Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.

Thallus: shrubby to subpendant, 2-10 cm long branching: both isotomic- and anisotomic-dichotomous, divergent basal part: usually conspicuously blackened on 2-4 mm or more branches: tapered, lateral branches not constricted at attachement point; annulations: numerous and conspicuous with white rings of medullary tissue segments: terete and cylindrical papillae: verrucous to cylindrical, usually numerous and irregularly to densely disposed on main and secondary branches tubercles: usually present and numerous, often eroded and white at their top, mainly on terminal branches bearing apothecia fibercles: rare, irregularly distributed fibrils: slender, 1-6 mm long isidiomorphs: absent pseudocyphellae: present on fibrils growing on branches or at the edge of apothecia cortex: thick (9-12 %) dull medulla: thin and compact, unpigmented axis: at least twice as much larger than the medulla Apothecia: 2-12 mm in diam., terminal and lateral ascospores: ellipsoid, 10-11 x 6-7 µm Spot tests: K+ yellow turning red, C-, KC-, P+ orangish yellow Secondary metabolites: salazinic acid; norstictic and salazinic (weak) acids. Substrate and ecology: mainly on bark of Pinus spp. and Quercus spp. in pine-oak forests, between 1800 and 2550 m World distribution: Europe, Taiwan, and Mexico Sonoran distribution: Sierra Madre Occidental. Notes: The Mexican specimens agree morphologically and anatomically well with the European specimens, but differ chemically. Although it is well known that, in species occurring both in Europe and North America, the secondary products diversity in North America is larger, it is still unsure whether such specimens with salazinic and/or norstictic acid can be accepted as chemical variants of the thamnolic/squamatic acid containing U. florida s. str. Usnea subfusca Stirt. with salazinic acid, occuring in northeastern U.S.A. and in central Mexico, is mophologically identical with U. florida but has smaller spores (7-9 x 5-6 µm).