Psoromidium Stirt.
=PSOROMARIA Nyl., 1888
Type : Psoromidium aleuroides (Stirt.) D.J.Galloway [=Lecidea aleuroides Stirt.]
Type : Psoromaria descendens (Nyl.) Nyl. [=Psoromidium aleuroides (Stirt.) D.J.Galloway]
Description : Thallus squamulose, orbicular to spreading, closely appressed to substratum, 1–5(–10) cm diam., 20–120(–150) μm thick. Photobiont green, protococcoid, cells spherical, 8–15 μm diam. Squamules orbicular to irregular, plane to convex, discrete or crowded-imbricate, sometimes distinctly areolate-diffract to secondarily lobulate. Margins entire, crenate or minutely lobulate. Upper surface bright-green when wet (see Kantvilas & Jarman 1999: 136–137), pale greenish yellow to olivaceous or somewhat greyish when dry, matt or glossy in parts, without isidia or soredia. Lower cortex absent, medulla of squamules attached directly to prothallus. Prothallus always present, noticeable at margins but also developed below squamules, thick, black, byssoid to thinly furfuraceous, pale-greyish, often inconspicuous in shaded habitats. Cephalodia always present, scattered, at margins of squamules or discrete on prothallus, sparse to numerous, orbicular, placodioid, flattened or convex, dark grey-blue to blue-black when wet, pale-grey, grey-white or yellow-grey when dry. Photobiont Nostoc in chains or clusters, cells 5–12 × 3.5–5 μm. Ascomata apothecia, scattered to frequent, biatorine, sessile or ±constricted at base, discrete to clustered-conglomerate (0.1–)0.4–2(–5) mm diam., concave to plane to convex; disc pale to dark red-brown, occasionally ±blackened, matt, epruinose; proper margin prominent, pale pinkish, ±translucent when wet, often excluded in older fruits, entire to ±crenulate, occasionally with short, white, silky hairs below. Hypothecium (35–)50–100(–150) μm thick, yellow-brown of densely interwoven, compacted hyphae. Hymenium I+ blue or I−, colourless, 70–125 μm tall. Hamathecium of paraphyses, simple, septate, 2–3 μm diam., apices thickened to 5 μm diam, red-brown, colour external to tips of paraphyses. Asci cylindrical-clavate, apices thickened, with I+ blue apical ring structure, outer layer of ascus wall amyloid, noticeably thicker at apex, ascus wall thin, non-amyloid, inner wall weakly amyloid. Ascospores biseriate, 8 per ascus, simple, colourless, subspherical to oval–ellipsoidal, apices rounded or bluntly apiculate, wall distinctly roughened-warted. Conidiomata pycnidia, rare or absent, ±densely clustered, swollen, globose, translucent brownish pink when wet, concolorous with thallus to somewhat blackened when dry. Conidia bacillar, colourless.
Psoromidium, a monospecific genus in the Pannariaceae (Eriksson et al. 2004; Pennycook & Galloway 2004; Eriksson 2005; Jørgensen 2004e) until recently, comprised two described species (Galloway & James 1985). It was realised for some time that as then defined, Psoromidium was probably heterogeneous, since two different ascus apex structures are present in the two taxa assigned to the genus. Accordingly, Jørgensen (2003c, 2004a) transferred P. versicolor to the newly described genus Degeliella (q.v.).
