Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.
Thallus: crustose, rimose-areolate or immersed, often reduced to scattered granules or areoles or when well-developed, +warted-areolate or almost squamulose, sometimes forming a +continuous, areolate crust; prothallus: if present, black areoles: irregular in outline or ±isodiametric, 0.1-0.3 mm in diam., weakly convex to irregularly wrinkled, ecorticate surface: pale yellow or greenish ochre or pale greenish yellow to pale gray-green or yellow-brown, dull to slightly glossy, esorediate Apothecia: +arising singly on areoles which are then obscured, scattered or often clustered, sessile, constricted below, round to irregular, 0.3-1(-1.5) mm in diam. disc: pale yellow to pale yellow-brown or pale yellow-green, occasionally greenish brown or pale brown, rarely darkening with age, concave and marginate at first, then convex and emarginate, waxy, glaucous when wet, epruinose or very indistinctly pruinose margin: pale yellow (paler than disc), entire to more or less crenulate, initially well developed but not very prominent, then becoming very thin or disappearing, without a parathecial ring amphithecium: present, with a poorly developed algal layer at the base or algae absent, laterally 40-110 µm wide, basally c. 150 µm wide or apothecia not constricted and margin intergrading with thallus, with short-celled, medullary hyphae with lumina 1-2 µm wide, corticate; cortex: with an outer layer of granules as in epihymenium, strongly gelatinized, composed of hyphae with lumina 2-3 µm wide parathecium: often forming a ±biatorine parathecium, 25-90 µm wide epihymenium: ochre, with granular crystals dissolving in K, 10-30 µm high hymenium: hyaline to pale yellow, 45-60 µm tall, interspersed; paraphyses: hyaline, weakly branched and anastomosing, with 1.2-1.5 µm, apically 1.2-2.0 µm wide lumina; subhymenium: hyaline, 40-85 µm thick; hypothecium: pale yellow or brown, 45-50 µm thick asci: clavate, 8-spored ascospores: hyaline, simple, often somewhat irregular, ellipsoid, (9-)10.5-13 (-14) x (4.5-)5.5-6.2(-7.5) µm Pycnidia: occasional, immersed, with a pale yellow-brown ostiole conidia: acicular to arcuate, (12-)1822(-25) x 1 µm Spot tests: thallus K-, C-, KC+ yellow, P- Secondary metabolites: rangiformic acid, usnic acid and zeorin. Substrate and ecology: on siliceous rocks (especially granite), in montane to alpine areas, rarely on wood; usually in full sun World distribution: cosmopolitan, known from all continents, including Antarctica Sonoran distribution: Arizona, southern California, and Baja California. Notes: As the specific epithet suggests, this is an extremely variable species, especially in terms of morphology of the thallus and apothecia; a thorough study of this and related species is needed. Forms with well developed thalli might be confused with L. geiserae or L. mazatzalensis; for comparisons see under those species. L. polytropa var. illusoria (Ach.) Th. Fr., reported from southern California, has apothecia only 0.3-0.6(-0.8) mm in diam. Some morphs of Lecanora mughicola might superficially resemble L. polytropa, but that species can easily be distinguished by its narrower ascospores, pigmented paraphysal tips, and isousnic acid as the only lichen substance. Lecanora symmicta has narrower ascospores, and together with Lecanora brucei contains xanthones.