Cotton (Gossypium spp.)
MALVACEAE, Mallow Family
Cotton, Gossypium, is a genus of the mallow family that has native species on every habitable continent, and several of the species occur on islands in the West Indies and in the Pacific Basin. Authorities currently recognize about 40 species in the genus, but only four species are economically important for their fiber. Cotton can be grown in areas where temperatures are high and water is relatively scarce; and, of course, much of the best cotton is grown under irrigation. In the tropics, certain cotton species are grown as perennial shrubs.
A plant typically has a central stem and many branches. The flowers have five separate petals, and like all members of the Malvaceae, the stamens are united into a column surrounding the style. The ovary develops into a capsule, which is a dry structure that splits open along four or five lines. Inside are the seeds and the fibers. Each fiber is actually a single cell hair that grows from the epidermis of the seed coat; the cell wall becomes thickened by adding layers of cellulose. There are two types of hairs, lint (long) and fuzz (short), and, of course, selection has been for the longest lint. Most of the wild species of cotton are lintless.
The cotton plant contains special glands that produce a chemical called gossypol. Gossypol is a natural defense against herbivores, but it is deleterious to livestock when consumed, Consequently, glandless cotton can be genetically produced if the seeds are to be fed to livestock, but of course this leaves the plant defenseless and the fields must be heavily sprayed with insecticides.
Cotton is the crop that requires frequent chemical sprays, first to control insects and later to defoliate the plants so that the cotton can be efficiently harvested via machines. Rows must be spaced to fit the machine, which vacuums and combs the boles from the plant and then separates the lint from the seeds. There may be some lint remaining on the plants after harvesting has been completed.
Many authors have discussed the mystery regarding the origin of cotton. Cotton was being cultivated in the Old World (Mohenjo Daro, Indus Valley of Asia at 2500 B.C.) and in the New World (Huaca Prieta, Peru in 8000 B.C. was nonagricultural Gossypium barbadense; Mexico in 3400 B.C. was G. hirsutum).
Old World Cotton (2n = 26). AA genome (13 pairs of large chromosomes).
New World Cotton (4n = 52). AADD genome (13 pairs of large and 13 pairs of small chromosomes)