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Lichens are an amazing group of plants that could be the oldest living things on Earth. Their colonies grow at steady and predictable rates, so their age can be estimated with a great degree of accuracy. Some lichens found in the Arctic are thought to be over 5,000 years old.

Lichens appear to be a single plant but in fact are two very different plants, an alga and a fungus, that live as one in a mutually beneficial relationship known as symbiosis. The alga lives among the fungus's mass of threadlike fibers, called hyphae. The fungus uses these hyphae to absorb minerals and moisture from its surroundings. In turn, the alga, which contains chlorophyll, converts these raw materials into food through photosynthesis. Together, the alga and fungus can survive extremely harsh conditions that neither partner could brave on its own.

In all, about 20,000 species of lichen have been identified. They inhabit just about everywhere, from scorching desert sands to icy mountain peaks. Some form colorful crusts or leaf–like structures on rock surfaces or bark. Others, such as "old man's beard," are hairlike and hang from tree branches. One of the showiest is the brilliant, red–crested lichen called "British soldier" that grows on the forest floor, rocks, and decaying wood.

Known as pioneer plants, lichens are active in the initial stages of soil making by colonizing bare rock and slowly breaking it down with acid. This creates pockets where mosses and other small plants can take hold and continue the process. A variety of wildlife depends upon lichens. Some birds use it to line and camouflage their nests, while insects and other invertebrates use it for food and shelter. In arctic regions, reindeer "moss," actually a lichen, is the chief winter food eaten by reindeer, caribou, and moose.

Despite their ability to live in the harshest of conditions, lichens are extremely sensitive to air pollution. About a century ago, polluted air from smokestacks caused lichens to disappear around many European cities, which became known as "lichen deserts." Today, the health of lichens is monitored in American cities to determine the effects of air pollution and acid rain, in the hope that someday this research may help to improve air quality—so that it's more to everyone's lichen.

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