Lichens are composed of fungi and algae. Varying in color from pale whitish green to brilliant red and orange, lichens usually grow attached to rocks and tree trunks and appear as thin, crusty coatings, as networks of small, branched strands, or as flattened, leaf-like forms. Some common lichens are reindeer moss and the red "British soldiers." There are approximately 20,000 known lichen species. Because they often grow under cold, dry, inhospitable conditions, they are usually the first plants to colonize barren rock surfaces.

The fungus and the alga form a symbiotic relationship within the lichen. The fungus forms the body of the lichen, called the thallus. The thallus attaches itself to the surface of a rock or tree trunk, and the fungal cells take up water and nutrients from the environment. The algal cells grow inside the fungal cells and perform photosynthesis, as do other plant cells, to form...