Grasses

Biology of grasses

Native grasses of North America

Grasses in agriculture

Wheats

Maize or corn

Rice

Other important agricultural grasses

Sugarcane

Pasture grasses

Other products obtained from grasses

Grasses in horticulture

Grasses as weeds

Resources

Grasses are monocotyledonous plants in the family Poaceae (also known as Gramineae). There are as many as 10,000 species of grasses distributed among more than 600 genera. The richest genera of grasses are the panic grasses (Panicum spp.) with 400 species, the blue grasses (Poa spp.) and love grasses (Eragrostis spp.) with 300 species each, and the needle grasses (Stipa spp.) with 200 species.

Species of grasses occur worldwide in virtually any habitat capable of supporting vascular plants. Grasses are the dominant species in some types of natural vegetation such as prairies and steppes, and they are an important source of forage for many species of herbivorous animals. Some species are grown as agricultural crops, and these are among the most important foods for humans and domestic livestock. The most important of the agricultural grasses are maize, wheat, rice, sorghum, barley, and sugar cane.

Biology of grasses

Most grasses are annual plants or are herbaceous perennials that die back to the ground surface at the end of the growing season and then regenerate the next season by shoots developing from underground rhizome or root systems. A few species, such as the bamboos, develop as shrub- and tree-sized, woody plants.

The shoots of grasses typically have swollen nodes, or bases, and they are often hollow between the nodes. The leaves are usually long and narrow and have parallel veins. A specialized tissue called a ligule is usually present at the location where a leaf sheaths to the stem. The flowers of grasses are typically small, monoecious or dioecious, and are called florets. The florets have various specialized tissues, and often contain a long bristle called an awn, which can be quite prominent in some species. The florets are generally arranged into an inflorescence, or cluster, which can be quite large in some species. Pollination of grasses occurs when grass pollen is shed to the wind and carried opportunistically to other grasses. The fruits of grasses are known as a caryopsis or grain, are one-seeded, and can contain a large concentration of starch.

Native grasses of North America

Hundreds of grass species are native to North America. Native grasses are present in virtually all habitats, and they are among the most dominant plants in prairies, some types of marshes, and similar herbaceous types of vegetation. In addition, many

species of grasses have been introduced by humans from elsewhere, especially from western Eurasia.

Although many rich varieties of form and function are represented by the native grasses of North America, only a few of the most prominent species of selected, grass-dominated habitats will be briefly mentioned.

The temperate prairies of North America are dominated by herbaceous perennial plants, many of which are species of grasses. In the tall-grass prairies, some of the grasses can grow as high as 6.5 ft (2 m). Examples of these tall species include the big blue-stem (Andropogon gerardi ), Indian grass (Sorghastrum nutans ), dropseed (Sporobolus asper ), needle grass (Stipa spartea ), panic grass (Panicum virgatum ), wild rye (Elymus virginicus ), and others. Somewhat drier sites support mixed-grass prairies containing shorter species, for example, little blue-stem (Andropogon sco-parius ), grama grass (Bouteloua gracilis ), wheat grass (Agropyron smithii ), and needlegrass (Stipa viridula ). The driest habitats support semiarid short grass prairies with species such as grama grasses (Bouteloua dactyloides and B. gracilis ), dropseed (Sporobolus cryptandrus ), muhly grass (Muhlenbergia torreyana ), and Junegrass (Koerlia comata ).

Some species of grasses can be abundant in marshes, including the reed (Phragmites communis ) which can reach a height greater than 13 ft (4 m) and is North America’s tallest grass. The reed is a very widespread species, occurring in marshes on all continents. Some seaside habitats can also develop perennial grasslands. Sandy habitats are typically dominated by species of grasses such as the beach grass (Ammophila breviligulata ), sand reed (Calamovilfa longifolia ), and beach rye (Elymus mollis ). Salt marshes are brackish estuarine habitats typically dominated by cord grasses such as Spartina alterniflora and S. patens, two species which segregate within the same salt marshes on the basis of salinity and moisture gradients.

Although it was actually introduced to North America from Europe, Kentucky bluegrass (Poa pratensis ) is now one of the most common species in lawns, and it also occurs widely in distributed habitats.

Grasses in agriculture

In terms of the economic and nutritional values of foods provided for humans and domestic livestock, no other plant family is as important as the grasses. All important cereals and grains are members of the grass family, and some of these have been cultivated for thousands of years. There are useful cereal species available for all of the climatic zones in which humans commonly live, and this has been one of the most important reasons why our species has been able to develop such large and prosperous populations during the past several thousand years.

Wheats

The bread wheat (Triticum aestivum or T. vulgare ) is a very important grain species. The origins of modern bread wheat are somewhat uncertain, because this species occurs in many hybrid varieties which have been selectively bred over time by complex, unrecorded hybridizations of various species of Triticum. Some botanists believe that the major progenitor species was an ancient cultivated wheat known as emmer (T. dicoccum ), which was grown in the Middle East at least 5,000 years ago. Other ancient wheats which have also contributed to the genetic make-up of the modern bread wheat include einkorn (T. monococcum ) from southwestern Asia and spelt (T. spelta ) and durum (T. durum ) from the Mediterranean region.

