Encyclopedia of Global Resources part 102 doc

mizing the yield per hectare of wooded land. He built a strong U.S. Forest Service, financed in part by the sale of mature timber. He actively opposedthe institu- tion of national parks to be used for recreation, con- sidering them a waste of natural resources. After leav- ing the U.S. Forest Service, Pinchot became active in the Progressive Party, founded by supporters of Theo- dore Roosevelt. Pinchot became chief forester of Pennsylvania in 1920, and he campaigned for and won election as governor of Pennsylvania in 1923 and again in 1931. Donald R. Franceschetti See also: Conservation; Forest management; Forest Service, U.S.; Leopold, Aldo; Roosevelt, Theodore. Placer deposits Categories: Geological processes and formations; mineral and other nonliving resources Placer depositsare mechanical concentrationsof debris weathered out of rocks. Commonly, economically important minerals have higher densities, sothey are concentrated as the lighter-density minerals are winnowed out by the action of water or wind. Background Placer deposits are found throughout the world wher- ever the mechanisms of concentration, water and wind, have been active and the resulting concentrates have notbeen redispersed by later processes. Thebest known types of placers occur in river channels and in beach sediments. The weathering and erosion of rocks release particles of varyingsize, shape,anddensity.Soluble materials are dissolved and removed in surface water or groundwater. Some minerals, such as feldspars, are hydrated and converted into clay minerals, which, being soft, small, and of lowdensity, are relatively readily removed in suspension. Quartz (SiO 2 ), common in many kinds of rocks, generally weathers out as roughly equant grains that, because of their hardness and insolubility, wash into streams and rivers, where they are moved by rolling, bouncing, and pushing along toward the oceans. The abundance of quartz and its resistance to mechanical and chemical weath - ering results in its being the most abundant placer mineral and the principal constituent of temperate and cold climate beaches throughout the world. During the weathering and erosional processes, other minor or trace minerals, which are resistant to breakdown, are also transported along with the quartz grains and pebbles in river channels to the ocean margins. If the mineral particles possess high densities, they may be selectively concentrated as the transporting agent (usually water) more readily re- moves the lighter-density minerals. Thus, gold nug- gets, with densities of 15-19 grams per cubic centimeter, are commonly concentrated in residual materials as thequartz grains,with adensity ofabout 2.65grams per cubic centimeter, are removed. The densities of several other valuable and resistant minerals are sufficiently higherthan thatof quartz toallow themto also be concentratedin placer deposits (examples include ilmenite, FeTiO 3 , 4.8; rutile, TiO 2 , 4.25; zircon, ZrSiO 4 , 4.7; cassiterite, SnO 2 , 7.0; and diamond, C, 3.5). Economically Important Placer Minerals Many types of minerals and rock materials can occur in placer deposits; among the most important are gold, titanium minerals, zircon, tin oxide, diamonds, platinum, and sand and gravel. Throughout history, gold has no doubt been the most important placer mineral. Gold is soft and malleable but is otherwise nearly inert in the weathering realm. Hence, once gold is weathered out of thelode deposits where itini- tially formed, the grains may survive transport in streams and rivers over long distances. It was the discovery of such gold grains that led to nearly all of the world’s major gold rushes, including the California gold rush in 1849. Gold placershave formed throughout geologic time; the world’s largest gold reserves in South Africa occur in placers formed 2 billion years ago. The titanium minerals, ilmenite and rutile, occur in minor amounts as small grains in many types of igneous and metamorphic rocks. These minerals are highly resistant toweathering andhence are liberated intact from their host rocks. Although their densities are less than twice that of quartz, they are quite effectively concentrated by flowing water in rivers and by the agitation of waves along beaches as the lower density quartz grains are winnowed out. The zirconium silicate zircon is a common accessory mineral in alka- line igneous rocks. It weathers out as the titanium minerals do and is generally found with those miner - als in river and beach deposits. 