A lichen is a compound organism built of a fungus intimately entwined about cyanobacteria or cells of an alga. From a distance, a lichen is a brightly colored coat on a tree, a low, bushlike structure, or greenish growths hanging from branches. Lichens are found in diverse places, from tropical rain forests to dry grasslands, shrinking where water is scarce and growing lushly where water is plentiful. They are particularly plentiful in the tundra, where they feed reindeer and are known as "reindeer moss." Some lichens even grow in association with a third organism, such as on the cuticle of an insect.
Each partner of a lichen contributes different synthetic capabilities. The cyanobacterium or algal cell, which comprises less than 10 percent of the mass of the dual organism, is vital to its survival because it can photosynthesize, capturing solar energy. The fungus secretes acids that release minerals and water from rocks. The fungus seems to benefit more from this living partnership, for it grows more slowly alone than when part of a lichen, but the situation is the opposite for the alga or cyanobacterium. Lichens may reproduce with knoblike structures that house sex cells from both components. These reach new sites carried by rain, wind, or animals.
Lichens play key roles in ecosystems . They can survive extremes of altitude and temperature that either component alone cannot. By growing within rock crevices, they contribute to soil formation, the first event as life comes to an area. Despite their hardiness, lichens are exquisitely sensitive to pollution because they cannot detoxify and excrete harmful chemicals.
Humans have used lichens in various ways. As a food, it might have been the biblical "manna from heaven." Various cultures have used lichens to create and dye fabrics, to tan leather, to poison arrows, and to treat infections. About 13,500 types of lichen are recognized.
Milius, Susan. "Yikes! The Lichens Went Flying." Science News 158, no. 9 (26 August 2000): 140.
Purvis, William. Lichens. Washington, DC: Smithsonian Institution Press, 2000.