Numerous varieties of wheat have been bred for various purposes and climatic regimes. Flowering wheat heads can have long awns (bristles) as in the so-called “bearded” wheats, or they can be awnless. Wheat can be sown in the spring or the autumn. The latter, known as winter wheat, generally has larger yields than spring wheat because it has a longer growing season. “Soft” wheats are used mostly for baking breads and pastries, while the “hard” or durum wheats are used to prepare pastas and other types of noodles.

Wheat is rarely grown in subtropical or tropical climates because it is too susceptible to fungal diseases under warm and humid conditions. The best climatic regime for growing wheat involves a temperate climate with soil moisture available during the spring and summer while the plants are growing, and drier conditions later on while the seeds are ripening and when the crop is being harvested.

Certain temperate zone landscapes that used to support natural prairies and steppes are now the best regions for wheat cultivation. These include the mixed- and short-grass prairies of North America and similar zones in the pampas of South America, the steppes of western Russia and Ukraine, as well as parts of central China and Australia. Winter wheat tends to be favored in places where the environmental regime is more moderate, while spring wheats are sown under more extreme climatic conditions.

Wheat grains are manufactured into various edible products. Most important is flour—finely milled wheat,—which is mostly used to bake breads, sweetened cakes, and pastries, and also for manufacturing into pastas and noodles. Wheat is also used to manufacture breakfast cereals, such as puffed wheat, shredded wheat, and fiber-rich bran flakes. Wheat grains are fermented in a mash to produce beer and other alcoholic beverages and also industrial alcohol. Wheat straw and hay are sometimes used as fodder for animals or as stuffing, although the latter use is now uncommon because so many synthetic materials are available for this purpose.

Maize or corn

Maize, corn, or mealies (Zea mays ) is derived from grasses native to Central America, probably from Mexico. Maize has a very distinctive, flowering structure, with a tassel of male flowers perched above the larger clusters of female flowers. Each of the several female flower clusters is an elongated, headlike structure known as a cob or ear, enclosed within sheathing leaves or bracts, known as husks. Each ear contains as many as several hundred female flowers, each of which may produce a seed known as a kernel. During the time that they are ripe for pollination, and the stigmas of the female flowers are borne outside of the sheathing leaves of the cob on very long styles known as corn silk.

As with wheat, maize exists in a wide range of cultivated varieties bred for particular uses and climates. The maize plant has been so highly modified by selective breeding for agriculture that it is now incapable of reproducing itself without human aid. The plant’s ripe grains are no longer able to detach from their husk or cob (this is known as shattering). Moreover, the ripe kernels are tightly enclosed within their sheathing husks so that they are trapped by those leaves when they germinate. Modern maize can only be propagated if humans remove the leaves and grains from the cob and sow the ripe seeds.

Some of the presumed wild ancestors of maize still occur in natural habitats in Mexico. One of these is teosinte (Zea mexicana ), a wild grass that does not form a cob encased in husks. Another possible progenitor is a grass called Tripsacum mexicanum that does not look much like corn but will readily hybridize with it. These wild relatives are of enormous importance because they contain genetic variation that no longer is present in the highly inbred strains that exist today, particularly the varieties that are used widely in modern industrialized agriculture. As such, some of the genetic information in the remaining wild species that participated in the cultural evolution of the modern maize plant may prove to be incredibly important in the future breeding of disease resistance, climatic tolerance, and other useful attributes of this critical food plant for humans.

Maize grows well in a hot and moist climates, and it can be cultivated in both tropical and temperate zones. Maize is used in many forms for consumption by humans. During the harvest season, much sweet corn is eaten after boiling or steaming. Maize is also eaten as a cooked porridge made of ground meal (in the southeastern United States, this is known as grits). Other foods include canned or frozen cooked kernels, cornflakes, tortillas, corn chips, and popcorn. The small unripe cobs can also be steamed or boiled and eaten whole as a nutritious vegetable. Corn seeds can also be pressed to manufacture an edible oil.

Much of the North American maize crop is fed to livestock. Its nutritional value is greatly enhanced if the plants are chopped up and fermented before being used for this purpose. This type of preparation, which can also be prepared from other grasses and from mixed-species hay, is known as silage.

In some regions such as the midwestern United States, much maize production goes into ethanol for use as a fuel in automobiles, usually blended with liquid petroleum hydrocarbons. Other products made from maize include cornstarch, corn syrup, and alcoholic beverages, such as some types of whiskey.

Rice

Rice (Oryza sativa ) is probably a native of south Asia, and it has been cultivated on that continent for more than 5,000 years. Its natural habitat is tropical marshes, but it is now cultivated in a wide range of subtropical and tropical habitats.

If rice is being cultivated under flooded conditions, its seeds are germinated, grown until they are about 6-12 in (15-30 cm) tall, and then planted in the sediment in shallow water. In Asia, this cultivation system is known as paddy. A variant of this system is also used in the southern United States where fields are flooded to plant and grow the crop and then drained for optimal ripening and harvest. Rice can also be cultivated under drier conditions, called “upland” rice, although the soil must be kept moist because the species is intolerant of drought. On moist, fertile sites in some parts of tropical Asia, two to four rice crops can be harvested each year, although this eventually could deplete the soil of its nutrient capital.

Rice is mostly eaten steamed or boiled, but it can also be dried and ground into a flour. Like most grains, it can be used to make beer and liquors. Rice straw is used to make paper and can also be woven into mats, hats, and other products.

