Xanthoria polycarpa (Hoffm.) Th. Fr. ex. Rieber
Polycauliona polycarpa (Hoffm.) Fröden, Arup & Sochting
Thallus foliose rosette forming less than 3 cm in diameter, formed of small very narrow lobes of 0.5-2 mm, often domed, enlarged at the ends which are divided into small lobules less than 1 mm in width, upper surface bright orange, bright yellow, but grayish-glaucous in shade, whitish underside with small holdfasts. (N.B. this thallus is sometimes very inconspicuous because it forms cushions with the apothecia which completely cover it). Apothecia in tight group which may cover the whole of the thallus, hide the margin and even overflow, sessile to almost stipitate, more or less deformed by their neighbors, 1-3 mm in diameter, flat disc, dark orange to orange-brown, yellow gold, margin fine, smooth, disappearing in old age, concolor or paler than the thallus. Spores elliptical, plurilocular, 11-15 x 6-8 µm. Photobiont: trebouxioide. Cortex: C-, K + purple, KC-, P-. Nitrophilic species uncommon in our prospecting region typically coming from the angles of twigs of deciduous shrubs or small branches of trees, in particular in urban areas or in the vicinity of farms. Rarer on the wood of pigsties with Xanthoria parietina..
This species has been variously interpreted and confused, we interpret it here ss. str. Easy to confuse with small specimens of Xanthoria parietina., an infinitely more common species which often comes in its company but which is larger and does not present apothecia gathered in a tight group which can cover the whole of the thallus and especially with larger lobes not convex, spread out at the margin of the thallus. Also watch out for the very fertile specimens of Candelaria concolor which are K-, and of Xanthoria candelaria which are sorediate.