Platismatia glauca (L.) W. L. Culb. & C. F. Culb.

    Thallus foliose, 5-15 cm across, composed of lobes, larger (1-4 cm) than longer (1-3 cm), markedly wrinkled and pleated, suggesting a crumpled paper ball, sometimes strappy, straps highly indented and in that case longer than larger (both types of lobes may be present on the same thallus), lobe margins ascending, erect, markedly rumply and curly, covered with coralloid isidia and/or greyish soralia; upper surface pale greyish, grey glaucous, grey-bluish, not turning greenish when moist, brown in shaded sections and pink-purplish when old and decaying, sometimes with greyish isidia and/or soralia; lower surface brown-reddish to blackish towards the center with a few rhizines (colour variable according to light intensity), paler towards margins. Apothecia very rare, marginal, more or less sessile but not stalked, 5-8 mm diam., disc concave, brown-red, exciple thin, irregular and isidiate (as well as the lower surface), soon excluded. Ascii urn-shaped, 30-40 x 10-15 µm according to our measures.  Ascospores colourless, simple, broadly ellipsoid, , 4-8 x 3-5 µm according to literature, 7-10 x 5-6 µm according to our measures. Photobiont: Trebouxioïd. Medulla C-, K-, KC-, Pd-, UV-; Cortex : C-, K+ yellow, KC-, Pd-, I+ indigo blue. Not especially maritime and showing montane trends, on bark of  trunks and main branches of deciduous or coniferous trees, seldom on rocks. N.B. The combination of grey-glaucous and brown-reddish on the same thallus may suggest the presence of this species. See also Cetraria chlorophylla  that is a smaller species with transluscent lobes and K- cortex turning olive-green when moist, lower surface pale cream-greenish, and Cetrelia olivetorum which differs by the presence of pseudocyphellae.