Lecanora argentata
Thallus continuous, pure white or with yellow or green tinges (in shade), smooth at the margins but uneven to warted centrally. Prothallus usually black and well-developed.
Anamorph: no information available.
Teleomorph: ascomata apothecial. Apothecia 0.4-0.8(-1) mm diam., sessile, dispersed or aggregated, constricted below. Thalline exciple persistent, smooth to crenulate, containing massive crystals not soluble in K. Discs red or dark brown, sometimes piebald. Epithecium red-brown, without granules, not pruinose. Hymenium (60-)75- 90(-100) µm tall. Paraphyses 1.5-2 µm wide, sparsely branched and anastomosed, apices to 3 µm wide, slightly swollen, faintly yellow. Asci 45-55 × 18-22 µm, broadly clavate, Ascospores (10.5-)11.5-14.5(-17.5) × (5.5-)6- 8.5 µm, broadly ellipsoid, walls 0.5-1 µm thick.
Chemistry: thallus C–, K+ yellow, Pd+ weakly yellow, UV– (atranorin, gangaleoidin, usually traces of californin).
Assessed by Woods & Coppins (2012) as of Least Concern.
A distinctive member of the L. subfusca group distinguished from L. chlarotera and allied species (but not from L. campestris or L. horiza) by the absence of granules in the epithecium. From L. campestris and L. horiza it can be distinguished by the presence of massive crystals in the thalline exciple.
Most other members of the Lecanora subfusca group (e.g. L. chlarotera, L. horiza and L. pulicaris) which grow on bark are superficially similar. The discs of L. chlarotera are usually rather more pale in colour than the others mentioned. The accurate determination of L. argentata usually requires collection for microscopical investigation.
Probably common (but overlooked) in W. British Isles, scattered elsewhere. It appears to be spreading back into Eastern England.
On bark of deciduous trees. In unpolluted regions L. argentata colonises young bark. In regions where atmospheric sulphur dioxide pollution were formerly high, L. argentata sometimes survives in relic communities on sheltered old bark along with Pertusaria species.