938 • Placer deposits Global Resources The tin oxide cassiterite, like the titanium miner - als, is heavy, hard, and extremely resistant to weathering. Consequently, where there are cassiterite-bearing lode deposits, the cassiterite weathers out and may be concentrated into economic placers. The hardness of diamonds and their resistance to normal weathering agents has allowed them to wash down rivers that drain from the areas of exposed diamond pipes and to occur in placer river and beach deposits. Because the density of the diamonds is only about one third greater than that of quartz, the diamonds are not as well selectively concentrated in these placers as are the heavier minerals noted above. Platinum, like gold, is chemically inert and has a high density, 15-19 grams per cubiccentimeter.Platinum lode deposits are much more restricted geologically than are gold deposits; hence, placer platinum deposits occur in only a few places in the world. The sand and gravel deposits found in rivers and lakes and on beaches constitute the largest placer deposits in that they represent mechanically concentrated residual materials. Although they have a much lower per unit value than many other placer materials, the large volumes of sand and gravel mined from these deposits actuallymake themthemost important placer deposits economically. Types of Placer Deposits Placer depositshave been classifiedinto severaldiffer- ent types on the basis of location of formation. The major types includeresidual placers, eluvial placers, stream or river placers, riverbank and flood placers, eolian placers, and beach placers. Residual placers are occur- rences of minerals at or near their point of release from the original source rocks. There has been some degree of enrich- ment ofthe placer minerals as the result of the removal of other portions of the host rocks by weathering. Eluvial placers are transitional placers in which concentra- tions of placer minerals occur downslope from the source rocks but where the valued minerals have not yet washed into streams and rivers that would transport them for long distances. Stream or river placers are the best known placers and are the types responsi- ble for most famous gold discoveries. The movement oftherunningwater, especially where there is turbulence, is effective in sorting rock fragments and mineral grains according to size and density. Because of their higher densities, gold grains and several other placer minerals settle out. They are readily trapped in crevices and irregu- larities on the stream bed or among larger boulders, as the lower density materials are more easily washed away. This type of placer sometimes grades into deltaic beds where a river drains into a lake or the ocean. Riverbank and flood placers are depos - its adjacent to streams and rivers that have been left as the rivers meander, cut down - Global Resources Placer deposits • 939 A nineteenth century magazine depiction of a miner using a “cradle” to mine a placer deposit in 1880’s California. (Library of Congress) ward, or overflow their banks in flood conditions. During the natural development of rivers, they commonly shift laterally across their floodplains, eroding banks on one side while depositing materials on the other side. In some areas, changes inbase levels result in riverscutting downwardthough the sediments they had previously deposited in their floodplains. In both these circumstances, valuable placer deposits may be left inthe riverbank sediments that areadjacent tothe present rivers. Flood placers also occur in the sediments adjacent to rivers. They form during episodic flooding when water flow is sufficiently rapid and tur- bulent to transport gold or other valued placer mineral grains up and out of the channels onto the adjacent floodplains. As the water spreads laterally and its velocity decreases, thegold grainsare leftas placerde- posits along the adjacent floodplains. Eolian placers are wind-formed placers that occur locally in desert regions where high winds have removed lighter mineral grains, thereby enriching the heavy minerals in the residuum. Beach placers are generally formed by the combined effects ofriver transport of weathered materials to coastal margins and the action of tides and storm waves along beaches. The ebb and flow of the waves and the generation of longshore currents, especially under storm conditions, can effectively winnow and sort beach materials such that certain areas are highly enriched in heavy minerals. Gold-bearing beach placers are known in many localities, but the most famous are probably those at Nome, Alaska, where the beaches were activelymined formany years. Diamond-bearing beach placers have been extensively mined along the west coast of central and southern Africa,where rivers draining the interior have transported diamonds into the beach sands. Beach placers containing ilmenite, rutile, and zircon are the world’s major sources of these minerals. Mining of Placer Deposits Gold panning is probably the best known method of exploiting placer deposits. A circular motion of water in a pan containing gold along with other sediments effectively separates the minerals on the basis of their densities. The same general principle isused in sluices, channel-like boxes with barriers to create turbulence in the water so that sorting can take place. On a large scale, modern placers are mined by the scooping up of the unconsolidated materials and the use of either spiral classifiers or heavy media to separate the heavy materials from the light materials. The differences in the densitiesof theminerals allowfor effective separa- tion. James R. Craig Further Reading Bell, Fred J., and Laurance J. Donnelly. “Placer De- posits and Mining.” In Mining and Its Impact on the Environment. New York: Taylor & Francis, 2006. Boggs, Sam, Jr. Principles of Sedimentology and Stratigra- phy. