Any of about 15,000 species of small, colourful, scaly plants that consist of a symbiotic association of algae (usually green) and fungi (see fungus ).
These extremely hardy, slow growers often are pioneer species in sparse environments such as mountaintops and the far North. Fungal cells, anchored to the substrate with hairlike growths (rhizines), form the base. In the body ( thallus ), numerous algal cells are distributed among fewer fungal cells. Through photosynthesis the algal cells provide simple sugars and vitamins for both partners in this symbiotic association. The fungal cells protect the algal cells from environmental extremes. Lichens may form a thin, crustlike, tightly bound covering over their substrate (e.g., cracks in rocks), or they may be small and leafy, with loose attachments to the substrate. Their colours range from brown to bright orange or yellow. In far northern Europe and Asia, lichens provide two-thirds of caribou and reindeer food. They have been the source of medicines and dyes.