Cotton

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Cotton

History

Cotton plant

Growing, harvesting, processing

Cotton is a fiber obtained from various species of plants, genus Gossypium, family Malvaceae (Mallow), and is the most important and widely used natural fiber in the world. Cotton is primarily an agricultural crop, but it can also be found growing wild. Originally cotton species were perennial plants, but in some areas cotton has been selectively bred to develop as an annual plant. There are more than 30 species of Gossypium, but only four species are used to supply the world market for cotton. Gossypium hirsutum, also called New World or upland cotton, and G. barba-dense, the source of Egyptian cotton and Sea Island cotton, supply most of the world’s cotton fiber.

G. barbadense was brought from Egypt to the United States around 1900. A hybrid of these two cotton species known as Pima cotton is also an important source of commercial cotton. These species have relatively longer fibers and greater resistance to the boll weevil, the most notable insect pest of cotton plants. Asian cotton plants, G. arboreum and G. herbaceum grow as small shrubs and produce relatively short fibers. Today, the United States produces one-sixth of the world’s cotton. Other leading cotton

producing countries are China (the world’s biggest producer), India, Pakistan, Brazil, and Turkey. The world production of cotton in the early 1990s was about 18.9 million metric tons per year. The world’s largest consumers of cotton are the United States and Europe.

History

Cotton was one of the first cultivated plants. There is evidence that the cotton plant was cultivated in India as long as 5, 000 years ago. Specimens of cotton cloth as old as 5, 000 years have been found in Peru, and scientists have found ancient specimens of the cotton plant dating 7, 000 years old in caves near Mexico City. Cotton was one of the resources sought by Columbus, and while he did not manage to find a shorter route to India, he did find species of cotton growing wild in the West Indies.

Cotton plant

The cotton plant grows to a height of 3-6 feet (0.9-1.8 m), depending on the species and the region where it is grown. The leaves are heart shaped, lobed, and coarse veined, somewhat resembling a maple leaf. The plant has many branches with one main central stem. Overall, the plant is cone or pyramid shaped.

After a cotton seed has sprouted (about four to five weeks after planting), two “seed” leaves provide food for the plant until additional “true” leaves appear. Flower buds protected by a fringed, leafy covering develop a few weeks after the plant starts to grow, and then bloom a few weeks later. The flower usually blooms in the morning and then withers and turns color within two to three days. The bloom falls off the plant, leaving a ripening seed pod.

Pollination must occur before the flower falls off. Pollen from the stamens (male part) is transferred to the stigma (female part) by insects and wind, and travels down the stigma to the ovary. The ovary contains ovules, which become seeds if fertilized. The ovary swells around the seeds and develops into a boll. The cotton boll is classified as a fruit because it contains seeds. As the bolls develop, the leaves on the plant turn red.

About four months are needed for the boll to ripen and split open. A cotton boll contains 27-45 seeds and each seed grows between 10, 000 and 20, 000 hairs or fibers. Each fiber is a single cell, 3, 000 times longer than it is wide. The fibers develop in two stages. First, the fibers grow to their full length (in about three weeks). For the following three to four weeks, layers of cellulose are deposited in a crisscross fashion, building up the wall of the fiber. After the boll matures and bursts open, the fibers dry out and become tiny hollow tubes that twist up, making the fiber very strong. The seed hairs or fibers grow in different lengths. The outer and longer fibers grow to 2.5 inches (6.4 cm) and are primarily used for cloth. These fibers are very strong, durable, flexible, and retain dyes well. The biological function of the long seed hairs is to help scatter the seeds around in the wind. The inner, short fibers are called linter.

Growing, harvesting, processing

Cotton requires a long growing season (from 180-200 days), sunny and warm weather, plenty of water during the growth season, and dry weather for harvest. Cotton grows near the equator in tropical and semitropical climates. The Cotton Belt in the United States reaches from North Carolina down to northern Florida and west to California. A crop started in March or April will be ready to harvest in September. Usually, cotton seeds are planted in rows. When the plants emerge, they need to be thinned. Herbicides, rotary hoes, or flame cultivators are used to manage weeds. Pesticides are also used to control bacterial and fungal diseases, and insect pests.

Harvesting

For centuries, harvesting was done by hand. Cotton had to be picked several times in the season because bolls of cotton do not all ripen at the same time. Today, most cotton is mechanically harvested. Farmers wait until all the bolls are ripe and then defoliate the plants with chemicals, although sometimes defoliation occurs naturally from frost.

Processing

Harvested cotton needs to be cleaned before going to the gin. Often, the cotton is dried before it is put through the cleaning equipment, which removes leaves, dirt, twigs, and other unwanted material. After cleaning, the long fibers are separated from the seeds with a cotton gin and then packed tightly into bales of 500 pounds

KEY TERMS

Boll— The fruit of the cotton plant that holds the fiber and seeds.

Cellulose— The main ingredient of plant tissue and fiber.

Cotton gin— Machine (invented by Eli Whitney) that separates the cotton fiber from the seeds.

Linter— The short, fuzzy fiber left on the seed after ginning.

Ovary— The lower part of the pistil where seeds develop.

Ovules— Small structures within the ovary that develop into seeds if fertilized.

Pollen— Fine powder made in the anthers of a flower that carries the male cells.

Stamen— Male reproductive organ of a flower that produces pollen.

Stigma— The part of the female organs of a plant flower (the pistil) upon which pollen lands in the first stage of fertilization.

(227 kg). Cotton is classified according to its staple (length of fiber), grade (color), and character (smoothness). At a textile mill, cotton fibers are spun into yarn and then woven or knitted into cloth. The seeds, still covered with linter, are sent to be pressed in an oil mill.

Cotton by-products

Cotton seeds are valuable by-products. The seeds are delinted by a similar process to ginning. Some linter is used to make candlewicks, string, cotton balls, cotton batting, paper, and cellulose products such as rayon, plastics, photographic film, and cellophane. The delinted seeds are crushed and the kernel is separated from the hull and squeezed. The cottonseed oil obtained from the kernels is used for cooking oil, shortening, soaps, and cosmetics. A semisolid residue from the refining process is called soap stock or foots, and provides fatty acids for various industrial uses such as insulation materials, soaps, linoleum, oilcloth, waterproofing materials, and as a paint base. The hulls are used for fertilizer, plastics, and paper. A liquid made from the hulls called furfural is used in the chemical industry. The remaining mash is used for livestock feed.

See also Natural fibers.