Dawn Johnston, age 11, of Tucson, Ariz., for her question:
HOW DOES A LICHEN GROW?
A lichen is a flowerless plant that consists of two plants living together as a single unit. These plants are an alga and a fungus. The alga can make its own food, but it needs water to grow. The fungus absorbs water rapidly, but it cannot make its own food.
There are about 18,000 different kinds of lichens. They can grow in soil or on such surfaces as tree bark or rock.
Because lichens have no roots, they can grow only when moistened by dew or rain. When a moist lichen absorbs sunlight, the alga part produces food for the plant by the process of photosynthesis. The food passes to the fungus and thus enables the entire plant to grow.
Dry lichens do not grow. But they can survive in extreme temperatures that kill wet lichens.
Most lichens add about 0.1 inch a year to their radius, but a few species grow about 10 times as fast. Many crustose lichens grow only .01 inch in radius yearly. Some of these slow growing lichens are about 4,000 years old.
Lichens reproduce in one of three ways.
In some lichens, the fungus releases spores into the air. A spore is a small body that can develop into a new organism. If a fungus spore lands next to a suitable alga, a new lichen may develop.
A second way that lichens can reproduce is by means of cells called soredia. Soredia consist of several alga cells surrounded by a few strands of fungus. They grow on the surface of the lichen and are broken off and carried away by the wind or water. If soredia get trapped in a crack of a tree or rock, they begin to grow into new lichens.
The third type of lichen reproduction occurs in species that have insidia. Insidia are tiny, peg shaped growths on the lichen's surface. They are broken off and distributed by wind.
Man uses lichens in many ways. Also, animals and the environment benefit from these tiny plants.
In the Arctic, lichens cover much of the ground surface. They keep the frozen ground from melting and thus prevent erosion.
Many species of lichens, including reindeer moss, provide winter food for caribou and reindeer in the Arctic. In other regions, many insects, as well as snails and slugs, eat lichens.
The Japanese use lichens in making soups and salads that they consider a delicacy. In the Middle East, people use lichens in bread and stew.
Some types of lichens are used as ingredients of perfumes and soaps in Europe.
For more than 2,000 years, doctors have used drugs made from lichens to treat certain lung and skin diseases. Lichen drugs are still used in parts of Finland, Germany and Russia. In other countries, antibiotics made from fungi have replaced lichen drugs.