Also called indogitin (C16H10O2N2),
a violet-blue dye obtained from leguminous plants
of the genus Indigofera. Indigo is produced by natural acidation
of a solution containing pieces of the plants. Cultivation of indigo plants
was once carried out on a large scale in India, but cheap synthetic indigo
(available since the early 1900s) is now mainly used. Indigo was first synthesized
by the German chemist Adolph von Baeyer.