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Grasses

Grasses

Grasses belong to one of the largest and most economically and ecologically important families of plants: the Poaceae, formerly called the Gramineae. There are over nine thousand species of grasses recognized by botanists. Grasses can be found on every continent and in a wide variety of habitats, both as the dominant plant type (in prairies and tundra) or as minor components of the plant community. Collectively, grasses domesticated as crops represent the world's most important source of food.

Grasses share a number of characteristics that differentiate them from other plant species. They typically have long, narrow leaves. The stems may be either flattened or round, and they are often hollow. Grasses can grow very tall (tropical bamboos can reach up to 100 meters [328 feet]) or they can grow prostrate along the ground. The root systems of grasses are highly branched (fibrous) and do not have a well-defined central taproot. Many grasses spread horizontally through the production of underground stems known as rhizomes, or prostrate stems aboveground known as stolons. New grass shoots can emerge from either rhizomes or stolons.

Grasses have evolved in environments where drought, grazing by large herbivores, and fires were common. Unlike many plants, the growing points (or meristems) of grasses are located near the base of the plant or below the ground, rather than at the tips of the plant. This characteristic allows grass plants to be grazed or burned without damage to the growing points. Additionally, grasses have large root systems that can store substantial food reserves that allow grasses to regrow quickly if aboveground parts are removed. These features also make grasses drought resistant and ideal for lawns that are repeatedly mowed. The large and fibrous root system of grasses has additional value for preventing soil erosion.

The flowers of grasses are small and inconspicuous. Grass flowers lack petals and other floral parts common in other plant families. Grass flowers are typically wind pollinated and therefore do not produce nectar, but they do produce pollen in large amounts. Grass flowers are so simple and small that they are sometimes referred to individually as florets. Florets are typically grouped or clustered along a central axis into units known as spikelets. The arrangement of florets and spikelets varies greatly among grasses, and individual grass species are often defined by these differences. The fruit of a grass flower is termed a caryopsis or a grain.

Grasses make up many of the most important crop species grown for human consumption. Three cereal cropscorn, wheat, and riceare the most important source of calories in all diets throughout the world. Sugarcane is a grass that supplies most of the world's sugar. Grasses, including several species of reed and bamboo, are used in many countries as construction material and as thatch for roofs, and the fiber from many grasses is used in making paper. Finally, native and planted grasslands are used worldwide in hay production and as grazing lands for animal production.

Economically Important Cereal Grasses
Global and U.S. production estimates (in millions of metric tons, 1998/1999) and the value of those crops produced in the United States (in millions of U.S. dollars, 1997). Data are from the U.S. Census Bureau and the U.S. Department of Agriculture.
  World Production U.S. Production Value
Corn 605.5 247.9 20,456
Wheat 588.4 69.3 8,926
Rice 394.0 5.8 1,657
Barley 136.8 7.7 799
Sorghum 59.2 13.2 1,619
Oats 26.0 2.4
Rye 20.3 0.3

As economically valuable as grasses are, the grass family, like all large plant families, also contains species that are considered pests or weeds and as such incur an economic cost. Crabgrass is a familiar example in lawns, but there are many agricultural weeds that are grasses and these consume resources meant for planted species, interfere with the harvest, and, ultimately, reduce crop yield.

see also Agriculture; Grain; History of Agriculture; Monocots

Alan K. Knapp

Bibliography

Chapman, G. P., and W. E. Peat. An Introduction to the Grasses. Melksham, UK: CAB International, Redwood Press, Ltd., 1992.

Heiser, Charles Bixler. Seed to Civilization. Cambridge, MA: Harvard University Press, 1990.

