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UIUC IB 102 - The Grasses

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Levetin−McMahon: Plants and Society, Fifth EditionIII. Plants As a Source of Food12. The Grasses © The McGraw−Hill Companies, 200818712THE GRASSES CHAPTER OUTLINE Characteristics of the Grass Family 188Vegetative Characteristics 188 The Flower 188 The Grain 188 Wheat: The Staff of Life 189Origin and Evolution of Wheat 190 Modern Cultivars 191 A CLOSER LOOK 12.1 The Rise of Bread 191Nutrition 192 Corn: Indian Maize 193An Unusual Cereal 193 Types of Corn 193 A CLOSER LOOK 12.2 BarbaraMcclintock and Jumping Genes in Corn 195Hybrid Corn 195 Ancestry of Corn 197 Value of Corn 198 Rice: Food for Billions 199A Plant for Flooded Fields 199 Varieties 201 Other Important Grains 201Rye and Triticale 201 Oats 201 Barley 203 Sorghum and Millets 203 Other Grasses 203Forage Grasses 203 Lawn Grasses 204 Chapter Summary 205 Review Questions 205 Further Reading 205 KEY CONCEPTS 1. Grasses are members of the monocot family Poaceae, whose characteristic grains are a vital food source. 2. Whole grains with the bran and germ intact are nutritionally superior to their refined counterparts, which contain only endosperm.3. Wheat, corn, and rice, the major cereals, outrank all other plants as food sources for human consumption. 4. Grasses are also indispensable components of forage crops and landscaping designs. CHAPTER Pearl millet, Pennisetum glaucum, is the most widely grown type of millet. It was domesticated in Africa approximately 4,000 years ago, and today it is grown throughout Africa and India as a food staple. Courtesy CGIAR/CRISATLevetin−McMahon: Plants and Society, Fifth EditionIII. Plants As a Source of Food12. The Grasses © The McGraw−Hill Companies, 2008188UNIT IIIPlants as a Source of Food The grass family is of greater importance to humanity than any other family of flowering plants. The edible grains of cultivated grasses, or cereals, are the basic foods of civilization, with wheat, rice, and corn the most extensively grown of all food crops. Other important cereals are barley, sorghum, oats, millet, and rye; most are among the top 25 food crops ( fig. 12.1 ). CHARACTERISTICS OF THE GRASS FAMILY There are approximately 600 genera and 8,500 species of grasses found throughout the world, making the grass family one of the largest and most widely distributed plant families. Grasses are the dominant plants in prairies and savannas but can also be found wherever plants can grow, under a wide variety of environmental conditions from arctic marshes to tropical swamps. In fact, 25% of the world’s vegetation belongs to the grass family. Vegetative Characteristics Members of the grass family, the Poaceae, are usually her-baceous, having linear leaves with parallel venation typical of monocots ( fig. 12.2 ). Leaves usually have an alternate arrangement, and the base of each leaf forms a sheath that wraps around the stem. The stems, or culms , are often hol-low between the nodes and usually unbranched. Many species may also have horizontal stems (either aboveground stolonsor underground rhizomes) that can propagate the plant veg-etatively by giving rise to new shoots (see Chapter 14). Both annual and perennial species of grasses occur, with most cere-als being annuals and most pasture and lawn grasses being perennials. The primary root system is fibrous, and adventi-tious roots may also form either from the lower nodes of erect stems (as prop roots) or from rhizomes or stolons. The Flower The flowers of grasses are borne in inflorescences, typically spikes, racemes, or panicles. The tassels of corn and the heads of wheat are common examples of grass inflorescences. The individual flowers are small, inconspicuous, and incomplete, with sepals and petals lacking completely or replaced by small structures called lodicules ( fig. 12.2 ). Each flower normally has three stamens, and the gynoecium has a single ovule in the ovary but two styles and stigmas. The stigmas are enlarged and feathery, and the mature stamens pendant; these features facilitate wind pollination. Surrounding each flower are two bracts, the outer lemma and inner palea; the flower and the two bracts together make up a floret . One to 12 florets are arranged on a spikelet , which also may be subtended by two bracts called glumes. Often, a slender bristle can be seen extending from either a glume or a lemma (occasionally the palea); this structure is known as an awn .The Grain The typical fruits for the grass family are grains, which are dry, single-seeded indehiscent fruits ( fig. 12.3 ). The bracts that surrounded the flower (or even the spikelet) now sur-round the grain and are called chaff. The outer wall of the grain, consisting of the fruit wall fused to the seed coat, is known as the bran. Interior to the bran is a layer of enlarged cells known as the aleurone layer, which is normally high in protein. If the seed is allowed to germinate, this layer provides the enzymes that break down stored food for the growing embryo. The majority of the seed is occupied by endosperm, which contains stored food mainly in the form of starch. The cotyledon transfers food to the embryo, which is surrounded by the coleoptile and coleorhiza sheaths. The embryo, with its sheaths, is often referred to as the germ.The large amount of stored food in the grain makes this family valuable as a food crop. In the economically important cereals, the endosperm is mostly starch, and in the refining process, the chaff, germ, and bran (usually with the aleu-rone layer attached) are removed, leaving only the starchy endosperm. Commercially, this refined product is available WheatRiceMaizePotatoesSugar beetCassavaBarleySoybeansSweet potatoesOil palm fruitTomatoesSorghumOrangesBananasApplesPulsesGrapesWatermelonsCoconutsRapeseedOnionsYamsCucumbersMetric TonsCottonseed42,638,50442,648,93748,891,20748,907,02654,254,23259,512,35659,722,08860,188,12160,908,79762,358,09566,901,41974,236,88597,497,401122,728,053124,663,053139,220,431172,840,720213,024,811213,976,284251,289,226324,491,141621,588,528630,556,602711,762,871Sugar cane1,285,388,292Figure 12.1 Annual world crop production figures (in metric tons) reveal that six of the top 25 crops are grasses (in boldface type).Source: FAO Production Yearbook, 2005.Levetin−McMahon: Plants and Society, Fifth EditionIII. Plants As a Source of Food12. The Grasses © The McGraw−Hill Companies, 2008CHAPTER 12The Grasses 189as white


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