Agonimia tristicula
Thallus minutely squamulose, often forming small, spreading, aggregated patches, squamules to 0.10-1 × 0.1-0.3(-0.5) mm, crowded, more or less contiguous and erect, sometimes nodulose and subgranular or rarely dispersed and elongate-finger-like, dull pale green-fawn to brown when dry, bright green when wet and fresh. Cells of upper cortex with small papillae 1.5-2 μm tall.
Anamorph: not known.
Teleomorph: ascomata perithecia, rather frequent, often in between and overgrown by squamules, 250-500 µm diam. and 250-600 µm tall, subglobose to broadly pyriform or barrel-shaped, very thick-walled, lacking an involucrellum, with a large and sometimes crateriform ostiole. Peridium black, matt, plicate-rugose. Interascal tissue absent, the ostiole lined with long periphyses within a gelatinous matrix. Asci clavate and fairly long-stalked, thick-walled with rostrate dehiscence, with an inconspicuous subapical apical ring, (1-) 2-spored. Ascospores (42-) 57-120 (-150) × 26-50 μm, elongate-ellipsoidal, sometimes slightly curved, the ends often apiculate, at first hyaline but eventually becoming golden to mid brown, muriform with a complex arrangement of angular cells, the septa irregular and frequently oblique, fairly thin-walled, ± smooth, without a perispore or gelatinous sheath.
Assessed by Woods & Coppins (2012) as of Least Concern. The species is widespread and reasonably common.
Sterile corticolous morphs are distinguished from Agonimia octospora by their larger, more distinctly flattened squamules which are darker olivaceous or brownish when dry and bright green when wet and fresh.
Agonimia vouauxii (Sérusiaux et al. 1999) is not yet reported from the British Isles but could occur here; like A. tristicula it has 2-spored asci, but the perithecia are smaller, 130-230 µm diam.
Throughout the British Isles. BLS map here.
On calcareous soil and dunes, or on mosses and lichens in crevices of more or less calcareous rocks and walls, including limestone, mortar, basalts and serpentine, more rarely on bark of base-rich trees, especially Acer, Ulmus and Fraxinus. Also occasional amongst bryophytes or overgrowing other lichens, particularly on periodically inundated siliceous rocks by rivers and lakes.