Grasses, family Gramineae, are the most variable, widespread, and useful plant groups and comprise over 600 genera and 7,500 species. The soft, velvety turf plants that form lawns bear little resemblance to the huge, woody bamboos towering 40 m (132 ft) or more in height. Submerged aquatic plants growing on the sea seem quite different from the small, tender shoots emerging from a snowbank in the alpine tundra above timberline, yet all these may be grasses. To the average urban dweller, grass may mean only the green carpet covering a lawn or courtyard. To a farmer in the U.S. Great Plains, on the other hand, grass cereal grains such as wheat may be the major cash crop. Grasses produce the bulk of forage for the world's livestock industry, and they provide building materials for houses in tropical areas. Grasses grow on all continents. They are found in the sea, in freshwater marshes, in deserts, above the tree line on high mountains, and in arctic areas. As a group grasses are among the most successful and actively evolving of all plants. Their great flexibility and adaptability permit them to live in most situations. Their adaptive radiation has evolved treelike forms, intermediate tufts that compete with understory in forests and savanna, and horizontally spreading perennials that form either dense sod or loose ground cover. Some species of grass produce large amounts of highly fertile seeds that can quickly colonize disturbed areas. Other species rely on specialized underground stems called rhizomes or creeping prostrate stems called stolons to move the plants into new environments. This wide variation in grasses has resulted in many uses. USES Human Consumption Grass seeds have a large quantity of nutrients stored in them that allow new shoots and roots to develop quickly upon germination. These large, nutritious seeds are ideally suited as food for animals and human beings alike. Grass seed, or \Tgrain\t, is grown on about three-fourths of the world's arable land. Three species of cereal grass--wheat, rice, and maize--form the basis of the staple human food for most of the world. Wheat (Triticum aestivum) is a grass of temperate origin, ideally suited to the cooler climates of the world. Although it probably originated in Europe, where many wild varieties occur, it was widely cultivated as early as 3000 BC in both Europe and Asia. Many specialized varieties have been developed through controlled breeding for bread, cereal foods, and animal feed, so that today it is not only a major food but an important trade item in world commerce. Rice (Oryza sativa) is a grass of tropical origin whose grain probably feeds more people than any other plant. Although rice is one of the oldest cultivated plants, production changed little for centuries. Then new varieties were developed by the International Rice Research Institute in the Philippines, and yields have been dramatically increased. Maize (Zea mays) probably originated in Mexico from tropical grasses. It was not known in Europe before Columbus's voyages to the New World. Now maize, or corn (as it is known in the United States), is a major crop on all continents. Yields have been increased dramatically following the development of hybrids and controlled breeding programs. Today hundreds of varieties are available. Some are bred especially for grain production, some as a table vegetable, some for popcorn, and others for silage or forage production. Recent development of a corn that is rich in the amino acid lysine offers the promise of a complete human food available in a single crop, maize. Other cool-season cereal grasses such as oats (Avena sativa), rye (Secale cereale), and barley (Hordeum vulgare) are important grain crops in temperate regions of the world. Tropical cereals such as sorghum (Sorghum bicolor) and pearl millet (Pennisetum americanum) form the basis of human diets in much of Africa and other tropical areas. Sugarcane (Saccharum officinarum) produces much of the world's refined molasses and sugar. It is cultivated in tropical areas throughout the world. The high production levels possible, combined with the large amount of tropical land suitable for sugar cane production, make the regulation of sugar production a topic for discussion in international trade. Animal Feed Although grasses produce a large part of the world's diet, only relatively small amounts are consumed directly by humans. Most grasses are used as animal feed. While animals are fed some grain, the bulk of animal feed comes from specialized varieties of grass that are not very palatable to human beings. In addition, grass varieties such as sorghum and maize are raised as fodder crops. Several tons per