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Gramineae

Gramineae The grass family; a very large and important family of monocotyledons, most of which are annual or perennial herbs, but a few genera (e.g. the bamboos) of which are woody. From an ecological viewpoint, it is the most successful family of flowering plants. Where forests have been destroyed by humans, grasses have tended to replace the trees as the dominant vegetation of many areas of the world. Economically, the Gramineae is the family of plants most important to humans, as it contains numerous cereal grasses (e.g. wheat, barley, oats, maize, rice, and millet, the staple foods of most of the world's population). Many grasses are important sources of fibres. Most grasses have hollow stems with solid points (nodes) capable of intercalary growth, and leaves in two opposite and alternating rows. Each leaf consists of a sheath round the stem, a blade, and usually a flap or ligule at the junction of sheath and blade. Inflorescences are very varied but are usually composed of units (spikelets) with a pair of sterile glumes at the base of each spikelet. Each spikelet consists of 1 to many florets, each floret normally having 2 subtending scales (the lower chaffy lemma and the upper membranaceous palea), 3 stamens with long filaments and flexible anthers (adapted to wind-pollination), and an ovary with 1 ovule and 2 long, feathery stigmas. There are 737 genera, with about 7950 species, distributed throughout the world.

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Grasses

Grasses

The grass family, known scientifically as either the Poaceae or Gramineae, is one of the four largest families of flowering plants, with approximately five hundred genera and ten thousand species. Grasses range from tiny inconspicuous herbs less than 5 centimeters tall to the giant bamboos, which grow to 40 meters tall. The family is undoubtedly the most important flowering plant family to humans, directly or indirectly providing more than three-quarters of our food. In addition, grasses are major producers of oxygen and a large component of environmental filtering processes due to the enormous geographic range, spatial coverage, and biomass of grasses on Earth. Grasses are the greatest single source of wealth in the world.

Morphology

All grasses have fibrous secondary roots (the primary root disappears early in development) and can be annual or perennial, in which case they usually have underground stems called rhizomes. These can be very short and knotty or very long. In some species the rhizomes can go for several meters. Sometimes these stems run horizontally above ground and are then called stolons. Grasses characteristically have stems that are round and usually hollow with a node (the swollen areas along the stem where the leaves and branches are attached) and internode (the part of the stem between the nodes) arrangement. Their leaves are attached at the nodes and consist of two parts. The sheath clasps the stem (also known as the culm) sometimes all the way up to or beyond the next node. The blade is the upper part of the leaf that is free from the stem. The fact that the top edges of the sheath may overlap each other around the stem but are not joined to each other is a defining characteristic of grasses. At the point where the blade joins the sheath, there may be a flap of tissue called a ligule. This structure keeps dirt and parasites from getting into the space between the sheath and the stem.

The tiny grass flower, called a spikelet, is actually a composite of one or more tiny flowers and is the most characteristic structure among grasses. It is generally composed of two bracts called glumes, with one to many tiny flowers called florets attached above them. Each floret consists of a bract called a lemma that generally wraps around a smaller and generally very thin bract called a palea. These two encase the nearly microscopic rudimentary petals called lodicules, the stamens (usually one to three), and the ovary, which can have two or three feathery stigmas at its apex. There may be only a few spikelets on a plant but usually there are many (sometimes hundreds) arranged variously in an inflorescence . The inflorescence is the plumelike structure that you see on sugarcane or the spike of a wheat plant. Corn is a special case, both the cob and the tassel are inflorescences, but the cob has only female flowers and the tassel only male. Some other grasses have separate male and female inflorescences and many have some of the spikelets with only male flowers, while other spikelets in the same inflorescence have hermaphrodite flowers. The grasses also have a very characteristic fruit (grain) called a caryopsis, which consists of the ovary with one or more of the floret bracts attached.

