Cotton - biology.

Cotton - biology. I INTRODUCTION Cotton, natural vegetable fiber of great economic importance as a raw material for cloth. Its widespread use is largely due to the ease with which its fibers are spun into yarns. Cotton's strength, absorbency, and capacity to be washed and dyed also make it adaptable to a considerable variety of textile products. See Textiles. II COTTON PLANTS Cotton is produced by small trees and shrubs of a genus belonging to the mallow family, which also includes hibiscus, okra, and the familiar swamp mallow. The immature flower bud, called a square, blossoms and develops into an oval boll that splits open at maturity, revealing a mass of long white seed hairs, called lint, that cover a large number of brown or black seeds. When fully mature and dry, each of these hairs is a thin flattened tubular cell with a pronounced spiral twist and is attached to a seed. The length of the individual fibers ranges from 1.3 to 6 cm (0.5 to 2.5 in). Shorter fibers that grow from the seeds are called linters. A few species are grown commercially; these range from a small tree of Asia, to the common American Upland cotton, a low, multibranched shrub that is grown as an annual. Another species includes the long-fiber Egyptian and Sea Island cottons botanically derived from the Egyptian species brought to the United States about 1900. Sea Island cotton thrives in the unique climate of the Sea Islands, located off the southeastern coast of the United States, and on the islands of the West Indies such as Barbados. As with Egyptian cotton, the fiber is white and lustrous but its fiber length is longer than that of any other type of cotton, which permits the spinning of extremely fine yarns. Pima, originally called American-Egyptian cotton, is a hybrid type. It is the only variety of long-fiber cotton now grown in commercially significant quantities in the United States, where it is cultivated under irrigation in the Southwest. It is almost impossible to determine the original habitats of the various species of cotton. Scientists have determined fiber and boll fragments from the Tehuacán Valley of Mexico to be about 7000 years old. The plant has certainly been grown and used in India for at least 5000 years and probably for much longer. Cotton was used also by the ancient Chinese, Egyptians, and North and South Americans. It was one of the earliest crops grown by European settlers, having been planted at the Jamestown colony in 1607. III CULTIVATION Successful cultivation of cotton requires a long growing season, plenty of sunshine and water during the period of growth, and dry weather for harvest. In general, these conditions are met within tropical and warm subtropical latitudes in the northern and southern hemispheres. The U.S. Cotton Belt stretches from northern Florida to North Carolina and westward to California. In the United States, production of the crop for a given year starts soon after harvesting the preceding fall, when many cotton farmers chop or shred the stalks with machines. The residue is plowed under and the land usually left rough until spring tillage. Planting time varies from the beginning of February in southern Texas to the beginning of June in the northern sections of the Cotton Belt. A number of methods, chemical and mechanical, have been used to control weeds and grass, including intensive spraying of herbicide before and after planting. The cultivator, rotary hoe, and flame cultivator are also used to destroy weeds. Approximately 95 percent of the cotton in the United States is now harvested mechanically with spindle-type pickers or strippers. Pickers are used extensively in irrigated lands. The picker has vertical drums equipped with wire spindles that engage and pull the cotton from open bolls. Strippers are used primarily in western Texas and western Oklahoma. They are "once-over" machines that pull the bolls from the plant. IV COTTON INSECTS AND DISEASES In addition to the flowers, the underside of each leaf of the cotton plant contains a small cuplike structure holding nectar. These deposits and the succulent stem make the plant attractive to a variety of insect pests. Chief among these is the boll weevil. The use of early maturing strains of cotton plus the application of several chemicals and control methods have greatly reduced losses from boll-weevil infestation. The bollworm, the pink larva of a small moth, is believed to have been a native of India but is now parasitic on cotton all over the world. The larvae burrow into the bolls and eat the seeds. In the United States the pink bollworm is largely confined to Texas and the western sections of the Cotton Belt. Quarantine, fumigation of seed, and destruction of trash removed from the cotton in