hectare of palatable forage can be produced, stored, and used at a later date. By far the largest amount of grass is consumed by grazing animals. Throughout history grasses and grazing animals have coevolved. In North America the fossil remains of the earliest known member of the horse family are found to have been present in the Eocene Epoch (154-38 million years ago), causing some scientists to theorize that grasses probably existed before that time, and grasslands apparently became abundant by the Miocene Epoch (26-12 million years ago). When the first Europeans arrived, the North American grasslands were stocked with bison, elk, and other grazing animals. Australia and New Zealand are the only major grassland areas that do not have a recent history of grazing by wild ruminant animals, and Australia does have wild grazing marsupials such as kangaroos. Grassland and savanna areas still provide feed for wild game herds in Africa, North America, and elsewhere. Native grasses provide a major source of feed for domestic livestock on all continents. The nomadic herd-keepers of Africa, station operators in outback Australia, dairy farmers in Norway, and ranchers in Nevada all depend on grass for their animal production. The species of grass growing in each area varies, but a sustained yield of products from these lands depends upon the application of the rules of ecology to perpetuate healthy populations of both plants and animals. The science of managing plants while observing ecological principles is called range management. Almost 30 percent of the earth's land surface is grazed by animals: grazing is the most widespread of all land uses. Some grasses are grown under cultivation and grazed by animals. These cultivated pastures are managed as farms. In cool temperate areas species of Poa (bluegrass), Festuca, Phleum (timothy), and Dactylis (orchard grass) are the most common forage plants. In tropical climates Sorghum, Paspalum, Panicum, and Cynodon are more important. Selective breeding has developed improved varieties and cultivars of most of the common species of forage grasses. In some species, such as Bermuda grass (Cynodon dactylon), hybrids have been developed that give much higher yields than the parent species. Turf Turf for aesthetic and recreation purposes is a major use of grass. Most lawns are of grass. The cool regions of the world regularly use plants of the genera Poa and Agrostis for turf. Kentucky bluegrass (Poa pratensis) is by far the most widely used lawn grass in the northern United States. Creeping bent grass (Agrostis palustris) is also used, but it is more popular for golf greens and specialty turfs than for home lawns. Bermuda grass is the popular lawn in much of the warm temperate to tropical areas. In hot areas, Saint Augustine grass (Stenotaphrum secundatum) is a popular grass, but because of its coarse leaves and high water use it has not challenged the finer-leafed varieties in areas where they can be grown. Other turf species include Zoysia, centipede grass (Eremochloa ophiuroides), and Bahia grass (Paspalum notatum) in warm areas and fescues (Festuca) and ryegrass (Lolium) in cool areas. Some native sod-forming grasses such as buffalo grass (Buchloe dactyloides) and curly mesquite grass (Hilaria belangeri) are used as minimum maintenance turfs in the drier portions of the U.S. Great Plains. Other Uses There are many nonfood or nonfeed uses of grasses. Grass skirts are used in the South Pacific Islands, and grass huts in many tropical regions. Other grasses, particularly bamboos, are used in construction and industry. Houses, bridges, and boat masts are constructed from the poles and posts of large bamboos. Furniture, window shades, mats and floor coverings, water pipes, and cooking utensils from grass are found in homes throughout the world. In addition, recreational equipment such as fishing poles, musical instruments, toys, and pipes have been made of grass. The grass plant has evolved to fill many human uses, and its widespread abundance in many forms will continue to contribute to civilization. STRUCTURE Stems The grass plant is composed of stems, leaves, roots, and fruiting structures. Stems of grasses have two parts: nodes and internodes. The node is a joint in the stem at the place where leaves are attached. The internode, or area between where leaves are attached, may be either pithy, hollow, or solid. The erect stem of a grass is called a culm. Lateral shoots immediately above the ground may produce creeping prostrate stems called stolons. Stoloniferous grasses may reproduce asexually by stolons growing into new territory and establishing roots at the nodes. Lateral buds immediately below the