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Physcia alba (Fée) Müll.Arg.
Family: Physciaceae
Physcia alba image
Thallus: orbicular to irregular, sometimes confluent with other thalli, up to 5 cm diam, ± loosely adnate, usually less than 150 µm thick and fragile lobes: radiating, flat to almost concave, truncate and sinuose, up to 1.5 mm wide but usually narrower, distinctly separate or sometimes overlapping, tips often ascending, eciliate upper surface: whitish gray to cream-colored, without pruina, occasionally with a weak maculation; soredia and isidia absent upper cortex: paraplectenchymatous medulla: white lower cortex: prosoplectenchymatous, in lower parts turning ± paraplectenchymatous and thick-walled, usually well delimited from the medulla lower surface: white to pale graphite gray; rhizines: concolorous with the lower surface Apothecia: abundant, variable in size, up to 2 mm diam. but usually around 1 mm; margin: ± thick (c. 0.2 mm) and weakly crenulate; disc: brown to black, rarely pruinose ascospores: brown, 1-septate, of Pachysporaria-type, (19.5-) 22-27.5 (-32) x (8.5-) 10-13 (-16) µm Pycnidia: sparse to common conidia: subcylindrical, 4-6 x 1 µm Spot tests: cortex and medulla K+ yellow, C-, KC-, P+ yellow Secondary metabolites: upper cortex with atranorin; medulla with atranorin, zeorin, occasionally leucotylin and/or other triterpenes. Substrate and ecology: mainly corticolous growing in open forests on solitary trees, and also found on rocks in ± exposed sites World distribution: scattered localities in northern South America and SW North America Sonoran distribution: scattered collections from southern Baja California Sur, Sonora and Chihuahua. Notes: This species is predominately tropical and is recognized by its narrow ± truncate lobes, the abundant apothecia with thick margins and the pale lower surface. It may be misidentified as P. stellaris that is similar in size and also has abundant apothecia. However, they differ in lobe shape, apothecial characters and chemistry.