1.3. Secondary chemistry

All Usnea species occurring in Europe and North America contain a yellow pigment, usnic acid, in the cortex (I–V). It is the only major pigment present in the East Fennoscandian Usneae, but some other pigments have been found in British Columbia species (IV): Usnea rubicunda Stirt. has a red pigment in the cortex, U. wirthii Clerc contains a red pigment in the cortex and a yellow pigment in the medulla and central axis, and U. ceratina Ach. produces a rose medullary pigment. The chemical structure of these pigments is not known.

The medulla has a complex chemistry (I–V). In addition to pigments, also present are various fatty acids (e.g., bourgeanic and caperatic acids, and the murolic acid complex) and terpenoids (e.g., zeorin). The majority of the secondary products, however, are closely related β -orcinol depsidones (e.g., norstictic, protocetraric, psoromic and salazinic acids) or β -orcinol depsides (e.g., baeomycesic, barbatic, squamatic and thamnolic acids).