Other important agricultural grasses

Barley (Hordeum vulgare ) is a relatively ancient crop species, having been grown in northeastern Africa and the Middle East for as long as 6,000 years. The environmental conditions favorable to barley’s growth are similar to those for wheat, although barley can be cultivated in somewhat cooler conditions and therefore farther to the north in Eurasia. Most barley is used as feed for domestic animals, but it is also used as a malt in brewing ale and other alcoholic beverages.

Rye (Secale cereale ) is an agricultural grass that originated in Asia. This species is mostly cultivated in northern temperate regions of central Asia and Europe. The flour is used to make rye breads and crisp breads, and it is sometimes used in a mash to prepare rye whiskey.

Oats (Avena sativa ) probably originated in western Asia, and they have been cultivated for more than 2,000 years. Unlike most temperate, agricultural grasses, oats are relatively tolerant of late-summer and autumn rains. Oats are mostly used as fodder for cattle and horses, but they are also used to prepare breakfast cereals, such as rolled oats and oatmeal porridge. The Turkish oat (A. orientalis ) and short oat (A. brevis ) are relatively minor cultivated species.

Sorghum (Sorghum bicolor ) is a small-grained cultivated species. Sorghum has been grown in Africa for at least 4,000 years and it is still probably the most important crop for the making of bread flour on that continent. Sorghum is also widely used in Africa to prepare a mash for the brewing of beer. Some varieties, known as broomcorns, are used to manufacture brushes, while others are used as forage crops.

Various other small-grain grasses are commonly known as millet. The most important species is the proso millet (Panicum miliaceum ), which originated in tropical Africa or Asia, and has been cultivated for more than 5,000 years. This species is relatively tolerant of drought, and it is most commonly cultivated in drier climates in Africa and Asia. Proso millet is commonly eaten as a cooked porridge, and it is also an important ingredient in commercial birdseeds. More minor species include pearl millet (Pennisetum glaucum ), foxtail

(Setaria italica ), Japanese (Echinochloa frumentacea ), shama (E. colona ), barnyard (E. crus-galli ), and ragi millets (Eleusine coracana ).

Wild rice (Zizania aquatica ) is a North American grass that grows naturally in shallow waters of temperate lakes and ponds, and has long been collected from the wild, usually by beating the ripe grains off their heads into a canoe, using a paddle. During the past several decades, however, this species has also been cultivated on farms in the southwestern United States. This grain is relatively expensive, and is mostly used as an epicurean food and served with fine meals often mixed with Oryza rice.

Sugarcane

Sugarcane (Saccharum officinarum ) is a very tall tropical grass that can grow as high as 23 ft (7 m) high, most likely derived from wild plants that grew in marshes in India. The stems or canes of this species can be as thick as 2 in (5 cm), and they have a sweet pith that typically contains 20% of a sugar known as sucrose. Sugar concentration varies greatly during the life cycle but is greatest when the cane is flowering, so this is when the harvest typically occurs. Sugarcane is propagated by planting sections of stems with at least one node, known as cuttings.

Sugarcane is grown widely in the subtropics and tropics; for example, in southern Florida, Cuba, and Brazil. Most of the harvest is refined into sucrose, or table sugar. Increasingly, however, sugarcane is used to manufacture ethanol as a fuel for vehicles.

Pasture grasses

Pasture grasses are species cultivated as nutritious fodder for agricultural animals such as cattle, sheep, horses, and goats. These grasses are often grown in combination with fodder legumes to provide better nutrition. Pasture foods can be eaten directly by the grazing animals, or may be harvested, baled, dried, and used later as hay. In recent decades, there has been a great increase in the use of hay silage, in which harvested pasture materials are stored under moist, oxygen-poor conditions for some time while microorganisms ferment some of the materials and develop a more nutritious product for the livestock.

Some pasture grasses that are commonly grown in North America include cocksfoot (Dactylis glomerata ), timothy (Phleum pratense ), meadow foxtail (Alopecurus pratensis ), and rye grasses (Lolium perenne and L. multiflorum ).

Other products obtained from grasses

The bamboos (Bambusa spp.) are fast-growing woody grass species. The largest bamboos can grow taller than 131 ft (40 m) and can have a diameter of 12 in (30 cm). The most important genera of the larger bamboos are Arundinaria, Bambusa, Dendrocalamus, Gigantochloa, and Phyllostachys. These tree-sized grasses occur in forests and in cultivation in subtropical and tropical parts of the world. Bamboo stalks are woody and strong and are widely used as a building and scaffolding material, especially in Asia. Bamboo canes are also split and used for thatch and for many other purposes. The young shoots can be steamed or boiled and eaten as a vegetable.

Some tropical grass species have essential oils in their tissues, and these can be extracted and used in the manufacturing of perfumes. Oil of citronella is distilled from the foliage of citronella grass (Cymbopogon nardus ) and is used as a scent and as an insect repellent. The lemongrass (C. citratus ) and gingergrass (C. martinii ) also yield aromatic oils that are used as scents and in medicine.

Sweet grass (Hierochloe odorata ) is an aromatic grass that grows in temperate regions of North America. This grass has long been used by Native Americans for basket weaving, and it is also smoked in culturally significant “sweetgrass” ceremonies.

