Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.

Thallus: usually entirely or mostly immersed and visible only as a stain, at most forming a thin, +smooth membrane, rather extensive, without marginal delimitation and dark prothalline line; superficial part of thallus 50-80 µm thick, composed of very thin, loose, somewhat inspersed hyphae; individual hyphal strands: +deeply penetrating the substrate surface: light gray (rarely blackish gray) soredia: absent goniocysts: numerous to almost absent, usually speckling the thallus brown, 17-40 µm in diam. photobiont: algae few, 6-8 µm wide, scattered within the wood Apothecia: numerous, mostly parallel-oriented following the grain of the wood, 0.4-2(-2.7) x 0.2-0.3 mm, ovoid to ellipsoid, oblong or linear, pointed to narrowly rounded at the ends, straight disc: pale brown to yellow-brown or usually dark brown or black, when moist brown-black or paler and redder with dark spots, closed and furrowed or becoming open, flat (or concave) or convex, epruinose or faintly gray pruinose margin: thin, concolorous with disc or usually paler, epruinose, often prominent when young (but not raised over the disc), later inconspicuous, sometimes disappearing exciple: narrow, at the base pale to +blacking, laterally pale yellow- to dark red-brown, towards the base inverted conical and disappearing, 10-15 µm wide, towards the outside widely divergent and +depressed from the disc, at the edge appressed to the thallus epihymenium: dark red-brown, 20-25 µm high, granular, K-, N- hymenium: +hyaline, 60-70 µm high, becoming yellowish brown above, I+ pure blue; paraphyses: filiform, dense and little agglutinated, the tips brown, clavate; hypothecium: hyaline, c. 60 µm high, towards the base narrowed asci: cylindrico-clavate, c. 60 x 12-15 µm, thin-walled, at the tips wall thickened to 10 µm, 8-spored ascospores: often absent or immature, hyaline, simple, +broadly ellipsoid to ovoid, (10-)11-17 x 5-7.5(-8.5) µm Pycnidia: frequent but inconspicuous (without lens), black, punctiform, resembling large goniocysts conidia: filiform or narrowly falcate, 13-17 x 0.5-1 µm Spot tests: thallus and apothecia K-, P- or medulla directly under lirellae K+ pale yellow; I+ blue Secondary metabolites: no secondary metabolites detectable (due to scarcity of thallus) or traces of stictic acid or norstictic acid [in var. rubescens (Räsänen) Räsänen, from Europe]. Substrate and ecology: on rotting logs or stumps, usually montane to subalpine (to alpine), in humid, often shaded situations, spruce-fir, pine-fir, and mixed conifer forests; occasionally in riparian, pine-quaking aspen communities World distribution: circumpolar arctic and boreal in Europe and North America (boreal and western) Sonoran distribution: throughout higher elevations in Arizona (2830-3140(-3650) m). Notes: The statement by Redinger (1937-1938) that the conidia of X. parallela are ellipsoid and 4-6 x 2-3 µm is erroneous; the conidia are filiform as in the rest of the genus. This is a rather variable species, and several infraspecific taxa have been described from Europe (e.g. var. elliptica Nyl. and f. pallens Nyl.); it is treated here in a broad sense. In many specimens from the Sonoran region, the apothecia are short and +unoriented (perhaps due to the lack of grain in the substrate), and/or the thallus has few or no goniocysts; in these features such material resembles X. hians Tuck., which occurs in California (south to Madera Co.), and differs especially in having a distinct K+ red reaction (norstictic acid) in the apothecia and thallus, mostly short and often irregularly contorted and confluent apothecia with usually pale discs. Some Arizona specimens (especially Nash 35206, from a very high elevation), in which several dozen apothecia are grouped into +discrete patches 3-35 mm long, 2-15 mm wide, with apothecia towards the ends of the patches mostly elongated and 1-2 times branched with the branches subulate and pointing along the grain away from the patch, while those towards the center of the patch are ellipsoid to elongate and +pointed at both ends and often irregularly arranged. In many specimens from Arizona, the pycnidia are very numerous and also form discrete patches separate from the apothecia and occasionally surrounded by a narrow blackish prothalline line. Deep cracks in the wood are often filled with pale globes (insect eggs), giving the false appearance of fissural soralia. The species is often intermixed with X. vitiligo, which has short, usually dark, soralia, and usually shorter apothecia. Other superficially similar species in North America are X. opegraphella Nyl. ex Rothr. (on driftwood; with short apothecia and containing norstictic acid), X. disseminata G.K. Merrill (with spores 14-25 x 4-6 µm) and X. trunciseda (Th. Fr.) Minks ex Redgr. (with somewhat smaller spores and with apothecia mostly short-ellipsoid to broadly fusiform and pale [at least in one Swedish specimen in ASU, with a peculiar pattern of one or more short dark apothecia overgrowing paler, more elongated ones]).