Roots have been important sources of food for humans since prehistoric times, and some, such as the carrot, have been in cultivation in Europe for at least 2,000 years. A number of cultivated root crops involve biennials (i.e., plants that complete their life cycles from seed to flowering and back to seed in two seasons). Such plants store food in a swollen taproot during the first year of growth, and then the stored food is used in the production of flowers in the second season. Among the best-known biennial root crops are sugar beets, beets, turnips, rutabagas, parsnips, horseradishes, and carrots. Other important root crops include sweet potatoes, yams, and cassava. Cassava (Fig. 5.18), from which tapioca is made, forms a major part of the basic diet for millions of inhabitants of the tropics. With a minimum of human labor, it yields more starch per hectare (about 45 metric tons, the equivalent of 20 tons per acre) than any other cultivated crop. Minor root crops, including relatives of wild mustards, nasturtiums, and sorrel, are cultivated in South America and other parts of the world.
Several well-known spices, including sassafras, sarsa-parilla, licorice, and angelica, are obtained from roots. Sweet potatoes are used in the production of alcohol in Japan. Some important red to brownish dyes are obtained from roots of members of the Madder Family (Rubiaceae), to which coffee plants belong. Drugs obtained from roots include aconite, ipecac, gentian, and reserpine, a tranquilizer. A valuable insecticide, rotenone, is obtained from the barbasco plant, which has been cultivated for centuries as a fish poison by primitive South American tribes. When thrown into a dammed stream, the roots containing rotenone cause the fish to float but in no way poison them for human consumption. In tobacco plants, nicotine produced in the roots is transported to the leaves. Other uses of roots are discussed in Chapter 24.
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