=Indigo.=--The well-known blue colouring matter termed indigo is produced by a great number and variety of plants, distributed throughout all the tropical countries of the globe. Commercially, it is obtained chiefly from species of _Indigofera_, as _I. tinctoria_, the cultivated species of India, furnishing the chief article of commerce, found also in Madagascar, St. Domingo, &c.; and _I. Anil_, in the Punjab, W. Indies, and on the Gambia river. Some is also obtained from _I. argentea_, in Africa and America: _I. Caroliniana_; _I. disperma_, the cultivated plant of Spain, America, and some of the E. Indies; _I. cærulea_, the “black indigo” of India; _I. glauca_, in Egypt and Arabia; _I. pseudo-tinctoria_, cultivated in some parts of the E. Indies, and said to yield the best dye; _I. cinerea_, _I. erecta_, _I. hirsuta_, and _I. glabra_, in Guinea. Considerable local supplies are obtained from the following plants:--_Isatis tinctoria_, in Europe and China (see Woad); _I. indigotica_, cultivated in some parts of China; _Amorpha fruticosa_, in Carolina; _Baptisia tinctoria_, wild, in the United States; _Gymnemia_ (_Asclepias_) _tingens_, in Burmah; _Polygala tinctoria_, in Arabia; _Polygonum Chinense_, _P. tinctorium_, _P. perfoliatum_, _P. barbatum_, _P. aviculare_, in China and Japan, and introduced into Belgium; _Ruellia indigotica_, largely cultivated in Assam, as well as in India, and at Che-king, in China; _Tephrosia tinctoria_, and _T. apollinea_, in India and Egypt; _Wrightia tinctoria_ (_Nerium tinctorium_), the Palas indigo of the Carnatic.