In Iceland and Greenland, and the vast tundras of the arctics, they are the dominant forms of plant life. The two exchange food materials. Fungus has remarkable power to absorb and store moisture. The alga, using that water, and using carbon dioxide from the air, manufactures food. The fungus absorbs the excess food and produces an acid which eats into the earth or wood or rock upon which it grows, anchoring it firmly in place.
Lichens grow very, very slowly.
A few are bright yellow or orange. Many are weirdly beautiful when studied under a hand lens, resembling miniature forests of dead trees. Lichens were used by the ancient Greeks, Romans and Hebrews, and are still used as a source of brown, blue and purple dyes. They are the pioneers of the plant world, the first living things to colonize barren earth or a rocky area. First come lichens, then moss, then grass and other flowering plants, and finally forest vegetation.