Where does cotton fiber come from

As you might know, the word cotton usually refers to the genus "Gossypium", which composes of over 50 species all over the world. Most of these species doesn't produce spinnable fiber, and therefore are "wild" species. The cotton as we know it in the industry are mainly the four domesticated species: Gossypium barbadense, Gossypium hirsutum, Gossypium herbaceum and Gossypium arboreum. The first two species are tetraploid cotton, and are mainly cultivated in the Americas, and hence the name "New World" cotton species; while the latter two species are diploid cotton, and are mainly cultivated in Asia, hence are called "Old World" cotton species.

The amazingly long cotton fiber in the cultivated species are mainly due to artificial selection, or domestication efforts. As is shown in the figure below, the Gossypium species are categorized into 8 different "genomes" according to their behavior in meiosis. The fiber producing cotton came from the A genome cotton, and the tetraploid New World cotton came from a polyploidization event merging the A genome and the D genome, which happened around 1 million years ago. You might ask about how the two cotton genome species A and D from two different continents can meet with each other to form the tetraploid genome. The answer is still anybody's guess.

Cotton fiber is a single cell expanding from the epidermal cells of the seed coat. It might be the longest single cell in the plant kingdom. Four different stages are involved in the development of cotton fiber: initiation, elongation, secondary cell wall formation, and maturation. The first two steps are quite self-explanatory, like blowing up a long balloon, the osmotic pressure pushes the cell outwards. The secondary cell wall start to form after that, which is a process of accumulating cellulose on the inside of the primary cell wall. The maturation is basically the drying up and dying of the cell, leaving the fine quality fiber behind. Much speculation has been raised as to the relationship between cotton fiber and other trichoms in plants. You can find more about that in one of the reviews i wrote here.