ground may produce subterranean stems called rhizomes. Rhizomes are a major source of asexual reproduction and allow a plant to persist even under heavy defoliation. Lateral shoots of cultivated grasses may be called tillers (usually in wheat, oats, and other cereals) or suckers (in maize and sorghum). Most perennial grasses have herbaceous culms that die back each year and are replaced by new shoots. The resulting colony of individual grass shoots is often called a "plant" or a clone. The clone grows from the center outward, with the older plants in the center becoming weakened from competition. In extreme cases the "plant" may actually develop into a ring of healthy plants with a dead open center. Not all perennial grasses die back each year; some have developed a woody stem that stores nutrients, and the grass behaves much like a woody shrub. Bamboos are the most common woody grasses. Leaves The leaves of grasses are two-ranked, a characteristic that separates them from other families that they closely resemble, such as sedges (Cyperaceae) with three-ranked leaves. The grass blade itself consists of a sheath and a blade, or lamina. The sheath, the lower portion of the leaf, tightly enfolds the culm from its attachment at the node almost to the next node. The blade is the flattened, elongated portion of the leaf that extends away from the culm. A membranous or hairy structure on the leaf at the side toward the axis where the sheath and blade come together is called a ligule. Lateral projections at the base of the leaf or the apex of the sheath are auricles. Photosynthesis and respiration are carried out in both the sheath and the blade. The growing point of the new shoot, the apical meristem, is usually enclosed by several older leaves that give the new shoot some protection from the extremes of a harsh environment. Roots Most grass roots are fibrous. When the seed or embryo first sprouts it produces primary, or seminal, roots. These roots are usually of little importance after the first few weeks. Early in the development of the grass plant, adventitious (roots growing from stems or leaves) roots are produced at the lower nodes of the shoot. The root systems of most native grass plants are almost entirely adventitious. The size of the root system is usually related to the amount of shoot growth. Plants with large root-to-shoot ratios appear to be more drought-tolerant than those with smaller ratios. Fruiting Structures The flowering and fruiting structure of a grass is called the inflorescence. Its basic unit is the spikelet, which may contain one or more flowers, or florets, on a shortened stem axis, or rachilla, enclosed by two flora, or "empty," bracts called glumes. There are three basic types of inflorescences: spikes, racemes, and panicles. A spike is an inflorescence in which all spikelets are attached directly to the main axis. Wheat and rye are good examples of this type. In a raceme, the florets are borne on short flower stalks, or pedicels, on the major axis; barley is an example. When the flower stalk branches, as in oats, it is called a panicle. The grass fruit itself varies greatly in size, from the very small grains of Sporobolus and Muhlenbergia to common grains of corn, wheat, and rice. Regardless of size, all have a high percentage of stored nutrients, making them attractive to insects, birds, and mammals. GRASSES AS WEEDS Grasses that invade new habitats and spread rapidly may become unwanted plants, or weeds. Crabgrass (Digitaria sanguinalis, D. adscendens) is a major weed in lawns and gardens throughout temperate areas. Quack grass (Agropyron repens), native to Europe, has become a major weed throughout temperate areas. Many of the annual grasses become weeds on overgrazed rangeland. Cheatgrass (Bromus tectorum) is an aggressive annual from the European area that has invaded the dry ranges of the western United States wherever fire or heavy grazing has left the soil bare. Other introduced annual grasses may be locally important. Grasses fill many ecological niches throughout the world. As humans create new niches, it is probable that grasses--either brought from other parts of the world or bred specifically for new conditions--will fill them. Thadis W. Box Bibliography: Bews, John W., The World's Grasses: Their Differentiation, Distribution, Economics (1977); Emmons, Robert D., Turfgrass Science and Management (1984); Gould, Frank W., and Shaw, Robert B., Grass Systematics, 2d ed. (1983); Hitchcock, A. S., Manual of the Grasses of the United States, 2d ed. rev. by Agnes Chase (1950; repr. 1971); Langer, R. H. M., How Grasses Grow, 2d ed. (1979). See also: \Tgrasslands\t; individual articles on grass species.