Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Life habit: lichenized Thallus: fruticose, erect, decumbent (prostrate), supendent or longly pendant, usually attached by a basal holdfast; branching isotomic- or anisotomic-dichotomous basal part: jet black to reddish-brown pigmented or concolorous with branches; continuous or with annular or longitudinal cracks branches: longitudinally cylindric, fusiform, tapered or irregular; transversally terete, flattened, striated, ridged or alate; constricted or not at attachment points; with or without annulations; divided into ±conspicuous segments which may be cylindrical, ±sausage-like or trapezoidal; with or without papillae, fibrils, tubercles, fibercles, pseudocyphellae; without true isidia soralia: present or absent; punctiform and indistinct to enlarged and conspicuous; even, excavate or stipitate; plane, convex or concave; rounded to longitudinally or transversally elliptical with or without a sharply delimited margin; crowded or widely spaced; originating initially on the cortex or on fibercles or on top of tubercles or in annular cracks between the segments Isidia: absent Isidiomorphs: absent, single or tufted cortex: radially or irregularly oriented leptodermatous or pachydermatous hyphae which are firmly or loosely conglutinated; gray to grayish-green, yellow-green when fresh, sometimes changing to brown in the herbarium; with or without red pigment; vitreous, shiny or mat medulla: lax, dense or compact, sometimes pink, red or yellow pigmented central axis: cartilaginous strand of solid, tough, compacted, prosoplectenchyma in which hyphae are conglutinated and longitudinally arranged, running throughout the thallus; fistulose and/or solid; I+ blue or I-; white, rarely brownish or yellowish photobiont: trebouxioid Ascomata: apothecial, lecanorine; lateral, subterminal or terminal; sessile to pedicellate; cup-shaped, flat or sinuose; margin: prominent, with thalloid rim and few to numerous well developed fibrils; disc: pruinose or not, pale to dark yellowish to brownish asci: lecanora-type, elongate-clavate, 8-spored ascospores: simple, ellipsoid to broadly ellipsoid, 7-11 x 5-7 µm, hyaline, not amyloid Conidiomata: pycnidia, mainly terminal or subapical conidia: simple, colorless, bacilliform or bifusiform, straight or curved Secondary metabolites: cortex with usnic acid; medulla with range of depside, depsidones, bisxanthones, terpenoids, aliphatic acids and unidentified substances Geography: widely distributed, occurring in polar zones, alpine, boreal, temperate and tropical regions Substrate: on trees, wood, or rocks. Notes: The fruticose thallus and the presence of the elastic central chord (seen by stretching a branch) readily separates Usnea from all other lichen genera. Each species of Usnea accepted here consists of a distinctive combination of morphological, anatomical and chemical characters [for discussion, see Clerc (1998)]. However, quite often one or more rarely two diagnostic characters are lacking, or are not interpretable, especially when specimens are young or growing in an extreme locality (for example with high insolation or with strong winds, or with very high humidity, etc.) or if they are infected by a lichenicolous fungus or when they are necrotic (in specimens collected on the ground). When such difficulties are encountered, chemistry should be investigated with t.l.c. and possibly specialists should be consulted.