Harry Resnick, age 12, of Pocatello, Ida., for his question:
HOW DO LICHENS GROW?
Lichens are flowerless plants that actually consist 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 in all parts of the world. Lichens live in many regions in which few other plants can survive.
Some lichens live in the extreme cold of the Arctic while others live in deserts or in the tropics. They can grow in soil, but, unlike most plants, they can also grow on such surfaces as rock and tree bark.
Lichens have no roots, so 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 one tenth of an inch a year to their radius, but a few species grow about 10 times as fast. Some of the more slow growing lichens are about 4,000 years old.
Lichens reproduce in one of three ways. In some types, 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 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 trapped in a crack of a tree or rock, they begin to grow into new lichens.
The third type of lichen reproduction occurs in types that have isidia. Isidia are small, peg shaped growths on the lichen's surface. Like soredia, they are broken off and distributed by the wind.
In the Arctic, lichens cover much of the ground surface. They keep the frozen ground from melting and thus prevent erosion.
Many types 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, some people use lichens in bread and stew.
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 used drugs made from lichens to treat certain lung and skin diseases. Lichen drugs are still used in Finland, Germany and Russia, but in most other countries, similar antibiotics are made from fungi rather than lichen.