Pollination and Dispersal

Most grasses are wind-pollinated. Their anthers are versatile, meaning that they pivot on their stalk (or filament ), and the stalks are very flexible, like a piece of string. At the appropriate time, usually early in the morning, the lodicules will swell with water and push the lemma and palea apart. The anthers will then pop out of the flower and dangle in the wind on their filaments, releasing their pollen as the breeze jostles them during the day. Most grass pollen is perfectly smooth and round with a single small hole in it. This is characteristic of many wind-pollinated plants. A few grassesespecially those that grow deep in the rain forestare pollinated by insects, probably because there is no wind. Grasses have a myriad of dispersal mechanisms for their seeds. Some rain forest grasses shoot their seeds several feet across the forest floor and others have flowers that bloom underground on the tips of long rhizomes and may have an association with ants. Many grasses have smooth fruits that get blown by wind or carried either inside

GRASSES CULTIVATED FOR FOOD
Common Name Scientific Name Geographic Origin and Area of Domestication Area of Cultivation
Wheat Triticum aestivum Southwest Asia, especially Turkey and environs Worldwide in temperate areas, especially in the Western Hemisphere
Maize or Corn Zea mays Mexico, Guatemala, Honduras Primarily in the Western Hemisphere in both temperate and tropical areas
Rice Oryza sativa Asia, probably domesticated in China and southern Asia Worldwide in mesic to wet tropical and warm temperate areas, especially in Asia
Sugarcane Saccharum officinarum New Guinea or Indonesia Worldwide in wet tropical and subtropical areas
Barley Hordeum vulgare Southwest Asia Primarily in temperate areas of the Western Hemisphere
Oats Avena sativa Probably Europe Primarily in temperate areas of the Western Hemisphere
Rye Secale cereale Southwest Asia Primarily in temperate areas of the Western Hemisphere
Millet Finger millet (Eleusine coracana ), proso millet (Panicum miliaceum ), foxtail millet (Setaria italica ), japanese barnyard millet (Echinochola crusgalli ), teff millet (Eragrostis tef ), pearl millet (Pennisetum glaucum ), koda millet (Paspalum scrobiculatum ) Mostly in Europe and North America Now important for food only in Asia and Africa although widely grown for birdseed in the United States and Europe
Sorghum Sorghum bicolor Africa Most important in Africa and other dry regions of the tropics and subtropics

or on the outside of animals. Most have some kind of hooks or hairs to catch passing animals. Many have very specialized fruits that have a hard, drilllike point on one end and one or more long, pinlike awns on the other. An awn can catch the wind and vibrate the point like a jackhammer (e.g., the genus Aristida ) or they can be twisted and sensitive to moisture (e.g., the genus Heteropogon ) so that it turns the point like a drill bit into sweaty animal fur or feathers or into the soil. Stiff, back-pointing hairs on the awn and on the hard point of the fruit help by only allowing the fruit to burrow in, not out. Other grass fruits are completely covered with long hairs that allow them to catch the wind and float for several kilometers. Still others have an inflorescence that breaks off and blows across the ground like a tumbleweed unit (e.g., Eragrostis spectabilis ), or is carried around by birds as nesting material (e.g., Panicum maximum ). The sandburs (the genus Cenchrus ) have a spiny bur around their seeds that sticks into your skin or an animal's fur while Job's Tears (Coix lacryma-jobi ) have a hard, white, shiny flask around the seeds that is used in many tropical regions for beads. Several bamboos have thorns; a very few species of grass have irritating hairs and the leaf edges can be sharp, but other than the high levels of silica found in the leaves of many savanna grasses (which wear down the teeth of grazers), protective structures are rare in the grasses.

