Trees do more than make life pleasant; they make life【C1】______ . Trees get【C2】______ through their roots and, primarily through their leaves, they 【C3】______ carbon dioxide【C4】______ the air. Then, with the action of sunlight on cells 【C5】 chlorophyll and other materials, chemical reactions 【C6】______ , and oxygen is 【C7】______ . Through photosynthesis, an acre of trees produces enough oxygen to sustain three 【C8】______ .Eating wood might seem odd,【C9】______ many low calorie breads and other baked goods now include powdered cellulose【C10】______ acts as a bulking agent to help hold the foods together.Trees have always been green machines,【C11】______ substances that humans【C12】______ to use. The ancient Greeks, for example, treated pain【C13】______ a tea made by boiling willow leaves and bark; a tea modern scientists now know contains silicon, a precursor of acetylsalicylic acid aspirin. For centuries, the Chinese have【C14】______ medicines from the ginkgo tree. More recently, researchers isolated and synthesized the chemical ginkgolide from the tree【C15】______ use in treating asthma, toxic shock and other ills.【C16】______ scientists unlock the secrets of trees, they 【C17】______ surprising facts. In the early 1980s David Rhoades, a chemical【C18】______ at the University of Washington, Seattle, discovered that trees send unseen signals to each other. When willows are attacked by webworms and tent caterpillars, they【C19】______ a chemical that alerts nearby willows. The neighboring trees respond by pumping more tannin into their leaves, making them more【C20】______ for the insects to digest.【C1】