Parmelia saxatilis (L.) Ach. forme saxicole

    Thallus foliose, often rosette-forming, to 10-15 cm diam., lobes loosely attached, short and trucated, 0.5-1 mm wide, upper surface apparently embossed with a whitish network of pseudocyphella, bluish, grey-greenish, grey-blue, lobe apices brownish, center sometimes blackish, covered, arising form the center, with isidia potentially spreading to the whole thallus notably on very exposed rocks, isidia tips darker; lower surface blackish with rhizines, simple, black. Apothecia rare, 0.5-1 mm diam., disc dark brown-orange, exciple isidiate. Ascospores broadly ellipsoid, simple, colourless, 16-18 x 9-11 µm. Cortex: K+ yellow; medulla : C-, K+ orange, KC+ orange, Pd+ orange. Not especially maritime, but common on acid rocks, also frequent on acid barks of trees inland. Specimen growing on rocks near the seashore are often sterile and conspicuously isidiate, the thallus sometimes blackish towards the center due to necrosis. N.B. One taxon, Parmelia ernstiae, that grows on bark has been described and is characterized by its lobes, markedly pruinose, and  by foliate isidia at the thallus center. See also Parmelia saxatilis corticolous form that frequently is covered with isidia. Not to be confused with Parmelia sulcata saxicolous form, similar but much more scarce, with soralia and not isidia.*