The Job’s tears (Coix lachryma-jobi ) of southeast Asia produces large white, lustrous seeds that can be eaten but are mostly used to make attractive necklaces, rosaries, and other decorations, often dyed in various attractive colors.

Grasses in horticulture

Some grass species are grown in horticulture as attractive foliage plants. Some varieties have been developed with variegated leaves, that is, with foliage that is mottled with green or white areas. Examples include reed canary grass (Phalaris arundinacea ) and bent grass (Agrostis stolonifera ). Various bamboo species, both large and small, are also cultivated in climates where the winters are not severe. Pampas grasses (Cortaderia spp.) are tall herbaceous grasses cultivated for their large, whitish fruiting heads.

Of course, grasses are also the most commonly cultivated plants to develop lawns around homes, public buildings, parks, and golf courses. Various species are favored as so-called turf-grasses, depending on the soil type, climate, amount of shading that the site has, and the type of use that the lawn is likely to

receive. Commonly used species include Kentucky bluegrass (Poa pratensis ), meadowgrass (P. palustris ), Canada bluegrass (P. compressa ), bent grass (Agrostis tenuis ), redtop (A. alba ), creeping red fescue (Festuca rubra ), tall fescue (F. arundinacea ), and rye grasses (Lolium perenne ) and (L. multiflorum ).

Grasses as weeds

Some people have developed allergies to grass pollen, which can be abundant in the atmosphere when these plants are flowering. Although many wind-pollinated plants contribute to hay fever, grasses are among the most important causes during the early and mid-summer seasons in temperate climates.

Some species may be deemed to be weeds for other reasons. Crabgrasses (Digitaria spp.), for example, are unwanted in lawns, and for that reason they are considered important weeds. Other grasses interfere with the productivity of agricultural crops, and they may be weeds for that reason. Barnyard grass (Echinochloa crus-galli ), for example, can be abundant in fields of cultivated rice, causing losses of economic yield in the form of rice grains. Another example is ylang-ylang (Imperata cylindrica ), a weed of various types of cultivated lands in tropical Asia. This grass can be such an aggressive plant that it is sometimes referred to as the world’s worst weed.

Other weed grasses are non-native species that have been introduced beyond their original range and have become seriously invasive in their new habitats. Sometimes these species can become dominant in natural communities and thereby seriously degrade the habitat for native plants and animals. In North America, for example, the reed canary grass (Phalaris arundinacea ) and giant manna grass (Glyceria maxima ) have invaded some type of wetlands, causing serious ecological damages in terms of habitat availability for native species. In semiarid parts of the Great Plains of the western United States, the downy bromegrass (Bromus tectorum ), a Eurasian species, has become abundant. The highly flammable late-season biomass of this grass has encouraged frequent fires in this habitat. This too-frequent disturbance regime has converted the naturally shrub-dominated ecosystem into a degraded one dominated by bromegrass, which supports few of the original, native species of plants and animals.

KEY TERMS

Awn —A sometimes long, bristle-like structure that extends from the tip of a leaf or floral part.

Caryopsis —A dry, one-seeded fruit in which the seed is tightly connected to its sheathing pericarp, a tissue derived from the ovary wall. (Also known as a grain.)

Essential oil —These are various types of volatile organic oils that occur in plants and can be extracted for use in perfumery and flavoring.

Inflorescence —A grouping or arrangement of florets or flowers into a composite structure.

Ligule —In grasses, this is a small hair- or scale-like tissue that develops where the leaf blade, leaf sheath, and stem all meet.

Malt —This is a preparation in which grain is soaked in water and allowed to germinate, and then fermentation by yeast is encouraged by removing the supply of oxygen. Malts are used in the preparation of ales, and they may be distilled to prepare a malt liquor or to manufacture pure grain alcohol, or ethanol.

Pith —A diffuse, spongy tissue that occurs inside of the stems of most herbaceous plants and is mostly used for storage of energy-rich nutrients such as carbohydrates.

Rhizome —This is a modified stem that grows horizontally in the soil and from which roots and upward-growing shoots develop at the stem nodes.

Tassel —A terminal spike-like inflorescence of male flowers, usually with one or more inflorescences of female flowers located beneath. The flowering structures of maize plants have this arrangement.

Weed —Any plant that is growing abundantly in a place where humans do not want it to be.

Clearly, the grass family contains species that are extraordinarily important to the welfare of humans and other creatures. Some grasses are consequential because they are such important food sources. Others are important because they can take advantage of ecological opportunities provided for them by human activities and disturbances. Especially important in this respect has been the dispersal of some species of grasses far beyond their native ranges. In their new colonized habitats the productivity and fecundity of these invasive grasses are not limited by the natural constraints that they experience in their original range such as diseases and herbivory. This is how these plants become invasive weeds.

See also Prairie.

Resources

BOOKS

Barbour, M.G., and W.D. Billings, eds. North American Terrestrial Vegetation. New York: Cambridge University Press, 1988.

Hvass, E. Plants That Serve and Feed Us. New York: Hippocrene Books, 1975.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Klein, R.M. The Green World. An Introduction to Plants and People. New York: Harper and Row, 1987.

OTHER

Palomar Community College: Wayne’s Word—An On-Line Textbook of Natural History “Photos of Some Important Cereal Grasses: Rye, Wheat, Sorghum, and Rice” <http://waynesword.palomar.edu/ ecoph12.htm> (accessed November 25, 2006).

