Polycoccum arnoldii
Anamorph: not known.
Teleomorph: ascomata perithecia (80-) 120-150 µm diam., subglobose, scattered on the host thallus or immersed within a gelatinous matrix on the apothecia, with the upper half externally visible, 3–12 per apothecium, without producing clear necrosis or bleaching. Ascomatal wall 5-10 µm thick, pale brown at the base, markedly darker and thicker in the upper part, composed of several layers of angular flattened cells which become almost hyphal in nature towards the ostiole. Interascal tissue composed of quite sparse pseudoparaphyses ca 2 µm diam., thick-walled, branched and anastomosing. Hymenial gel not blueing in iodine. Asci 75-90 x 10-12 µm, cylindrical, short-stalked, fissitunicate, the apex rounded with a distinct ocular chamber, not blueing in iodine, mostly 8-spored. Ascospores strictly or obliquely uniseriate, 12-14 x 6-7 µm, ellipsoidal to slightly clavate, the ends rounded, strongly constricted at the single ± median septum, the lower cell usually slightly narrower, dark brown, thick-walled, verruculose, without an epispore, gelatinous sheath or appendages.
Not evaluated by Woods & Coppins (2012), but listed as Nationally Rare.
On thalli and apothecia of Diploschistes and Rhizocarpon species. Further work is needed to establish whether the populations on different host lichens represent separate species.
Known from single sites in mid Wales, the Yorkshire Dales, Cumbria, SW and central Scotland. BLS map here.