ALGAE. Some algae are simple, one-celled plants visible only under a microscope; others are large seaweeds up to 300 feet long. Most of nearly 18,000 species of algae live in water. Since light is necessary for manufacturing food, algae generally grow in fairy shallow water. A few kinds of seaweeds, however, grow at depths of more than 400 feet where the light is very dim. Algae contain chlorophyll, the green pigment enables plants to manufacture food. Algae may also have red, blue, brown, or yellow pigments which also give some groups of algae their common names: blue-green, yellow-green, green, red, and brown. Other groups (classes) of algae include diatoms, dinoflagellates, stoneworts, and euglenoids. Each is discussed separately in these volumes. Some kinds of algae, such as Nostoc and Vaucheria, grow on or in moist soil. A teaspoon of such soil may contain as many as a million of these tiny one-celled plants. Some algae live in hot springs; others grow on the surface of snow. Protococcus and others grow on trees, rocks, or tile roofs, especially in shaded areas. A covering of gelatin over their cells filters out harmful or dying rays of the sunlight. There are kinds of algae that grow in the roots of air plants. Some live inside the bodies of small animals or in the intestines of larger animals. Some cling to the bodies of aquatic animal, such as crustaceans and turtles, and one kind grows on the hair of a sloth, giving the animal a greenish tinge. Algae also vary greatly in form. Some live as independent single cells. The cells of others are loosely attached to make long filaments. Some algae grow as flat, leaf like sheets; others are greatly branched. No algae have roots, stems and leaves like the flowering plants. Some kinds float int the water, while others are attached to roots, sticks, or other plants. Some kinds have whip like flagella which they use in swimming. They may also have sensitive eye spots. Kinds that both plant and animals characteristics are sometimes called “plantimals,” or some biologists put them in a special group called the Protista to distinguish them from living things that are definitely either plant or animal. Algae reproduce in several ways. Some of the one-celled kinds merely divide into two cells. Other kinds produce spores. Many also have sexual reproduction or may have and alternation of generations between asexual and sexual stages. Floating and swimming algae (phytoplankton) are the basic food of animals in both fresh and salt water. Some large fishes and even whales eat only plankton. Ordinarily the plankton is eaten by small animals which in turn are eaten by larger animals. This is called a food chain, and algae are the basic link in most food chains. People in various parts of the world also eat algae-the seaweeds. These are eaten raw in salads, cooked into soups or puddings, or brewed to make tea-like tea. Algae are also processed to make food for livestock. Piles of seaweeds are allowed to to decay partially, then are spread over fields as fertilizer. Agar and algin are important products obtained from seaweeds. Sometimes algae grow so thickly over the leaves of trees that the interfere with photosynthesis. The red tide, which has killed millions of fish in the Gulf of Mexico, is said to be caused by a dinoflagellate and the pollution of water by algae is also often a serious problem. The decay of tremendous growths or “blooms” of algae in ponds or lakes may use up the oxygen and cause the fish to suffocate.