Bill Freedman

Grasses

Grasses are monocotyledonous plants in the family Poaceae (also known as Gramineae). There are as many as 10,000 species of grasses distributed among more than 600 genera. The richest genera of grasses are the panic-grasses (Panicum spp.) with 400 species, the bluegrasses (Poa spp.) and love-grasses (Eragrostis spp.) with 300 species each, and the needle-grasses (Stipa spp.) with 200 species.

Species of grasses occur worldwide in virtually any habitats that are capable of supporting vascular plants. Grasses are the dominant species in some types of natural vegetation such as prairies and steppes, and they are an important source of forage for many species of herbivorous animals. Some species of grasses are grown as agricultural crops , and these are among the most important foods for humans and domestic livestock . The most important of the agricultural grasses are maize, wheat , rice , sorghum , barley , and sugar cane.

Biology of grasses

Most grasses are annual plants or are herbaceous perennials that die back to the ground surface at the end of the growing season and then regenerate the next season by shoots developing from underground rhizome or root systems. A few species, such as the bamboos, develop as shrub- and tree-sized, woody plants.

The shoots of grasses typically have swollen nodes, or bases, and they are often hollow between the nodes. The leaves are usually long and narrow and have parallel veins . A specialized tissue called a ligule is usually present at the location where a leaf sheaths to the stem. The flowers of grasses are typically small, monoecious or dioecious, and are called florets. The florets have various specialized tissues, and often contain a long bristle called an awn, which can be quite prominent in some species. The florets are generally arranged into an inflorescence, or cluster, which can be quite large in some species. Pollination of grasses occurs when grass pollen is shed to the wind and carried opportunistically to other grasses. The fruits of grasses are known as a caryopsis or grain, are one-seeded, and can contain a large concentration of starch.

Native grasses of North America

Hundreds of species of grasses are native to North America . Native grasses are present in virtually all habitats, and they are among the most dominant plants in prairies, some types of marshes, and similar, herbaceous types of vegetation. In addition, many species of grasses have been introduced by humans from elsewhere, especially from western Eurasia.

Although many rich varieties of form and function are represented by the native grasses of North America, only a few of the most prominent species of selected, grass-dominated habitats will be briefly mentioned.

The temperate prairies of North America are dominated by herbaceous perennial plants, many of which are species of grasses. In the tall-grass prairies, some of the grasses can grow as high as 6.5 ft (2 m). Examples of these tall species include the big blue-stem (Andropogon gerardi), indian grass (Sorghastrum nutans), dropseed (Sporobolus asper), needle grass (Stipa spartea), panic grass (Panicum virgatum), wild rye (Elymus virginicus), and others. Somewhat drier sites support mixed-grass prairies containing shorter species, for example, little blue-stem (Andropogon scoparius), grama grass (Bouteloua gracilis), wheat grass (Agropyron smithii), and green needlegrass (Stipa viridula). The driest habitats support semi-arid, short-grass prairies with species such as grama grasses (Bouteloua dactyloides and B. gracilis), dropseed (Sporobolus cryptandrus), muhly grass (Muhlenbergia torreyana), and Junegrass (Koerlia comata).

Some species of grasses can be abundant in marshes, including the reed (Phragmites communis) which can reach a height greater than 13 ft (4 m) and is North America's tallest grass. The reed is a very widespread species, occurring in marshes on all of the continents. Some seaside habitats can also develop perennial grasslands . Sandy habitats are typically dominated by species of grasses such as the beach grass (Ammophila breviligulata), sand-reed (Calamovilfa longifolia), and beach rye (Elymus mollis). Salt-marshes are brackish , estuarine habitats that are typically dominated by cord grasses, such as Spartina alterniflora and S. patens, two species which segregate within the same salt-marshes on the basis of salinity and moisture gradients.

Although it was actually introduced to North America from Europe , the so-called Kentucky blue-grass (Poa pratensis) is now a very widespread species. Kentucky blue-grass is one of the most common species in lawns, and it also occurs widely in disturbed habitats.

Grasses in agriculture

In terms of the economic and nutritional values of foods provided for humans and domestic livestock, no other plant family is as important as the grasses. All of the important cereals and grains are members of the grass family, and some of these agronomic species have been cultivated for thousands of years. There are useful cereal species available for all of the climatic zones in which humans commonly live, and this has been one of the most important reasons why our species has been able to develop such large and prosperous populations during the past several thousand years.

Wheats

The bread wheat (Triticum aestivum or T. vulgare) is a very important grain species. The origins of the modern bread wheat are somewhat uncertain, because this species occurs in many hybrid varieties which have been selectively bred over time by complex, unrecorded hybridizations of various species of Triticum. Some botanists believe that the major progenitor species was an ancient cultivated wheat known as emmer (T. dicoccum) which was grown in the Middle East at least 5,000 years ago. Other ancient wheats which have also contributed to the genetic make-up of the modern bread wheat include einkorn (T. monococcum) from southwestern Asia and spelt (T. spelta) and durum (T. durum) from the Mediterranean region.

The numerous varieties of wheat have been bred for various purposes and climatic regimes. The flowering heads of wheats can have long awns as in the so-called "bearded" wheats, or they can be awnless. Wheat can be sown in the spring or in the previous autumn, known as winter wheat. Winter wheat generally has larger yields than spring wheat because it has a longer growing season. The so-called "soft" wheats are mostly used for baking breads and pastries, while the "hard" or durham wheats are used to prepare pastas and other types of noodles.