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006. Dixon, Colin J. Atlas of Economic Mineral Deposits. Ithaca, N.Y.: Cornell University Press, 1979. Guilbert, John M., and Charles F. Park, Jr. The Geology of Ore Deposits. Long Grove, Ill.: Waveland Press, 2007. Hartman, Howard L., and Jan M. Mutmansky. Intro- ductory Mining Engineering. 2d ed. Hoboken, N.J.: J. Wiley, 2002. McCulloch, Robin, et al. AppliedGold Placer Exploration and Evaluation Techniques. Butte: Montana Bureau of Mines and Geology, 2003. Macdonald, Eoin H. Alluvial Mining: The Geology, Tech- nology, andEconomics of Placers. New York:Chapman and Hall, 1983. Valentine, David. “Chinese Placer Mining in the United States: An Example from American Can- yon, Nevada.” In The Chinese in America: A History from Gold Mountain to the New Millennium, edited by Susie Lan Cassel. Walnut Creek, Calif.: AltaMira Press, 2002. Wells, John H. Placer Examination: Principles and Prac- tice. Washington, D.C.: U.S. Department of the In- terior, Bureau of Land Management, Phoenix Training Center, 1989. Wenk, Hans-Rudolf, and Andrei Bulakh. Minerals: Their Constitution and Origin. New York: Cambridge University Press, 2004. Web Site U.S. Geological Survey Prospecting for Gold in the United States http://pubs.usgs.gov/gip/prospect2/ prospectgip.html See also: Diamond; Gold; Marine mining; Residual mineral deposits; Sand and gravel; Tin; Titanium; Weathering; Zirconium. 940 • Placer deposits Global Resources Plant domestication and breeding Category: Plant and animal resources Plant domestication and breeding refers to the process by which wild plants are intentionally bred and grown to meet human food, fiber, shelter, medicinal, or aesthetic needs. Background Perhaps any nation’s greatest resource is its ability to sustain an agricultural system with the capacity to feed, shelter, and clothe its population. The development of an agricultural system depends on an ability not only to cultivate wild plants but also to selectively breed plants to increase orimprove the production of products that are useful for food, clothing, shelter, medicines, or aesthetic purposes. No one knows exactly when the first crop was cultivated, but most authorities believe that it occurred at some time between eight and ten thousand years ago. For centuries prior to that time, humans had known that some wild plants and plant parts (such as fruits, leaves, and roots) were edible. These plants ap- peared periodically (usually annually) and randomly throughout a givenregion. Eventuallyhumans discov- ered not only that these wild plants grew from seed but also that the seed from certain wild plants could be collected, planted, and later gathered for food. This most likely occurred at about the same time in both the Sumerian region between the Tigris and Eu- phrates riversin the OldWorld and in Mexicoand the Central Americanregion ofthe NewWorld.While the earliest attempts at domesticating plants were primarily to supplement the food supply provided by hunting and gathering, people soon improved their ability to domesticate and breed plants to the point that they could depend on an annual supply of food. This food supply allowed the development of permanent settle- ments and decreased reliance on hunting and gathering. Early Crop Domestication By six thousand years ago, agriculture was firmly es- tablished inAsia, India, Mesopotamia,Egypt, Mexico, Central America, and South America. Even before recorded history, these areas had domesticated some of the world’s most important food (corn or maize, rice, and wheat) and fiber (cotton, flax, and hemp) crops. The placeof originof wheatis unknown,but manyau - thorities believe that it may have grown wild in the Tigris andEuphrates Valleys and spread from there to the rest of the Old World. Wheat was grown by Stone Age Europeans and was reportedly produced in China as far back as 2700 b.c.e. Wheat is the major staple for about one third of the people of the world. The earliest traces of the human utilization of corn (or maize, as it is also called), dates back to about 5200 b.c.e. It was probably first cultivated in the high plateau region of central or southern Mexico and represented the basic food plant of all pre-Columbian advanced cultures and civilizations, including the Inca of South America and theMaya of CentralAmerica and Mexico. Botanists believe that rice originated in southeast Asia. Rice was cultivated inIndia as early as 3000b.c.e. and spread from there throughout Asia and Malaysia. In modern society, rice is one of the world’s most important cereal grains and is the principal food crop of almost half of the world’s people. Hemp, most likely the first plant cultivated for its fiber, was cultivated for the purpose of making cloth