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Physcia spp.
Family: Physciaceae
Physcia image
Robin Schoeninger  
Life habit: lichenized Thallus: foliose, often circular in outline, ± loosely adnate, lobate lobes: discrete or overlapping, short to elongate mostly < 3 mm wide, tips with or without cilia upper surface: whitish gray or bluish gray to gray, dull or somewhat shiny, sometimes maculate or white pruinose; with or without soredia or isidia upper cortex: paraplectenchymatous medulla: white photobiont: primary one a trebouxoid alga, secondary photobiont absent lower cortex: proso- or paraplectenchymatous (lumina < 2.5 µm wide or 4-7 µm wide, respectively) lower surface: white, pale gray, pale tan or occasionally pinkish, sparsely to densely rhizinate; rhizines simple or furcate Ascomata: apothecial, lecanorine, sessile or shortly stipitate: disc: brown to black, sometimes pruinose; epithecium: pale brown; hymenium and hypothecium colorless; paraphyses: simple or forked above, apices clavate, pale brown with a dark brown cap asci: cylindrical, 8-spored, Lecanora-type ascospores: brown, 1-septate, Physcia to Pachysporaria type, usually 15-25 µm long Conidiomata: pycnidial, immersed, walls colorless except for a dark region around the ostiole conidia: simple, subcylindrical, colorless Secondary metabolites: cortex always with atranorin, medulla with or without atranorin, zeorin or other triterpenes Geography: world-wide, found on all continents, particularly common in temperate regions Substrate: common on bark, wood and rocks; less common on soil and artificial substrates. Notes: Apart from the fact that most Physcias are substantially smaller than Heterodermias, a major difference is that the upper cortex of Physcia is always paraplectenchymatous whereas the upper cortex of Heterodermia is prosoplectenchymatous. In contrast, Phaeophyscia is similar in size to Physcia, but it never has atranorin in the upper cortex and hence reacts K-.