Parmelina tiliacea (Hoffm.) Hale

    Thallus foliose, to 15 cm across, loosely attached to the substratum, lobes 5-10 mm wide, more or less imbricate, notably towards the margin, sometimes slightly ciliate, apices rounded, little indented, upper surface smooth, pale grey-blue, pale grey, lower surface black towards the center and margins brown and with a few simple rhizines. Upper surface covered, arising from the center, with grey-brown isidia, simple, cylindrical, clavate or coralloid, tips darker, potentially, in exposed areas, forming a thick layer and covering the whole thallus. Seldom fertile, or apothecia scarce, sessile or cup-shaped, 4-7 mm diam., disc brown-red, exciple more or less isidiate and indented. N.B. Some specimen have numerous apothecia and in that case very few isidia, restricted to sections without apothecia; these speciemen are easily mistaken with Parmelina carporrhizans that is devoid of isidia and bears countless apothecia (see pictures). Ascospores simple, colourless, broadly ellipsoid to subglobose, 9-10 x 6-9 µm. Cortex: K+ yellow; medulla: C+ carmine, K-, KC+ red, Pd-. Rather rare thermophilic species in our prospection region, coming on the bark of trees and more rarely on the sunny siliceous rocks by the sea (see Parmelina tiliacea saxicolous form). N.B. Not to be confused with similar species and notably infertile Parmelina pastillifera with large bud-shaped isidia and with Parmelina quercina and Parmelina carporrhizans both devoid of isidia.