Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Life habit: lichenized Thallus: fruticose, semi-erect and caespitose to subpendent or pendent, 40-80 (-100) mm long, pliant (to rather stiff and cartilaginous in very coarse specimens), solid, with a basal attachment disc, irregularly and richly branched, branching isotomic to anisotomic branches: principally radial and irregularly rounded in cross-section, yet at least the main branches usually angular or flattened, 1-2 mm wide surface: usually intensely green-yellow to yellow ("fluorescent" yellow-green or chartreuse when fresh), the older parts turning light yellowish brown; dull, epruinose, or occasionally slightly yellowish to whitish pruinose on ridges; weakly to often wrinkled or ridged and lacunose or foveolate, especially on the smallest branches; irregularly shaped abraded areas ± present, slightly paler and brighter yellow than the cortex; soredia and/or isidia present or absent cortex: present on all sides, c. 50 micrometer thick, densely filled with yellowish crystals (soluble in K) apparently concentrated around the cell walls; hyphae in K ± anticlinal, branched, with ± thick-wall, short, roundish cells c. 5.-7.5 micrometer across medulla: white, arachnoid, solid or with a loosely filled center, with solid medullary strands or filaments: variable in number (c. 5-12), often together, relatively thin in the upper branches, thicker near the base, fusing into a single large strand that almost fills the inside of the branch; loose hyphae: c. 4-7 micrometer wide with lumina 1-2 micrometer wide; hyphae in bundles: strongly conglutinated, narrower, periclinal; cell walls with Cetraria-type lichenan (Common, pers. comm.) photobiont: primary one a Trebouxia, secondary photobiont absent; algal layer: under the cortex on all sides, in rounded groups of up to 20 (-30) cells, each c. 10-20 micrometer diam. Ascomata: apothecial, sessile to short-stipitate, lateral or terminal, roundish to rather irregular in outline, bowl-shaped when young, ± soon shallow and flattened; disc: deep or dark red-brown to grayish brown; thalline exciple: concolorous with the thallus, prominent, raised or level with the disc, lobulate or cornute on the outer edge, with elongated, attenuate projections similar in structure and appearance to the thallus; exciple: hyaline above, lower layers hyaline or pale yellowish, not strongly gelatinized but the hyphae rather strongly conglutinated and thick-walled, at least partly periclinal to the disc; epihymenium: yellow-brown; hymenium: thin, hyaline to pale yellowish; paraphyses: thick, coherent, strongly conglutinate and difficult to see clearly, septate, not branched or anastomosing; ostiolar filaments absent asci: broadly clavate, thick-walled, Lecanora-type, amyloid reaction in the tholus moderate (compared to that in Vulpicida), 8-spored ascospores: biseriate or triseriate, simple, hyaline, narrowly to broadly ellipsoid or subglobose, obtuse at the poles, 5-7 (-8) x (3.5-) 4-5 micrometer, wall smooth, without distinct endospore thickening, not amyloid Conidiomata: pycnidial, mainly towards the tips or spreading downward somewhat along ridges, scattered or crowded, laminal, immersed, but usually within low, rounded elevations, c. 0.1-0.2 mm diam., internally almost spherical, ostioles dark brown to black, punctiform (< 0.05 mm diam.), conspicuous under moderate magnification of a dissecting microscope [but not appearing as raised black structures as in the illustration by Keissler (1960)]; conidiophores ± type V of Vobis (1980) conidia: borne pleurogenously, on tips of ± elongated projections from conidiogenous cells (see Keissler 1960, fig. 27c), acicular or sublageniform, 8-9 (-10) x < 1 micrometer, straight Secondary metabolites: cortex always with pulvinic acid derivatives (vulpinic acid), ß-orcinol depside (atranorin), ß-orcinol depsidone (accessory), usnic acids, and aliphatic acids Geography: cool temperate to temperate, western North America, Europe, southern Asia Substrate: bark or wood, rarely , rarely on acidic rock; the species are good indicators of the height of the snowpack on tree trunks, since they do not grow under snow. Notes: This is the most conspicuous lichen genus in the montane forests of California because of the large bushy, brightly colored thallus, which covers branches and tree trunks profusely, at least in areas away from the major cities. Today in many mainland areas of southern California (e.g., the San Gabriel Mountains) the thalli are often few and very small and poorly developed. The thalli, which are somewhat poisonous from the yellow pigment vulpinic acid, and have been known to cause allergic reactions, are often gathered in quantity for crafts or decorations.
Preliminary studies by Kroken and Taylor (2000 & 2001) suggest that, based on on-going molecular, chemical, and morphological/anatomical investigations, may support a new taxonomy of the taxa currently treated in Letharia. Although these authors provide a little information on distribution, morphology, and apothecial chemistry of the tentatively named variants, much further study is needed before these variants can be adequately circumscribed and distinguished from each other.