in China as early as the twenty-eighth centuryb.c.e. It was usedas thecordage or rope on almost all ancient sailing vessels. Linen made from flax is one of the oldest fabrics. Traces of flax plantshave been identifiedin archaeological sites dating back to the Stone Age, and flax was definitely cultivated in Mesopotamia and Egypt five thousand years ago. Cotton has been known and highly valued by people throughout the world for more than three thousand years. From India, where a vigorous cotton industry waspresent asearly as 1500 b.c.e., thecultiva- tion of cotton spread to Egypt and then to Spain and Italy. Inthe West Indies andSouth Americain the New World, a different species of cotton was grown long before the Europeans arrived. Other important plants that have been under domestic cultivation since antiquity include dates, figs, olives, onions, grapes, ba- nanas, lemons,cucumbers, lentils, garlic,lettuce, mint, radishes, and various melons. Modern Plant Breeding Genetic variability is prevalent in all sexually reproduc- ing organisms, and like all other sexually reproduc- ing organisms, plants produce spontaneous mutants. Throughout most of history, plant domestication and breeding were primarily based on the propagation of these mutants. When a grower observed a plant with a potentially desirable mutation (such as a change that produced a characteristic such as bigger fruit, Global Resources Plant domestication and breeding • 941 brighter flowers, or increased insect resistance), the growerwould collectseedor takecuttings(if theplant could be propagated vegetatively) and produce addi- tional plants with the desirable characteristic. Advances in the understanding of genetics in the early part of the twentieth century made it possible to breed some of the desirable characteristics resulting from mutation into plants that previously had lacked the characteristic. The obvious advantages of producing plants with improved characteristics such as higher yield made plant breeding highly desirable. As popu- lations continued to grow, there was a need to select and produce higher-yieldingcrops. Thedevelopment and widespread successful use of new high-yield varieties of crop plants in the 1960’s is often referred to as the Green Revolution. Basic information sup- plied by biological scientists allowedplant breeders to fuse a variety of characteristics from different plants to produce new, higher-yielding varieties of numerous crops—particularly the seed grains that supply most of the calories necessary for maintenance of the world’s population. When a plant characteristic is identified as desir - able, it isstudied bothmorphologically and biochemically to determine the mechanism of inheritance. If it is determined that the mechanism is transferable, attempts are made to incorporate the trait into the tar- get plant. If the plants are closely related, traditional breeding techniques are used to crossbreed the plant with the desirable trait with the plant that lacks the characteristic. Although this process is often tedious, is sometimes difficult to accomplish, and requires considerable patience and hard work, it is based on a fairly simple concept.Basically, pollenfrom one ofthe plant types is used to fertilize the other plant type. This process often requires specialized handling techniques to ensure that only the pollen from the plant with the desired characteristic is allowed to fertilize the eggs of the recipient plant. Sometimes this process involves the use of bags or other materials to isolate the recipient flowers, which are then pollinated by hand. Another technique involves theintroduction of a gene for male sterility into the recipient plant. In these cases, only pollen from another plant can be used to fertilize the egg. Once plants with the desirable characteristics are developed, the lines are often inbred to maintain large numbers of progeny with the desired traits. In many cases, inbred lines will lose vigor after several genera- tions. When this occurs, two inbred lines are often crossed to produce hybrids. A majority of the hybrid offspring will still contain the desired characteristics but will be more vigorous. Until relatively recently, the use of traditional breeding techniques between two closely related species was theonly means of transferring heritablechar- acteristics from one plant to another. The advent of recombinant deoxyribonucleic acid (DNA) technol- ogy, however, made it possible to transfer genetic characteristics from any plant (or, in actuality, from any organism) to any other plant. The simplest method for accomplishing this transfer involves the use of a vector, usually apiece of circular DNAcalled a plasmid. The plasmid is removed from a microorgan- ism such as bacteria and cut open by an enzyme called a restriction endonuclease or restriction enzyme. A section of DNA from the plant donor cell that contains the gene for a previously identified desirable trait is cut from the donor cell DNA by the same restriction endonuclease. The section of plant donor cell DNA with the gene for the characteristic of inter - est isthen combined with the openplasmid DNA,and 942 • Plant domestication and breeding Global Resources Agriculturist Edgar E. Hartwig, the “soybean doctor,” has spent more than fifty years breeding crops resistant to diseases and pests. (United States Department of Agriculture) the plasmid closes with the new gene as part of its structure. The recombinant plasmid (DNA from two sources) is placed back into the bacteria where it will replicate and code for protein just as it did in the do- nor cell. The bacteria is then used as a vector to transfer the gene to another plant, where it will also be transcribed and translated. D. R. Gossett Further Reading Acquaah, George. Horticulture: Principles and Practices. