lichen

An organism that forms as a thin layer of fungal and algal substance on soil, rocks, and trees. A lichen is a symbiotic life form in which algae lives inside the filaments of a fungus, and the two grow and reproduce in a mutually beneficial existence. Lichens can be found in every part of the world, but are especially abundant in dry climates such as semi-arid zones and arctic tundra. There is a large number of classified lichens that grow in a variety of shapes and colors.