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

Thallus: erect-shrubby to subpendulous, very stiff branching: anisotomic-dichotomous, divergent basal part: concolorous with branches or dark brown to black branches: not segmented, only here and there with thin and ±incomplete annular cracks, cylindrical or irregular, terete; lateral branches: not narrowed at attachment points segments: absent papillae: absent tubercles: nearly absent to numerous, low-verrucous to truncate-conic, paler at top, sometimes eroded, irregularly distributed mainly on secondary branches fibercles: nearly absent fibrils: long (2-6 mm) and slender, irregularly distributed mainly on main branches soralia: punctiform, of irregular outline, smaller than half the diameter of the branch, even or slightly stipitate, developed from the top of tubercles or initially from the cortex, mainly on secondary and terminal branches isidiomorphs: nearly absent to numerous and conspicuous, giving a spinulous appearance to the thallus (lens at 10x) pseudocyphellae: absent cortex: thin (2-4%), medulla: thin and compact, with a pink-red pigment in the area close to the axis axis: very thick (usually > 80%), fistulose (whole thallus) with loose hyphae inside, sometimes with a yellow pigment Apothecia: not seen Spot tests: medulla K+ yellowish, C+ yellow, KC+ yellow, P- Secondary metabolites: diffractaic acid (major), ±barbatic acid (minor), eumitrins. Substrate and ecology: on bark and on rock World distribution: pantropical Sonoran distribution: one old collection from the San Gabriel Mountains in southern California, probably extinct. Notes: The stiff thallus, the thin pink pigmented medulla and the fistulose axis clearly define Usnea baileyi and make it easily recognizable.

Usnea baileyi (Stirt.) Zahlbr.
Denkschr. Kaiserl. Akad. Wiss., Math.-Naturwiss. Kl. 83: 183 (1909). Usnea antillarum (Vain.) Zahlbr., Cat. Lich.
Univ. 6: 536 (1930). Eumitria antillarum Vain., Ann. Acad. Sci. Fenn., Ser. A 6(7): 9 (1915)

Short Description. In Galapagos easily recognized as the only eumitridioid Usnea (i.e., with a tubular axis); first reported as U. antillarum, considered a synonym by Truong 2012 and Truong & Clerc 2013; for a detailed description see Ohmura 2001, Clerc (2007) and Herrera-Campos (2016).

Chemistry. Medulla with eumitrin, salazinic and norstictic acid [P+ yellow, K+ yellow turning deep red, C–, KC–].

Ecology and distribution. Distributed worldwide, throughout (sub-)tropical regions (Swinscow & Krog 1974; Rogers & Stevens 1988; Brodo et al. 2001; Ohmura 2001; Clerc 2007, Truong 2012 and Truong & Clerc 2013). In continental South America the species has been found in a wide range of habitats, mostly at low altitudes (sea level – 2300m). In the Galapagos U. baileyi is among the most common species, together with U. clerciana and U. rubicunda. All three species are particularly abundant throughout the transition zone, occasionally found also in the humid zone, but they are less frequently observed in the dry zone. Usnea clerciana and U. rubicunda have also been collected in the high-altitude dry zone, an area where U. baileyi has not yet been documented. All three species most commonly grow corticolous, mostly on native and endemic trees and shrubs; they occasionally have also been found on cacti, rarely on wood or rock.