Thallus: erect-shrubby to subpendulous, 3-8 cm long branching: main branches and secondary ones anisotomic-dichotomous; apices: ±isotomic-dichotomous basal part: concolorous with the branches to pale brown or black branches: irregular to tapering; lateral branches: distinctly narrowed at attachment points segments: terete, ±sausage-like papillae/tubercles: low, hemispherical to verrucose, primarily on main branches, ±densely and regularly distributed fibercles: absent to few on main branches fibrils: rather long, 2- 4 mm, slender, unevenly distributed, sparse to numerous soralia: punctiform to enlarged and thus wider than half the diameter of the branch, stipitate, developing mainly on top of tubercles typically on main and secondary branches or initiated on the cortex (apices), plane or often efflorescent-excrescent, circular with a distinct and sharply delimited cortical rim (especially on main branches), sometimes confluent but mainly remaining distinct isidiomorphs: sparse to typically numerous and long (up to 0.3 mm) pseudocyphellae: absent cortex: thin to moderately thick (5-9%), shiny medulla: thick, lax to dense central axis: thin to moderately thick, unpigmented Apothecia: not seen Spot tests: K+ slowly yellowish to dull yellow, C-, KC-, P+ deep yellow, or K+ yellow slowly turning orangish, C-, KC-, P+ deep orange, or K+ dull yellow turning reddish orange, C-, KC-, P+ deep yellow Secondary metabolites: psoromic acid (major), ±2-O-demethylpsoromic acid (minor), or stictic acid (major), norstictic acid (major), ±constictic acid (minor), ±cryptostictic acid (minor), ±menegazziaic acid (minor), or norstictic acid (major), ±connorstictic acid (minor); salazinic acid (major) Substrate and ecology: on bark of Quercus spp. and Pi-nus spp. or on scrubs, rarely on dead wood or on rock; in low Quercus forests on ridges, in oak-pine woodlands, pine forests or in the chapparal, between 30 and 550 m World distribution: northwestern Europe, eastern and western North America, and Japan Sonoran distribution: common on the Channel Islands (especially Santa Cruz Island) and coastal areas of southern California and scattered in Baja California. Notes: When well developed, Usnea fragilescens is easy to recognize. It is distinguished by its large, ±circular and even to slightly concave soralia with numerous isidiomorphs present on main and terminal branches, its constricted branches at attachment points, its glossy and thin cortex and the presence of psoromic acid (main chemotype in the area). When soralia are not well developed, distinction from U. cornuta requires careful examination (soralia usually absent on main branches of U. cornuta) and chemical analysis. There is, for instance, a morphotype with psoromic acid collected on rocks in Santa Rosa Island that is morphologically close to U. fragilescens, but that has numerous minute and irregularly shaped soralia covered with isidiomorphs on the main and secondary branches. Such a morphotype can be interpreted as being an intermediate between U. cornuta and U. fragilescens.