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2009. Adams, C. R.,K. M.Bamford, and M.P. Early.Principles of Horticulture. 5th ed. Boston: Butterworth-Heine- mann, 2008. Fennema, Owen R., ed. Principles of Food Science. New York: Dekker, 1975. Hartmann, Hudson T., et al. Hartmann and Kester’s Plant Propagation: Principles and Practices. 7th ed. Upper Saddle River, N.J.: Prentice Hall, 2002. Janick, Jules. Horticultural Science. 4th ed. New York: W. H. Freeman, 1986. Kipps, M. S. Production of Field Crops: A Textbook of Agronomy. 6th ed. New York: McGraw-Hill, 1970. Martin, John H., Richard P. Waldren, and David L. Stamp. Principles ofField Crop Production.4th ed. Up- per Saddle River,N.J.: PearsonPrenticeHall, 2006. Metcalfe, Darrel S., and Donald M. Elkins. Crop Pro- duction: Principles and Practices. 4th ed. New York: Macmillan, 1980. Rather, Howard C., and Carter M. Harrison. Field Crops. 2d. ed. New York: McGraw-Hill, 1951. Vaclavik, Vickie A., and Elizabeth W. Christian. Essen- tials of FoodScience. 3d ed.New York:Springer, 2008. See also: Agricultural products; Agriculture industry; Biotechnology; Corn; Green Revolution; Horti- culture; Monoculture agriculture; Plant fibers; Rice; Wheat. Plant fibers Category: Plant and animal resources Fiber crops provide a natural source of the raw materi - als used to produce textiles, ropes, twine, and similar materials. Background The major fiber plants are cotton, flax, and hemp. Less important crops such as ramie, jute, and sisal are produced in small amounts. Cotton With a total annual production approaching 25 mil- lion metric tons in 2008, cotton (Gossypium species) is by far the most important fiber crop in the world. Since humans rely heavily on cotton for clothing and other textiles, it enters the daily life of more of the world’s people than any other product except salt. Cotton fiber has been known and highly valued by people throughout the world for more than three thousand years. As is true of most crop plants that have been in cultivation for long periods of time, the early history of cotton is obscure. A vigorous cotton industry was present in India as early as 1500 b.c.e. From India, the cultivation of cotton spread to Egypt and then toSpain andItaly. IntheNew World, adiffer- ent species of cotton was grown in the West Indies and South America long before the Europeans arrived. In the United States, cotton is grown from the East Coastto theWestCoast in the nineteen southern- most states. Botanically, cotton is in the mallow family, which includes such plants as okra, hollyhock, hibiscus, and althea. The plant has a taproot and branching stems. Flowers form at the tips of fruiting branches, and the ovary within each flower develops into a boll which contains the seed,fiber, andfuzz.The fiber,most commonly referred to as lint, develops from epidermal cells in the seed coat of the cottonseed. The fiber reaches maximum length in twenty to twenty-five days, and anadditional twenty-fivedays are required forthe fiber to thicken. Fiber length from 2.0 to 2.4 centimeters is referred to as short-staple cotton, and fiber length from 2.4to 3.8 centimeters is called long-staple cotton. The boll normally opens forty-five to sixty-five days after flowering. Cotton is native to tropical regions but has adapted to the humid, subtropical climate, where there are warm days (30° Celsius), relatively warm nights, and a frost-free season of at least 200 to 210 days. There are eight species of cotton in the genus Gossypium, but only three species are of commercial importance. Gossypium hirsutum, also known as up- land cotton, has a variable staple length and is pro - duced primarily in North and Central America. Gossypium barbadense, a long-staple cotton, is primarily Global Resources Plant fibers • 943 produced in South America and Africa. Gossypium herbaceum is ashorter-staple cotton native to India and eastern Asia. Cotton is one of the more labor-intensive and ex- pensive crops to produce. The most opportune time to plant cotton is at least two weeks after the date of the region’s last killing frost. Prior to seeding, the field is prepared by plowing to a depth of 2.5 centimeters. Fertilizer, which is applied before seeding or at the same time the seeds are planted, is placed to the side andbelow thecotton seed.Once