, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/ALAN SIRULNIKOFF)