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).