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Rhizocarpon distinctum Th. Fr.
Family: Rhizocarpaceae
[Buellia distincta (Th. Fr.) H. Olivier,  more]
Rhizocarpon distinctum image
André Aptroot  
Life habit: lichenized, not lichenicolous Thallus: crustose, areolate; prothallus: usually present, black, dendritic areoles: mainly angular (not peltate), flat to weakly convex, usually contiguous, 0.2-0.5 mm in diam. surface: gray-brown to brown or occasionally red-brown cortex: thin, 5-15 µm thick, +with a thin epinecral layer up to 8 µm thick medulla: white, I+ blue Apothecia: round to angular, 0.3-1 mm in diam. disc: black, remaining +flat, usually epruinose margin: +immarginate or with thin persistent margin exciple: K+ red epihymenium: carbonaceous incrusted, in section brown-black, opaque, K+ red or reddish violet hymenium: hyaline, 70-130 µm tall; paraphyses: capitate, with dark tips; hypothecium: violet-brown or olive-black, K- ascospores: hyaline or sometimes greenish brown with age, (1-)3(-5)-septate to submuriform, ellipsoid, 24-32 x 11-15 µm, halonate Pycnidia: frequent, black, almost globose, on the prothallus or immersed in the areoles conidia: hyaline, filiform, 6-12 x 0.4-0.6 µm Spot tests: medulla K+ yellow, C-, KC-, P+ yellow Secondary metabolites: stictic acid according to Runemark (1956) but not confirmed by Thomson (1997). Substrate and ecology: on siliceous rocks and walls, in low, moist and cool places World distribution: Greenland, North and South America, and widespread in Europe Sonoran distribution: Arizona.