Lichens
LichensLICHEN. On places like tree trunks, rocks, old boards, and also on the ground grow strange splotches of various-colored plant life called lichens. They are of great scientific interest because they are not single plants; instead, each lichen is formed of a fungus and an alga living together so intimately as to seem a single plant. The lichens are one of the best illustrations of symbiosis, the intimate living together of two different kinds of organisms. The fungus makes the bulk of the body with its interwoven threads, and in the meshes of the threads live the algae. The special fungi that take part in this arrangement are almost never found growing separately, but the algae are found growing free.
There are about 15,000 different kinds of lichens distributed worldwide. They are especially suited for growth in harsh regions, where few plants can survive. They grow farther north and south than most plants, as well as higher on mountains. One unusual type grows completely submerged in the cold coastal waters of Antarctica. Some lichens inhabit the Earth's driest deserts, where they grow almost entirely underground and obtain light and moisture through small openings in the ground. Few grow near cities because most cannot survive in industrial air pollution. There are notable exceptions, however: in England, for example, Lecanora conizaeoides is actually confined to areas of high pollution.
The body of the lichen, the thallus, has three basic growth forms. These forms are crustose, foliose, and fruticose. Each form is adapted to live under different moisture conditions. The crustose lichens resemble a crust that has become attached to a surface and are well suited to dry areas. Foliose, or leafy, lichens need much greater amounts of water. Some of them even grow...
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