Placopsis parellina
Placopsis parellina is a species of lichen-forming fungus in the family Trapeliaceae. It grows on soil and rocks, forming small, spreading patches made up of tightly packed scale-like lobes, and bears dark cyanobacteria-containing structures scattered across its surface. The fruiting bodies have a conspicuously swollen rim and a pale to dark reddish-brown. The species has a Southern Hemisphere distribution, occurring from Ecuador southward through the Andes to southern Chile, with additional populations in the Australia–New Zealand region and an isolated record from Java.
Taxonomy
It was described by William Nylander in 1855 as Lecanora parellina, based on material growing on clay soil and characterised by a grey-white, granular crust with small, flat, brick-coloured apothecia set in a thick. He reported 8-spored cylindrical asci with ellipsoid spores about 21–24 × 11–12 μm and an iodine-positive hymenium, and contrasted it with Lecanora parella by its smaller spores and much longer, needle-like conidia. Elke Mackenzie reclassified it in Placopsis in 1940.Description
Placopsis parellina is a small, spreading lichen that forms a low, patchy thallus made of thickened, tightly packed. On soil the squamules tend to overlap like roof-tiles, while on rock the thallus is usually flatter and more clearly rosette-forming. It does not produce isidia or soredia. Darker, cyanobacteria-containing cephalodia are typically scattered across the thallus as flattened to low-domed structures about 0.5–1.2 mm across. Their surface texture may range from finely ridged to smooth, and they can be so closely set that they appear almost immersed among the thalline squamules.The lichen forms sessile apothecia about 0.5–2 mm wide, with a conspicuously swollen, thallus-coloured rim reminiscent of some species of Ochrolechia. The is pale to dark red-brown, sometimes deepening to brown-black, and lacks pruina. Microscopically, the spore-bearing layer is colourless and about 200–240 μm tall, and the colourless ascospores are broadly ellipsoid, about 16–20 × 10–15 μm.