As Kavaler concludes, ""There seems to be no end to the uses for algae. . . . The simplest and the oldest of all plants existing today, . . . someday they might feed the hungry of the world, improve farmland, produce energy, and help in the conquest of space."" The book touches on the ways these goals might be accomplished--and have been by other cultures, such as the far-apart Aztecs and Kanembou who skimmed the spirulina slime from Lake Texcoco and Lake Chad and made it into loaves. You too can use spirulina or chlorella, mixing it into cookies according to a recipe provided. Nearly a thousand years ago a monk in North Vietnam found that planting ferns in rice paddies made the rice grow. The explanation is that algae attached to the fern leaves fix nitrogen; using the same principle an Ohio high school student won the grand prize at an International Science and Engineering Fair in 1979 with her algae fertilizer. On space ships algae can provide oxygen and use up human wastes, and might be used to grow other plants for food on long trips. Algae might even make the desert bloom, says Kavaler, by ""turn[ing] bad water into good so that. it could be used for crops."" This is not well explained, but Kavaler is less interested throughout in clarifying processes than in previewing possibilities. Readers drawn in by her come-on opening (""Have you ever seen a green polar bear?"") might be bored by her abrupt background briefs on fossils (to support the information that algae fossils revealed life on earth to be probably about three-and-a-half billion years old), satellites (used to locate plankton), food chains (compared here to ""magic boxes"" in a discussion of red-tide poisoning), and the like. Otherwise it's an undemanding if somewhat formless survey.