Dawn Anderson, age 13, of Dodge City, Kan., for her question:

HOW DO LICHENS 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 species of lichens. They can grow in soil, but, unlike most plants, they can also grow on such surfaces as tree bark and 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 radii, but a few species grow about 10 times as fast. Some of the 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.

Lichens also reproduce by means of cells called soredia. Soredia consist of several algal 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 isidia. Isidia are tiny, peg shaped growths on the lichen's surface. Like soredia, they are broken off and distributed by the wind and water.

There are three groups of lichens: crustose lichens that lie flat on the surface on which they grow; foliose lichens that resemble leaves; and fruiticose lichens that look like small shrubs.

Lichens actually have no leaves, roots or stems. Most have an outer layer of green, brown, yellow or gray fungal cells, which cover a layer of green or blue green algal cells. Below the algal cells lies a whitish storage zone that contains food. This layer consists mainly of thread' like fungal structures called hyphae.

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.

One type of lichen is an important ingredient of perfumes and soaps made in Europe. It has a mossy fragrance and prevents scents from evaporating quickly.

For more than 2.000 years, doctors have used drugs made from certain lichens to treat certain lung and skin diseases.