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Rhizocarpon polycarpum (Hepp.) Th. Fr.
Family: Rhizocarpaceae
[Buellia atroalbella (Nyl.) Mong.,  more]
Rhizocarpon polycarpum image
Life habit: lichenized, not lichenicolous Thallus: crustose, areolate; prothallus: black, marginal and among the areoles areoles: usually contiguous and angular or crescent-shaped around the apothecia, sometimes dispersed, flat to convex, 0.3-0.8 mm wide surface: pale to dark brown or reddish-brown, rarely grayish brown cortex: few cells of hyphae, the uppermost with a dark cap, with a thin layer of polysaccharides medulla: white lower cortex: KI+ intensely blue Apothecia: dispersed to aggregated, rounded to polygonal, slightly constricted at the base, sessile, 0.4-0.8 mm wide disc: black, mostly flat, rarely convex, epruinose margin: thin or lacking exciple: red-brown to brown to brownish-black, becoming ligher to the interior epihymenium: brown to violet-brown, K+ red-violet or red, rarely K- hymenium: hyaline to faintly greenish blue above, 80-100(-130) µm tall; paraphyses: coherent, branched, 2-3 µm wide below, 3-5 µm wide apically, with brownish black tips; hypothecium: reddish-brown to brown, K- asci: clavate, c. 80 x 20 µm, 8spored ascospores: predominately hyaline but becoming pale brown, 1(-3) septate, 17-30 x 8-11(-14) µm, halonate Pycnidia: not seen Spot tests: medulla mostly K-, rarely K+ very light yellow, C-, KC-, P- yellow; epihymenium K+ red, exciple K+ red solution Secondary metabolites: norstictic acid and stictic acid. Substrate and ecology: on siliceous rocks in moderately cold and humid areas World distribution: holarctic, North America, Greenland and Eurasia Sonoran distribution: eastern Arizona.