theseeds germi- nate and emerge from the soil, they often have to be thinned, and shortly afterward, the producer begins to apply irrigation water as needed. After the plants have developed a stand, weed control becomes crucial. Weeds are controlled both by cultivation and chemical herbicides. Cotton plants are subject to invasion by a variety of insect pests such as the boll worm and boll weevil; therefore considerable attentionis givento insectcontrol, typicallyusing a number of different insecticides. When the bolls ripen with mature fiber, the leaves of the plant are removed by theapplication of a chemicaldefoliant, and the cotton fiber is harvested. Harvesting was once done almost entirely by hand, but today mechanical pickers harvest almost all the cotton produced inthe United States. The pickedcot- ton isginned to remove the seed and compressed into bales. The bales are transported to the cotton mill, where thecotton is cleanedand spun into yarn,which is then woven into fabric. One kilogram of fiber is suf- ficient to produce up to 10 square metersof the fabric used for shirts and simple dresses. Flax Flax (Linum sitatissimum) is the natural fiber used to make linen. While some flax is still produced for the purpose of producing this fabric, much of the flax, particularly that grown in the United States, is used to produce the flaxseed from which linseed can be extracted. Linen made from flax is one of the oldest fabrics. Flax was definitely cultivated in Mesopotamia and Egypt five thousand years ago, and traces of flax plants havebeenidentified inarchaeological sites dating back to the Stone Age. Flax was one of the first crops brought to North America by the early settlers. Today, most of the flax produced in the United States is grown in the north-central states. Flax, anannual plant, grows to a height of60 to100 centimeters and bears five-celled bolls or capsules with ten seeds each at the ends of fertile branches. Since the flax fiber is found in the stems from the ground to the lowest branches, varieties that are long- stemmed withlittle branching are grown forfiber production. Selection of high-quality, disease-free seed is essential in flax production. Flax fields are usually prepared in the fall to allow the soil to settle before planting. Flax is usually sown in early spring two to three weeks prior to the date of the last killing frost of the region. Con- siderable attention is given to con- trolling weeds in a flax field. When the crop is harvested for fiber, the plants are pulled from the soil, the seeds are removed, andthe flaxstraw is “retted” to separate the fiber from the woody part of the stem. When the straw is completely retted, it is dried and thenbrokenapart toremove the fifty-centimeter fibers which can be woven into fabrics. Hemp Hemp (Cannabis sativa), a term used to identify the plant and the fiber it produces, is used to make the stron- gest and mostdurable commercial fi - bers available. Hemp was most likely the first plant cultivated for its fiber. 944 • Plant fibers Global Resources A hemp stem featuring the plant’s fibers. It was cultivated for the purpose of making cloth in China as early as the twenty-eighth century b.c.e. It was also used as a drug by the ancient Persians as early as 1400 b.c.e. It was used to make the cordage or rope on almost all ancient sailing vessels. Today hemp is commercially produced for heavy textiles in numerous countries, including Canada, China, Australia, Russia, and France. Hemp production is problematic in theUnited Statesbecause itis illegalto grow Canna- bis sativa, as it is the source of marijuana. Hemp is an annual plant in the mulberry family. The plant is dioecious, meaning that it has staminate or “male” flowers and pistillate or “female” flowers. It has a rigid stalk which can reach a thickness of more than 2.5centimeters in diameter and aheight of5 me- ters. The plant has a hollow stem, and the bark or “bast” located outside the woody shell is used to make the bast fiber, which is then used to make hemp twine, ropes, and other textiles where strength and durabil- ity are desired. Humid climates with moderate tem- peratures and a period of at least 120 frost-free days are necessary for hemp production. Unlike flax, hemp requires that the soil be plowed and thoroughly disked or harrowed prior to planting. The entire above- ground portion of the plant is harvested when the male plants are in full flower. After two to three days the plants are tied inbundles and setin shocks. Hemp fiber is retted and prepared for the mills in a manner similar to that described for flax, except that heavier machines are usedtohandle thestrongerhemp stalks. Minor Crops As forthe minor fibercrops, ramie(Boehmeria nivea)is produced primarily in Asia and is used to makestrong cloth suchas Chineselinen. Jute (Corchoruscapsularis) is grown primarily in India andPakistan and is used to manufacture burlap for bags and sacks. Sisal (Agave sisalana) is produced in East Africa and the West In- dies and is used to make different types of cordage, such as baler twine. D. R. Gossett Further Reading Bourrie, Mark. Hemp: A Short History of the Most Misun- derstood Plant and Its Uses and Abuses. Buffalo, N.Y.: Firefly Books, 2003. Franck, Robert R., ed. Bast and Other Plant Fibres. Boca Raton, Fla.: CRC Press, 2005. Kipps, M. S. Production of Field Crops: A Textbook of Agronomy. 