Wheat is rarely grown in subtropical or tropical climates because it is too susceptible to fungal diseases under warm and humid conditions. The best climatic regime for growing wheat involves a temperate climate with soil moisture available during the spring and summer while the plants are actively growing, and drier conditions later on while the seeds are ripening and when the crop is being harvested.

Certain landscapes of the temperate zones that used to support natural prairies and steppes are now the best regions for the cultivation of wheat. These include the mixed-grass and short-grass prairies of North America and similar zones in the pampas of South America , the steppes of western Russia and Ukraine, parts of central China and Australia , and elsewhere. Winter wheat tends to be the favored type grown in places where the environmental regime is more moderate, while spring wheats are sown under more extreme climatic conditions.

Wheat grains are manufactured into various edible products. Most important is flour, finely milled wheat, which is mostly used to bake breads, sweetened cakes, and pastries, and also for manufacturing into pastas and noodles. Wheat is also used to manufacture breakfast cereals, such as puffed wheat, shredded wheat, and fiber-rich bran flakes. Wheat grains are fermented in a mash to produce beer and other alcoholic beverages and also industrial alcohol . Wheat straw and hay are sometimes
used as fodder for animals or as stuffing, although the latter use is now uncommon because so many synthetic materials are available for this purpose.

Maize or corn

Maize, corn, or mealies (Zea mays) is derived from grasses native to Central America, probably from Mexico. Maize has a very distinctive, flowering structure, with a tassel of male flowers perched above the larger clusters of female flowers. Each of the several female flower clusters is an elongated, head-like structure known as a cob or ear , enclosed within sheathing leaves or bracts, known as husks. Each ear contains as many as several hundred female flowers, each of which may produce a seed known as a kernel. During the time that they are ripe for pollination, and the stigmas of the female flowers are borne outside of the sheathing leaves of the cob on very long styles known as corn silk.

As with wheat, maize occurs in a wide range of cultivated varieties bred for particular uses and climates. The maize plant has been so highly modified by selective breeding for agriculture that it is now incapable of reproducing itself without the aid of humans. The ripe grains of the plant are no longer able to detach from their husk or cob (this is known as shattering). Moreover, the ripe kernels are tightly enclosed within their sheathing husks so that they are trapped by those leaves when they germinate. Modern maize can only be propagated if humans remove the leaves and grains from the cob and sow the ripe seeds.

Some of the presumed, wild ancestors of maize still occur in natural habitats in Mexico. One of these is teosinte (Zea mexicana), a wild grass that does not form a cob encased in husks. Another possible progenitor of maize is a grass called Tripsacum mexicanum that does not look much like corn but will readily hybridize with it. The wild relatives of maize are of enormous importance because they contain genetic variation that no longer is present in the highly inbred races of maize that exist today, particularly the varieties that are used widely in modern, industrialized agriculture. As such, some of the genetic information in the remaining wild species that participated in the cultural evolution of the modern maize plant may prove to be incredibly important in the future breeding of disease resistance, climatic tolerance, and other useful attributes of this critical food plant for humans.

Maize grows well under a hot and moist climatic regime, and it can be cultivated in both the tropical and temperate zones. Maize is used in many forms for direct consumption by humans. During the harvest season, much sweet corn is eaten after boiling or steaming. Maize is also eaten as a cooked porridge made of ground meal (in the southeastern United States, this food is known as grits). Other foods include canned or frozen cooked kernels, corn flakes, tortillas, corn chips, and popcorn. The small, unripe cobs of maize can also be steamed or boiled and eaten as a nutritious vegetable. Corn seeds can also be pressed to manufacture an edible oil.

Much of the maize crop in North America is fed to livestock. The nutritional value of the maize is greatly enhanced if the plants are chopped up and subjected to a fermentation process before being used for this purpose. This type of preparation which can also be prepared from other grasses and from mixed-species hay is known as silage.

In some regions such as the midwestern United States, much of the maize production is utilized to manufacture ethanol for use as a fuel in automobiles, usually blended with liquid petroleum hydrocarbons as a mixture known as gasohol. Other products made from maize include corn starch, corn syrup, and alcoholic beverages, such as some types of whiskey.

Rice

Rice (Oryza sativa) is probably a native of south Asia, and it has been cultivated on that continent for more than 5,000 years. The natural habitat of rice is tropical marshes, but it is now cultivated in a wide range of subtropical and tropical habitats.

If rice is being cultivated under flooded conditions, its seeds are germinated, grown until they are about 6-12 in (15-30 cm) tall, and then out-planted into the sediment in shallow water . In Asia, this cultivation system is known as paddy. A variant of this system is also used in the southern United States where fields are flooded to plant and grow the crop and then drained for optimal ripening and the harvest. Rice can also be cultivated under drier conditions, called "upland" rice, although the soil must be kept moist because the species is intolerant of drought . On moist, fertile sites in some parts of tropical Asia, two to four rice crops can be harvested each year, although this eventually could deplete the soil of its nutrient capital.

Rice is mostly eaten steamed or boiled, but it can also be dried and ground into a flour. Like most grains, rice can be used to make beer and liquors. Rice straw is used to make paper and can also be woven into mats, hats, and other products.

