Helene Newton, age 12, of Clarksville, Virginia, for her question:

What exactly are lichens?

Lichens are pretty little plants that thrive where other plants cannot grow. Some look like dainty finger paintings on the surfaces of bare rocks, others are mistaken for mosses. About 15,000 lichen species have been identified. Each is actually two humble plants, an alga and a fungus, living together in a give and take partnership called symbiosis. The alga has chlorophyll to synthesize plant food from sunlight, air and water. But it has no roots to absorb moisture from the soil. The fungus has no chlorophyll to manufacture its food, but its spongy fibers can trap and hold moisture from the air.     

The two plants grow together as one in order to solve their problems. The fungus provides moisture for the alga, the alga manufactures enough plant food for itself and its partner. In barren regions, lichens break down hard rocks and mix their organic material with the earth's minerals. They start the vital process of building the soil on which all living things depend