Tara Larson, age 13, of Casper, Wyo., for her question:
HOW DO LICHENS GROW?
Lichens are flowerless plants that are actually two plants living together as a single unit. These plants are an alga and a fungus. Because neither of these plants have roots, lichens 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 in radius yearly. It seems a bit hard to believe, but some of the slow growing lichens of the world 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 consists of several algae cells surrounded by a few strands of fungus. They grow on the surface of the lichen and are broken off and carried by the wind or water. If soredia get trapped in a crack of a tree or rock, they may start to grow into new lichens.
The third type of lichen reproduction happens in species that have isidia. Isidia are tiny, peg shaped growths that can be found on the lichens's surface. Like soredia, the isidia are sometimes broken off and distributed by the wind and water.
There are about 18,000 species of lichens. They can grow in soil, but unlike most plants, they can also grow on such unusual planting surfaces as rock and tree bark.
Lichens live in many parts of the world where few other plants can survive. Some species live only in the tropics or on deserts while others can survive only in the extreme cold of the Arctic.
Botanists recognize three groups of lichens according to their general appearance: crustose, foliose and fruticose lichens.
Crustose lichens are crustlike plants that lie flat on the substratum or surface on which they grow. Foiliose lichens resemble leaves that stick up somewhat from the substratum. Fruticose lichens look like small shrubs growing on the substratum.
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 in the Arctic provide winter food for caribou and reindeer.
The Japanese use a type of lichen in making soups and salads. They consider it a great delicacy. In the Middle East, people use lichens in bread and as a seasoning in stew.
Lichen is an important ingredient of some European perfumes. It has a mossy fragrance and prevents scents from evaporating quickly.
For more than 2,000 years, doctors have used drugs made from lichens to treat certain lung and skin diseases.