Other important agricultural grasses

Barley (Hordeum vulgare) is a relatively ancient crop species, having been grown in northeastern Africa and the Middle East for as long as 6,000 years. The environmental conditions favorable to the growth of barley are similar to those for wheat, although barley can be cultivated in somewhat cooler conditions and therefore farther to the north in Eurasia. Most barley is used as feed for domestic animals, but it is also used as a malt in brewing ale and other alcoholic beverages.

Rye (Secale cereale) is an agricultural grass that originated in Asia. This species is mostly cultivated in north-temperate regions of central Asia and Europe. The flour is used to make rye breads and crisp breads, and it is sometimes used in a mash to prepare rye whisky.

Oats (Avena sativa) probably originated in western Asia, and they have been cultivated for more than 2,000 years. Unlike most of the temperate, agricultural grasses, oats are relatively tolerant of late-summer and autumn rains. Oats are mostly used as fodder for cattle and horses , but they are also used to prepare breakfast cereals, such as rolled oats and oatmeal porridge. The Turkish oat (A. orientalis) and short oat (A. brevis) are relatively minor cultivated species.

Sorghum (Sorghum bicolor) is a small-grained cultivated species. Sorghum has been grown in Africa for at least 4,000 years, and it is still probably the most important crop for the making of bread flour on that continent. Sorghum is also widely used in Africa to prepare a mash for the brewing of beer. Some varieties of sorghum, known as broom-corns, are used to manufacture brushes, while others are used as forage crops.

Various other small-grain grasses are commonly known as millet. The most important species is the proso millet (Panicum miliaceum), which originated in tropical Africa or Asia, and has been cultivated for more than 5,000 years. This species is relatively tolerant of drought, and it is most commonly cultivated under drier climatic regimes in Africa and Asia. Proso millet is commonly eaten as a cooked porridge, and it is also an important ingredient in commercial birdseeds. More minor species of millets include pearl millet (Pennisetum glaucum), foxtail millet (Setaria italica), Japanese millet (Echinochloa frumentacea), shama millet (E. colona), barnyard millet ( E. crus-galli), and ragi millet (Eleusine coracana).

Wild rice (Zizania aquatica) is a North American grass that grows naturally in shallow waters of temperate lakes and ponds, and has long been collected from the wild, usually by beating the ripe grains off their heads into a canoe, using a paddle. During the past several decades, however, this species has also been cultivated on farms in
the southwestern United States. This grain is relatively expensive, and is mostly used as an epicurean food and served with fine meals often mixed with Oryza rice.

Sugar cane

Sugar cane (Saccharum officinarum) is a very tall, tropical grass which can grow as high as 23 ft (7 m), most likely derived from wild plants that grew in marshes in India. The stems or canes of this species can be as thick as 2 in (5 cm), and they have a sweet pith that typically contains 20% of a sugar known as sucrose. The concentration of sugar varies greatly during the life cycle but is greatest when the cane is flowering, so this is when the harvest typically occurs. Sugar cane is propagated by planting sections of stems with at least one node, known as cuttings.

Sugar cane is grown widely in the subtropics and tropics; for example, in southern Florida, Cuba, and Brazil. Most of the harvest is manufactured into refined sucrose, or table sugar. Increasingly, however, sugar cane is used to manufacture alcohol as a fuel for vehicles.

Pasture grasses

Pasture grasses are species that are cultivated as nutritious fodder for agricultural animals such as cattle, sheep , horses, and goats . These grasses are often grown in combination with fodder legumes to provide better nutrition for the livestock. The pasture foods may be eaten directly by the grazing animals, or they may be harvested, baled, dried, and used later as hay. In recent decades, there has been a great increase in the use of hay silage in which harvested pasture materials are stored under moist, oxygen-poor conditions for some time while microorganisms ferment some of the materials and develop a more nutritious product for the livestock.

Some of the pasture grasses that are commonly grown in North America include cock's-foot (Dactylis glomerata), timothy (Phleum pratense), meadow fox-tail (Alopecurus pratensis), and rye-grasses (Lolium perenne and L. multiflorum).

Other economic products obtained from grasses

The bamboos (Bambusa spp.) are fast-growing, woody species of grasses. The largest species of bamboos can grow taller than 131 ft (40 m) and can have a diameter of 12 in (30 cm). The most important genera of the larger bamboos are Arundinaria, Bambusa, Dendrocalamus, Gigantochloa, and Phyllostachys. These treesized grasses occur in forests and in cultivation in subtropical and tropical parts of the world. Bamboo stalks are woody and strong and are widely used as a building and scaffolding material, especially in Asia. Bamboo canes are also split and used for thatching and for many other purposes. The young shoots can be steamed or boiled and eaten as a vegetable.

Some tropical species of grasses have essential oils in their tissues, and these can be extracted and used in the manufacturing of perfumes. Oil of citronella is distilled from the foliage of citronella grass (Cymbopogon nardus) and is used as a scent and as an insect repellent. The lemon-grass (C. citratus) and ginger-grass (C. martinii) also yield aromatic oils which are used as scents and in medicine.

Sweet grass (Hierochloe odorata) is an aromatic grass that grows in temperate regions of North America. This grass has long been used by Native Americans for basket weaving, and it is also smoked in culturally significant "sweetgrass" ceremonies.

