Physcia caesia (Hoffm.) Fürnohr var. caesia

    Thallus foliose, rosette-shaped, 3-8 cm diam., lobes elongate, radiating, separate and indented, 0.5-1 mm wide, convex, tightly appressed, overlapping, upper surface typically pale grey-bluish, dull whitish, dotted with white spots (pseudocyphella), with globose and farinose soralia forming small heaps, 1-2 mm diam., grey-bluish dotted whith white; lower surface whitish to pale brownish with simple rhizines, greyish to blackish. Apothecia scarce or absent, 1-2 mm diam., disc black, blue pruinose, thalline exciple prominent, concolourous to the thallus. Ascospores ellipsoid, brown, 1-septate, 18-25 x 6-10 µm. Cortex and medulla : C-, K+ yellow. Uncommon in our prospection area, on nutrient-rich substrates, rather tolerant to poluttion including in cities, rocks, cement, concrete, artificial substrates like plastics, organic glasses, etc. see Physcia caesia sur substrats artificiels, more rarely on the bark of the trunks or the wood, see Physcia caesia forme lignicole.

See Physcia dubia that is devoid of pseudocyphella and is K-medulla and Physcia subalbinea = Physcia caesia var. caesiella of which the lobes are discrete and not overlapping (notably towards the margin), with a bluish-grey upper surface very heavily dotted with pseudocyphellae, labriform soralia and dark blackish-brown underside, and which is less tolerant to pollution.