From Ernst Haeckel's ''Kunstformen der Natur," 1904.

Lichens are composite organisms consisting of a fungus and an algae, living together in a symbiotic relationship. The algae is the photosynthetic partner, while the fungus provides physical protection and the chemical products that provide a vast array of colorants. 

Historically, lichens were important dyestuffs, the two most important being orchil and crottle lichens.

Orchils produce violet dyes and were used in ancient Rome to dye silk and wool as an imitator of Tyrian purple, as well as, used in combination with indigo and woad to economize the valuable and scarce Tyrian purple.

Crottles produce russet, brown and orange dyes and are colorfast. They are found on the rocks in the costal areas of Ireland, the Scottish Highlands and the Outer Hebrides. Historically, they have been used in the production of Irish and Scottish tweeds and up until the 1970s were still being used in Donegal and Connemara.