Where They Grow

Grasses are not only in your backyard. They are more geographically distributed than any other family of flowering plants. The southernmost recorded flowering plant is the Antarctic hair grass, Deschampsia antarctica, and several species of grasses are among the most northern growing tundra plants as well. They are very common in alpine areas and lowlands, in swamps, and in some deserts. When forest is cleared, grasses usually dominate the landscape. They bind the soil and prevent loss of topsoil all over the world. Grasses are planted as cover crops when land is cleared. Taken together, they cover more area on Earth than any other flowering plant family. They are the dominant plant in the savannas that ring Earth at the boundaries of the tropics, and they dominate the boreal steppe (cold temperate grasslands) and the prairies of North America. Tropical savannas currently cover some 23,000,000 square kilometers, or about 20 percent of Earth's land surface. Dominance of grasses in these habitats is usually maintained either because there is not enough water for trees to survive, there is heavy grazing pressure, or because there are fires frequently enough to keep the trees out. Grasses adapted to fire-prone areas have their growing tip either below ground or well protected within a tight clump of leaf bases. When they burn, only the leaves or the old flowerstalks are lost; the growing tip stays safe. They are also generally fast growers; for example, a bamboo has been measured growing 120 centimeters in twenty-four hours. Some grasses are actually stimulated to grow by grazing. The huge herds of more than a hundred different grazing animals in Africa, bison on the great plains of the United States, cattle all over the world, and billions of termites on the savannas of South America are all supported by grass.

There are two major photosynthetic pathways in grasses, C3 and C4 (with the exception of the bamboos, which are all C3, and are common in the tropics and some temperate areas of Asia). Almost all grasses at high latitudes are C3, while most of those at the equator are C4. In general, C4 grasses can work at higher temperatures and light levels than C3 grasses but require higher temperatures and/or light levels to begin photosynthesizing.

Economic Importance

The economic importance of grasses can hardly be overstated. They provide the majority of food. Grasses provide much of the starch (e.g., rice, cornmeal, bread, cereal, pasta) and a certain amount of protein, in most human diets. Although a few grasses absorb selenium and other harmful substances from the soil and others have potentially poisonous cyanogenic compounds in their shoots and leaves, most are not poisonous. The grains are naturally low in fat and rich in complex carbohydrates. Remember that most livestock eat primarily grass or grass products so leather, wool, meat, and milk also indirectly come from grass. Grasses sweeten what you drink and eat with cane sugar, molasses, and high fructose corn syrup. Corn byproducts also provide the raw material for many chemicals used in industry and everyday life. Grasses provide the raw material for most alcohol products (e.g., sake from rice, rum from sugarcane, beer from barley, bourbon from corn, and other whiskeys from barley, wheat, and rye). Although bamboo shoots are enormously important as a food crop in Asia, the real economic contribution of the bamboos is as a building material and a raw material for paper and furniture. More than three thousand uses have been listed for bamboos in Japan alone. There is even a bamboo culture in Honduras that is based on the giant Guadua bamboo. Of course, because corn and rice are the staple foods of many of the world's people, cultures can be defined by them as well. Rice is a sacred plant in many Asian cultures. In contrast to the enormous economic benefit of grasses, it must also be noted that they make up a large percentage of the world's worst weeds, which cost millions of dollars every year to manage. Cogon grass (Imperata cylindrica ) and Bermuda grass (Cynodon dactylon ) are two of the most common. Bermuda grass is also the most common grass on Earth.

see also Bamboo; Corn; Grains; Monocots; Photosynthesis, Carbon Fixation and; Rice; Savanna; Seed Dispersal; Wheat.

Gerald F. Guala

Bibliography

Clark, L. G., and R. W. Pohl. Agnes Chase's First Book of Grasses, 4th ed. Washington, DC: Smithsonian Institution Press, 1996.

Cole, M. M. The Savannas: Biogeography and Geobotany. London: Academic Press, 1986.

Farrelly, D. The Book of Bamboo. San Francisco: Sierra Club Books, 1984.

Guinness Media. The Guinness Book of World Records. Stamford, CT: Guinness Media.1996.

Heiser, C. B. Seed to Civilization: The Story of Food. Cambridge, MA: Harvard University Press, 1990.

Holm, L., J. Doll., E. Holm, J. Pancho, and J. Herberger. World Weeds: Natural Histories and Distribution. New York: Wiley & Sons, Inc., 1997.

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Grasses

188. Grasses

See also 54. BOTANY ; 319. PLANTS

agrostography
a description of grasses. agrostographer, n.
agrostology
the branch of systematic botany that studies grasses. Also called graminology. agrostologist, n.
graminology
agrostology.

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