Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: shrubby, rigid, 2-12 cm long branching: anisotomic-dichotomous, divergent basal part: short (1-3 mm), variously pigmented but never jet black, sometimes barely developed, often rugose to ±decorticated branches: usually tapering, conspicuous with few ramifications, distinctly segmented (especially the main branches) with conspicuous everted medulla between the segments; lateral branches: sometimes slightly narrowed at point of attachment segments: terete to slightly ridged, cylindrical to slightly sausage-like; foveoles: absent; transversal furrows: sometimes present papillae: minute, hardly distinct tubercles: absent fibercles: large (7-10 x 10-20 µm), conspicuous, and numerous over the whole thallus fibrils: mature ones rather rare and only present near the basal part, short (1-3 mm), ±slender; short (0.5 mm) isidiomorph-like fibrils: produced (sometimes in bundles) on top of fibercles isidiomorphs: absent cortex: ±glossy, distinctly longitudinally, and transversally cracked with conspicuous medullary tissues coming out from the cracks, thin to moderately thick (5-10%) medulla: dense, moderately thick axis: sometimes fistulose Apothecia: not seen Spot tests: K-, C-, KC-, P+ orangish yellow Secondary metabolite: protocetraric acid (major). Substrate and ecology: growing on rock, at 1950-2800m World distribution: endemic to Mexico; Sonoran distribution: only one locality in the Sierra Madre Occidental. Notes: Usnea nashii is distinguished by the large fibercles producing short isidiomorph-like fibrils at their summit and the presence of protocetraric acid in the medulla.