6th ed. New York: McGraw-Hill, 1970. Mauney, Jack R., and James M. Stewart, eds. Cotton Physiology. Memphis, Tenn.: Cotton Foundation, 1986. Metcalfe, Darrel S., and Donald M. Elkins. Crop Pro- duction: Principles and Practices. 4th ed. New York: Macmillan, 1980. Muir, AlisterD.,and NeilD. Westcott, eds. Flax:The Ge- nus Linum. New York: Routledge, 2003. Rather, Howard C., and Carter M. Harrison. Field Crops. 2d ed. New York: McGraw-Hill, 1951. Schreiber, Gisela. The Hemp Handbook. 2d ed. Trans- lated by Angela Hounam. London: Vision, 2003. Wakelyn, PhillipJ.,et al.Cotton Fiber Chemistry and Tech- nology. Boca Raton, Fla.: CRC Press, 2007. Yafa, Stephen H.Big Cotton: How a Humble Fiber Created Fortunes, Wrecked Civilizations, and Put America on the Map. New York: Viking, 2005. Web Sites Ameriflax http://www.ameriflax.com/ Cotton 24/7 http://www.cotton247.com/ E. F. Legner, professor emeritus, University of California, Riverside Fibers and Fiber Plants http://www.faculty.ucr.edu/~legneref/botany/ fibers.htm International Cotton Association http://www.ica-ltd.org/ See also: Agricultural products; Agriculture industry; Cotton; Flax; Hemp; Horticulture; Monoculture agriculture; Plant domestication and breeding; Tex- tiles and fabrics. Plants as a medical resource Category: Plant and animal resources Because plants are so biochemically diverse, they produce thousands ofnatural products commonlyreferred to as secondary metabolites, and many of these second - ary metabolites have medicinal properties that have proven to be beneficial to humankind. Global Resources Plants as a medical resource • 945 Background The use ofplants for medicinal purposes predates the recorded history of humankind. Primitive people’s use of trial and error in the constant search for edible plants inevitably led them to the discovery of plants that contained substances that caused appetite sup- pression, stimulation, hallucination, or other side effects. Written records show that drugs such as opium have been in use for more than five thousand years. From antiquity until fairly recent times, most practic- ing physicianswere also botanists or atleast herbalists. In contemporarysocietymedicinal plants areperhaps one of the most overlooked natural resources. Be- cause modern commercial medicines are obtained in neat packages in the form of pills, capsules, or bottled liquids, most people do not realize that many of these drugs were first extracted from plants. In some cases, chemists have learned how to duplicate synthetically the natural product that was initially identified in a plant, but in many cases, a plant may still be the only economically feasible source of the drug. Plant-Derived Medicines There are numerous ways to categorize medicinal compounds from plants. For this discussion, medicinal drugs will be categorized as antibacterial substances, anti-inflammatoryagents, drugs affecting the reproductive system, drugs affecting the heart and circulation, drugs affecting the central nervous system, antiasthma drugs, drugs affecting the gastrointestinal tract, antiparasitic agents, and anticancer agents. The first effective antibacterial substance was car- bolic acid, but the first truly plant-derived antibacterial drug was penicillin, which was extracted from an extremely primitive plant, the fungus Penicillium,in 1928. The work with penicillin led to the discovery of other fungal and bacterial compounds that have antibacterial activity. The most notable of these are cephalosporin and griseofulvin. Inflammation can be caused by mechanical or chemical damage, radiation, or foreign organisms. For centuries poultices of leaves from coriander (Coriandrum sativum), thornapple (Datura stramon- ium), wintergreen (Gaultheria procumbens), witchhazel (Hamamelis virginiana), and willow (Salix niger) were used to treat localized inflammation. In the seven- teenth and eighteenth centuries, cinchona bark was used as a source of quinine, which could be taken in - ternally. In 1876, salicylic acid was obtained from the salicin produced by the willow leaves. Today, salicylic acid, also known as aspirin, and derivatives such as ibuprofen, are the most widely used anti-inflammatory drugs in the world. The most effective home remedy for preventing pregnancy wasa tea madefrom theleaves of theMexi- can plant zoapatle (Montana tomentosa). The drug zoapatanol and its derivatives were extracted from this plant to produce the first effective birth control substance—which has not been used in human trials, however, because of potential harmful side effects. Other plant compounds that affect the reproductive system include diosgenin, extracted from Dioscorea species and used as a precursor for the pro- gesterone used in birth control