[96] Hofenk De Graaff differentiates between ‘indigotin’, a term used for the main colouring matter, and ‘indigo’, used for the dyestuff as such. Indigo develops from precursors contained in plants, e.g., as isatan B in woad (Isatis tinctoria L.) cultivated on a large scale in pre-modern Europe and as indican in tropical indigo plants (Indigofera tinctoria L.), Judith H. Hofenk de Graaff, The Colourful Past: Origins, Chemistry and Identification of Natural Dyestuffs (London: Archetype Publ, 2004), 240; see also CJ Cooksey, “Indigo: An Annotated Bibliography,” Biotechnic & Histochemistry 82, no. 2 (January 2007): 105–25.