The Job's tears (Coix lachryma-jobi) of southeast Asia produces large, white, lustrous seeds that can be eaten but are mostly used to make attractive necklaces, rosaries, and other decorations, often dyed in various attractive colors.

Grasses in horticulture

Some species of grasses are grown in horticulture as attractive foliage plants. Some varieties have been developed with variegated leaves, that is, with foliage that is mottled with green or white areas. Examples include reed canary-grass (Phalaris arundinacea) and bent-grass (Agrostis stolonifera). Various species of bamboos, both large and small, are also cultivated in gardens in climates where the winters are not severe. The pampas grasses (Cortaderia spp.) are tall, herbaceous grasses that are cultivated for their large, whitish fruiting heads.

Of course, grasses are also the most commonly cultivated plants to develop lawns around homes, public buildings, parks, and golf courses. Various species are favored as so-called turf-grasses, depending on the soil type, climate, amount of shading that the site has, and the type of use that the lawn is likely to receive. Commonly used species include Kentucky bluegrass (Poa pratensis), meadowgrass (P. palustris), Canada bluegrass (P. compressa), bent-grass (Agrostis tenuis), red-top (a. alba), creeping red fescue (Festuca rubra), tall fescue (F. arundinacea), and ryegrasses (Lolium perenne and L. multiflorum).

Grasses as weeds

Some people have developed allergies to grass pollen which can be very abundant in the atmosphere at times when these plants are flowering. Although many wind-pollinated plants contribute to hay fever, grasses are among the most important causes during the early and mid-summer seasons in temperate climates.

Some species of grasses may be deemed to be weeds for other reasons. Crabgrasses (Digitaria spp.), for example, are unwanted in lawns, and for that reason they are considered to be important, aesthetic weeds. Other grasses interfere with the productivity of agricultural crops, and they may be weeds for that reason. The barnyard grass (Echinochloa crus-galli), for example, can be abundant in fields of cultivated rice, causing losses of economic yield in the form of rice grains. Another example is ilang-ilang (Imperata cylindrica), a weed of various types of cultivated lands in tropical Asia. This grass can be such an aggressive plant that it is sometimes referred to as the world's worst weed.

Other weed grasses are non-native species that have been introduced beyond their original range and have become seriously invasive in their new habitats. Sometimes these species can become dominant in natural communities and thereby seriously degrade the habitat for native plants and animals. In North America, for example, the reed canary-grass (Phalaris arundinacea) and giant manna-grass (Glyceria maxima) have invaded some type of wetlands , causing serious ecological damages in terms of habitat availability for native species. In semi-arid parts of the Great Plains of the western United States, the downy brome-grass (Bromus tectorum), a Eurasian species, has become abundant. The highly flammable, late-season biomass of this grass has encouraged frequent fires in this habitat. This too-frequent disturbance regime has converted the naturally shrub-dominated ecosystem into a degraded system dominated by the brome-grass, which supports few of the original, native species of plants and animals.

Clearly, the grass family contains species that are extraordinarily important to the welfare of humans and other creatures. Some of these grasses are consequential because they are such important sources of food. Others species are important because they have been able to take advantage of ecological opportunities provided for them by human activities and disturbances. Especially important in this respect has been the dispersal of some species of grasses far beyond their native ranges. In their new, colonized habitats the productivity and fecundity of these invasive grasses are not limited by the natural constraints that they experience in their original range such as diseases and herbivory. This is how these plants become invasive weeds.

Resources

books

Barbour, M.G., and W.D. Billings, eds. North American Terrestrial Vegetation. New York: Cambridge University Press, 1988.

Hvass, E. Plants That Serve and Feed Us. New York: Hippocrene Books, 1975.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Klein, R.M. The Green World. An Introduction to Plants and People. New York: Harper and Row, 1987.

Bill Freedman

KEY TERMS

Awn

—A sometimes long, bristle-like structure that extends from the tip of a leaf or floral part.

Caryopsis

—A dry, one-seeded fruit in which the seed is tightly connected to its sheathing pericarp, a tissue derived from the ovary wall. (Also known as a grain.)

Essential oil

—These are various types of volatile organic oils that occur in plants and can be extracted for use in perfumery and flavoring.

Inflorescence

—A grouping or arrangement of florets or flowers into a composite structure.

Ligule

—In grasses, this is a small hair- or scale-like tissue that develops where the leaf blade, leaf sheath, and stem all meet.

Malt

—This is a preparation in which grain is soaked in water and allowed to germinate, and then fermentation by yeast is encouraged by removing the supply of oxygen. Malts are used in the preparation of ales, and they may be distilled to prepare a malt liquor or to manufacture pure grain alcohol, or ethanol.

Pith

—A diffuse, spongy tissue that occurs inside of the stems of most herbaceous plants and is mostly used for storage of energy-rich nutrients such as carbohydrates.

Rhizome

—This is a modified stem that grows horizontally in the soil and from which roots and upward-growing shoots develop at the stem nodes.

Tassel

—A terminal, spike-like inflorescence of male flowers, usually with one or more inflorescences of female flowers located beneath. The flowering structures of maize plants have this arrangement.

Weed

—Any plant that is growing abundantly in a place where humans do not want it to be.

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 grasses—especially those that grow deep in the rain forest—are 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

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.

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 crops—corn, wheat, and rice—are 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.

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.

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.