pills; gossypol from cotton (Gossypium species.), which has been shown to be an effective birth control agent for males; ergome - trine, extracted from the ergot fungus (Claviceps spe - 946 • Plants as a medical resource Global Resources Factory workers on a production line inspect samples of penicillin, an antibiotic derived from fungi. (SSPL via Getty Images) cies.) and used to control postpartum bleeding; and yohimbine from the African tree (Corynanthe yo- himbe), which apparently has some effect as an aphro- disiac. Through the ages, dogbane (Apocynum cannabi- num) and milkweeds (Asclepias spp) have been prized for their effects on the circulatory system because of the presence of a group of compounds called cardiac glycosides, but foxglove (Digitalis species) has produced the most useful cardiac glycosides, digitalis and digoxin. Opiate alkaloids such as opium extracted from the poppy (Papaver sonniferum) and its derivatives such as morphine, as well as cocaine from Erythroxylumcoca andErythroxylum truxillense, have long been known for their analgesic (pain-relieving) properties through their effects on the central nervous system. Boththese drugscan also produce harmful side effects, however, and both have addictive properties. The major antiasthma drugs come from ephed- rine, extracted from the ma huang plant (Ephedra sinaica), and its structural derivatives. Plant-derived drugs that affect the gastrointestinal tract include castor oil, senna, and aloes as laxatives, opiate alkaloids as antidiarrheals, and ipecac from Cephaelis acuminata asan emetic.The mostuseful plant-derived antiparasitic agent is quinine, derived from the bark of the chinchona plant (Chinchona succirubra). Qui- nine has been used to control malaria, a disease that has plagued humankind for centuries. The primary plant-derived anticancer agents are vincristine and vinblastine, extracted from Catheranthus roseus, may- tansinoids from Maytentus serrata, ellipticine and related compounds from Ochrosia elliptica, and taxol from the yew tree (Taxus baccata). The Future Many as-yet-unknown plant-derived medicinal drugs await discovery, particularly in the tropical rain for- ests. The threats to many plant species, and biodiver- sity in general, from development and industrializa- tion may compromise the ability of humankind to take advantage of the unique compounds offered by these plants. Modern biotechnology has provided the methods by whichplants canbe bioengineeredto producenew and novel pharmaceuticals. Progress toward the production of specific proteins in transgenic plants pro - vides opportunities to produce large quantities of complex pharmaceuticals and other valuable prod - ucts in traditional farm environments rather than in laboratories. These novel strategies promise a broad array of natural or nature-based products, ranging from foodstuffs with enhanced nutritive value to the production of biopharmaceuticals. D. R. Gossett Further Reading Evans, William Charles. Trease and Evans Pharmacog- nosy. 15th ed. New York: W. B. Saunders, 2002. Foster, Steven, and Rebecca L. Johnson. Desk Reference to Nature’s Medicine. Washington, D.C.: National Geographic Society, 2006. Hanson, Bryan. Understanding Medicinal Plants: Their Chemistry and Therapeutic Action. New York: Ha- worth Herbal Press, 2005. Kar, Ashutosh. Pharmacognosy and Pharmacobiotechnol- ogy. 2d ed. Tunbridge Wells, England: Anshan, 2008. Lewis, Walter H., and Memory P. F. Elvin-Lewis. Medi- cal Botany: Plants Affecting Human Health. 2d ed. Hoboken, N.J.: J. Wiley, 2003. Mann, John.Murder, Magic,and Medicine.Rev.ed. New York: Oxford University Press, 2000. Plotkin, Mark J. Medicine Quest: In Search of Nature’s Healing Secrets. New York: Viking, 2000. Sneader, Walter. Drug Discovery: A History. Hoboken, N.J.: Wiley, 2005. Stockwell, Christine. Nature’s Pharmacy: A History of Plants and Healing. London: Century, 1988. Web Sites E. F. Legner, professor emeritus, University of California, Riverside Medicinal Plants http://www.faculty.ucr.edu/~legneref/botany/ medicine.htm Medicinal Plant Working Group http://www.nps.gov/plants/MEDICINAL/ index.htm World Health Organization WHO Guidelines on Good Agricultural and Collection Practices (GACP) for Medicinal Plants http://whqlibdoc.who.int/publications/2003/ 9241546271.pdf See also: Agricultural products; Agriculture indus - try; Animals as a medical resource; Biotechnology. Global Resources Plants as a medical resource • 947 . of minerals at or near their point of release from the original source rocks. There has been some degree of enrich- ment ofthe placer minerals as the result of the removal of other portions of. waste of natural resources. After leav- ing the U.S. Forest Service, Pinchot became active in the Progressive Party, founded by supporters of Theo- dore Roosevelt. Pinchot became chief forester of Pennsylvania. combined effects ofriver transport of weathered materials to coastal margins and the action of tides and storm waves along beaches. The ebb and flow of the waves and the generation of longshore currents,