Levetin−McMahon: Plants and Society, Fifth EditionIII. Plants As a Source of Food13. Legumes © The McGraw−Hill Companies, 200820713 LEGUMES CHAPTER OUTLINE Characteristics of the Legume Family 208 Important Legume Food Crops 209Beans and Peas 209 A CLOSER LOOK 13.1The Nitrogen Cycle 210 Peanuts 212 Soybeans 213 A CLOSER LOOK 13.2Harvesting Oil 214Other Legumes of Interest 216 A Supertree for Forestry 216 Forage Crops 217 Beans of the Future 217 Chapter Summary 219 Review Questions 219 Further Reading 219 KEY CONCEPTS 1. The legumes are second only to the cereals in their importance in human nutrition and are an excellent source of high-quality protein. 2. Nitrogen fixation is important for generating nitrogen compounds that can be used by plants in both natural and agricultural ecosystems. 3. Due to the wonders of chemistry, the soybean has been transformed into a variety of food products and has an ever-increasing role in the Western diet. CHAPTER The hyacinth bean ( Dolichos lablab ), a vine within the Fabaceae or bean family.Levetin−McMahon: Plants and Society, Fifth EditionIII. Plants As a Source of Food13. Legumes © The McGraw−Hill Companies, 2008208UNIT IIIPlants as a Source of Food Legumes are members of the bean family, Fabaceae, which includes all types of beans and peas as well as soybeans, peanuts, alfalfa, and clover. This large, widely distributed family also includes various trees and ornamentals such as black locust, wisteria, lupine, and the Texas bluebonnet. CHARACTERISTICS OF THE LEGUME FAMILY Most members of this dicot family share a very similar flower and fruit structure ( fig. 13.1 ). The five-petaled flower is irregu-lar, with bilateral symmetry, and has been described as either butterfly shaped or boat shaped. The fruit is a pod, or legume, with one row of seeds; the seeds contain two prominent food-storing cotyledons. The two halves of a peanut are clear examples of cotyledons. Although the leaves of some legumes are simple, most are pinnately or palmately compound. The seeds of many legumes are an important food staple worldwide because they are rich in both oil and protein. They are higher in protein than any other food plant and are close to animal meat in quality. In fact, they are often called “poor man’s meat” because they are an inexpensive source of high-quality protein. The high protein content of legumes is correlated with the presence of root nodules( fig. 13.2 ), which contain nitrogen-fixing bacteria. These bacteria, which are species of the genus Rhizobium, are able to convert free atmospheric nitrogen into a form that can be used by plants in the making of protein and other nitro-gen-containing compounds (see A Closer Look 13.1—The Nitrogen Cycle). Because of the presence of nitrogen-fixing bacteria, the cultivation of legumes enriches the soil. For this reason farm-ers often rotate legumes with crops that deplete soil nitrogen. Soybeans are often rotated with corn in the cornbelt region of the United States. Sometimes leguminous crops may even be plowed under as a “green manure” instead of being harvested. Before the advent of commercial fertilizers, these practices were more common than they are today, but they may gain renewed importance as fertilizer costs continue to rise and environmental awareness increases. Without the need for massive application of fertilizers, legumes can be cultivated worldwide, in even the poorest soils. Ecologists have even recommended planting fast-growing leguminous trees to reclaim eroded or barren areas. Figure 13.1(a) Flowers of the hyacinth bean resemble sailboats, a shape common to many species in the legume family. (b) Peas (seeds) in an opened pod.(b)(a)Figure 13.2 Nitrogen-fixing bacteria, Rhizobium spp., inhabit nodules on the roots of many legumes.Levetin−McMahon: Plants and Society, Fifth EditionIII. Plants As a Source of Food13. Legumes © The McGraw−Hill Companies, 2008CHAPTER 13Legumes 209Concept QuizOne of the goals of genetic engineering is to develop crops that are able to provide their own supply of nitrogen through symbiosis with Rhizobium, as do the legumes.What benefits to agriculture and the environment would be realized if cereals and other staples became nitrogen-fixing crops? IMPORTANT LEGUME FOOD CROPS As you recall from the discussion on the origin of agriculture, legumes have been cultivated for thousands of years in both the Old and New Worlds (see Chapter 11). A possible reason for their long history as food crops may be that their seeds, which are easily harvested, have a low water content and, when dry, are easily stored for long periods of time. These features, plus their high protein content and ease in growing, make legumes ideal crops. Beans and Peas To most people the word legume brings to mind beans and peas; these are, in fact, some of the oldest and most common food crops ( table 13.1 ). Beans come in all shapes, sizes, and colors: kidney beans, lima beans, pinto beans, navy beans, green beans, wax beans, and butter beans are just a few of the many types familiar to us in the United States ( fig. 13.3 ). Add to this list the hundreds of varieties found in other parts of the world and you can see the diversity implicit in the term bean.Beans are a good source of protein, with values ranging from 17% to 31% and the average about 25%. Although the dry seeds were considered the only edible part for thousands of years, some of the most popular varieties today, such as green beans and wax beans, have edible pods. Beans are warm-season annuals requiring a modest amount of rainfall. Like all legumes, they can tolerate most types of soil and can be grown worldwide. One bean of particular interest is Vicia faba, the broad bean or Windsor bean, which is an Old World species. It has been cultivated and eaten for several thousand years in the Mediterranean region; however, a disease called favism is associated with its consumption. In susceptible individuals, eating broad beans or even inhaling the pollen can produce favism—technically, hemolytic anemia (the lysis of red blood cells). The disease is actually caused by Common Name Scientific Name ValueAdzuki beans Vigna angularis Popular in Japan and often used in deserts and confectionsAnasazi beans Phaseolus vulgaris Dramatic red and white heirloom bean of the American southwestBlack-eyed peas Vigna unguiculata Popular favorite