Cultivated American cotton, Gossypium hirsutum. This species has 26 pairs of chromosomes: 13 are large and 13 are much smaller. Old World cotton had only 13 pairs of large chromosomes, while wild American cotton has 13 pairs of small chromosomes. The origin of the cultivated cotton was reconstructed experimentally by treating a hybrid with colchicine to double the chromosome number and produce a fertile amphidiploid with characteristics similar to the cultivated variety.