Physcia magnussonii is a foliose lichen, meaning it has a leaf-like structure. The thallus—the main body of the lichen—is heteromerous and dorsiventral, with narrow lobes that are closely pressed against the substrate. These lobes form tightly packed, circular rosettes that can reach up to 8 cm (3.1 in) in diameter. The lobes themselves are up to 3 mm wide, generally overlap each other, and are slightly convex. The colour of the lobes ranges from whitish grey to brownish grey, and they are uniformly covered with a dense, white powdery coating (pruina). The underside of the thallus is white to brownish, featuring a few thick and simple rhizines, which are root-like structures that help anchor the lichen.[1]
The upper layer of the thallus, or cortex, is made of densely packed cells (paraplectenchymatous), while the inner layer, or medulla, is white. Physcia magnussonii commonly produces apothecia (fruiting bodies) that are up to 3 mm across. These have a black disc that is usually white-pruinose and are bordered by a smooth, thallus-like margin. The fruiting bodies contain spore-producing structures called asci, which are club-shaped, very thin-walled, and feature a tall, blue-staining tholus with an indistinct amyloid ring when treated with iodine (K/I+ blue), characteristic of the Lecanora-type asci. Each ascus holds eight ascospores.[1]
The spores are one-septate (divided into two cells by an internal partition, or septum), brown, and narrowly ellipsoid, measuring 16–24 by 6–9 μm. They have a unique verrucose (warty) ornamentation and have thickened walls at the apex and septum, typical of the genus Physcia. Pycnidia, small fruiting bodies containing asexual spores (conidia), are also common and appear as immersed black dots. The conidia are almost cylindrical in shape.[1]
In terms of chemical properties, the cortex tests K+ (yellow), indicating the presence of atranorin, a common lichen product, whereas other chemical spot tests on the medulla may show negative (K−) or a pale pink reaction (K+ pale pink), with no reaction to other tests: (C−), (KC−), and paraphenylenediamine (P−).[1]
Variolaric acid is a secondary metabolite (lichen product) that has been shown to occur in some Norwegian specimens of Physcia magnussonii. This rare depsidone substance is know to occur in only a few lichen species, mostly in the order Pertusariales.[5]