ginning are control measures. The bollworm-tobacco budworm also is one of the most damaging cotton pests in terms of losses and control costs. Armyworm, thrips, lygus, and red spider are among other significant pests. Among the serious diseases to which the cotton plant is subject is the wilt caused by a fungus which enters the roots from the soil and manufactures a poison. No treatment is known, but wilt-resistant strains of cotton have been developed. Another fungus disease is boll rot or anthracnose, caused by sac fungus. The best control is using seed from fungus-free fields. V PROCESSING When cotton arrives at the cotton gin, it is sucked into the building through pipes placed in the trailers or trucks. In many plants it first enters driers that reduce the moisture content for easier processing. The cotton travels next to equipment that removes burrs, sticks, dirt, leaf trash, and other foreign matter. It then moves to the gin stand, where lint is separated from the seeds. After separation from the seeds, the lint is packed tightly into bales. For the processing of cotton fiber to make yarn, cloth, and cordage, see Spinning; Textiles. VI MARKETING In determining the value of cotton, workers class samples from each bale according to staple, grade, and character. Staple refers to fiber length. Short staple accounts for about 25 percent of the domestic crop and is used in many coarser textiles. Medium staple accounts for about 70 percent of U.S. cotton, and long staple and extralong staple for about 5 percent. Grade refers to color, brightness, and amount of foreign matter. Standards for grading U.S. cotton are established by the U.S. Department of Agriculture and revised from time to time. The highest of the grades are Good Middling and Strict Middling. Six color groupings indicate the degree of whiteness from white to gray. Character refers to the diameter, strength, body, maturity (that is, ratio of mature to immature fibers), uniformity, and smoothness of the fibers. VII COTTONSEED Once a waste-disposal problem for gins, cottonseed is now a valuable by-product. The seed goes to oil mills, where it is delinted of its linters in an operation similar to ginning. The bare seed is then cracked and the kernel removed. The meal that remains after the oil has been extracted is high in protein. Linters are used for padding in furniture and automobiles, for absorbent cotton swabs, and for manufacture of many cellulose products such as rayon, plastics, lacquers, and smokeless powder for munitions. The hulls, or husks, are used as feed for cattle. Kernels, or meats, provide cottonseed oil. The cake and meal are used for feed and flour. Foots, the sediment left by cottonseed oil refining, provides fatty acids for industrial products. VIII PRODUCTION Cotton ranks just behind corn, soybeans, wheat, and hay among the leading cash crops of United States agriculture and is among the nation's principal agricultural exports. The leading cotton-producing states are Texas, California, Georgia, Mississippi, Arkansas, Louisiana, North Carolina, and Arizona. Surplus stocks of cotton on hand in the United States fluctuated widely during the 1970s. The world economic recession of 1973-74 ushered in a period during which both production and consumption of cotton dropped. Production, however, fell faster than consumption, and by the mid-1970s the U.S. surplus had been reduced to the lowest level in 50 years in order to compensate. Toward the end of the decade, rising prices caused by the shortages had stimulated increased production, but at the same time these higher prices made domestic cotton more vulnerable to competition from artificial fibers and imported cotton goods. World demand for cotton continued to be erratic, and some groups lobbied for increased price-supports, but an upward trend began in the 1980s. Cotton is still a principal raw material for the world's textile industry, but its dominant position has been seriously eroded by synthetic fibers. In the United States, cotton accounts today for about 35 percent of the materials processed in textile mills, as against 80 percent before World War II. Net per capita consumption of cotton fibers in the United States, after declining by more than one-third between 1950 and 1970, increased during the 1980s and by the early 1990s was about 12 kg (about 27 lb) per year. World production of cotton in the early 1990s stood at 18.9 million metric tons annually. In the 1930s, the United States produced more than half the world's cotton; by the early 1990s it was turning out about a sixth. The other leading producers included China, India, Pakistan, Brazil, and Turkey. Contributed by: National Cotton Council of America Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.