Polygonum tinctorium

Indigo In"di*go, n.; pl. {Indigoes}. [F. indigo, Sp. indigo, indico, L. indicum indigo, fr. Indicus Indian. See {Indian}.] [1913 Webster] 1. A kind of deep blue, one of the seven prismatic colors. [1913 Webster]

2. (Chem.) A blue dyestuff obtained from several plants belonging to very different genera and orders, such as, the woad, {Isatis tinctoria} (family {Cruciferae}), {Indigofera suffroticosa}, {Indigofera tinctoria} (family {Leguminosae}), {Indigofera Anil}, {Nereum tinctorium}, {Polygonum tinctorium} Ait. (family {Polygonaceae}), etc.; called also {natural indigo}. It is a dark blue earthy substance, tasteless and odorless, with a copper-violet luster when rubbed. Indigo does not exist in the plants as such, but is obtained by decomposition of the glycoside {indican}. [1913 Webster]

Note: Commercial indigo contains the essential coloring principle indigo blue or indigotine, with several other dyes; as, indigo red, indigo brown, etc., and various impurities. Indigo is insoluble in ordinary reagents, with the exception of strong sulphuric acid. [1913 Webster]

{Chinese indigo} (Bot.), {Isatis indigotica}, a kind of woad.

{Wild indigo} (Bot.), the American herb {Baptisia tinctoria} which yields a poor quality of indigo, as do several other species of the same genus. [1913 Webster]