Slideshow image
Placidium michelii A. Massal.
Family: Verrucariaceae
[Catapyrenium michelii (A. Massal.) R. Sant.,  more]
Placidium michelii image
Thallus: squamulose, squamules: 1-5 mm in diam., rather thin (up to 0.25 mm thick), dispersed or in groups, appressed to the substrate, roundish or slightly lobed upper surface: pale to dark chestnut brown, dull upper cortex: 20-40 µm thick; epinecral layer: very thin or lacking medulla: white, with many spherical cells (c. 9-14 µm in diam.); algal layer: c. 50-80 µm thick lower cortex: composed of 2-3 rows of roundish-angular cells (outermost ones: dark brown) lower surface: black, fastened by a dense rhizoidal weft; rhizohyphae: hyaline, 4-6 µm thick Perithecia: pyriform, up to 0.35 mm broad, bulging the lower side of the squamules; wall: dark brown, thinner than in the other Placidium species (hardly more than 25 µm wide); periphyses: 30-40 x 2.5 µm asci: cylindrical, c. 60-70 x 10-13 µm, 8-spored ascospores: uniseriate, ellipsoid, 11-15 x 5-7 µm Pycnidia: laminal, immersed conidia: ellipsoid-oblong, 2.5-3.5 x 1.5 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: soil World distribution: widely distributed but scattered in Europe, Asia and North America, but not known from the Southern Hemisphere Sonoran distribution: rare in Arizona and southern California (San Diego Co.). Notes: It is very similar to Placidium squamulosum, from which it differs primarily in its dark, thin perithecial walls and the thinner periphyses. The squamules tend to be smaller and are always black on the lower side.