5. Gossypium L. (cotton)

About 50 species, nearly worldwide in tropical and warm-temperate regions.

Several species of Gossypium have long been cultivated in different parts of the world as a source of cotton fiber, cottonseed oil, and seed meal. The cytogenetics and evolution of the cultivated taxa have been studied in detail. Those grown in the New World are tetraploid taxa that originated from past hybridization between diploid parents native to the Old World and New World tropics respectively, whereas those cultivated in the Old World are diploids originating from Old World diploid taxa. In both areas, human selection has resulted in a large number of races and cultivars. For a fascinating review of the natural history of this group, see Fryxell (1979).