Parmelia sulcata Taylor forme corticole.
Thallus foliose, rosette-forming, to 5-12 cm diam., lobes to 2-5 mm wide, apices truncated and incised, upper surface conspicuously embossed by an incomplete network of whitish pseudocyphella, grey-bluish, grey-greenish, pale grey when dry, lobe apices sometimes brownish, greenish, green-bluish when moist; numerous elongate and greyish soralia arise from the center, along the pseudocyphella network, to sometimes cover almost the entire thallus, lower surface blackish with rhizines, simple or forked. Often sterile or apothecia scarce and scattered, markedly sessile, disc brown-red, exciple concolourous to the thallus with soredia. Asci with 8 spores, 50-60 x 25-35 µm according to our own measurements. Simple spores, colorless, elliptical, 11-14 x 6-8 µm according to literature, 10-15 x 6-9 according to our measurements. Cortex: K+ yellow > red; medulla and soredia: C-, K+ red-orange, KC+ orange, Pd+ orange (slow reaction). Not especially maritime but common on bark of deciduous trees, seldom on wood:see Parmelia sulcata lignicolous form and seldom on acid rocks, see Parmelia sulcata saxicolous form. N.B. Very similar to Parmelia saxatilis but the latter is covered with isidia and not soralia. N.B. This species is highly variable and many different taxa, difficult to distinguish, coexist. N.B. this species is highly variable and many close taxa are hardly distinguishable. Not to be confused with some forms of Punctelia borreri and Punctelia subrudecta that have different chemical reactions: medulla C+ pink-red or C+ carmine, and white to whitish soredia.*