Arthothelium ruanum

Thallus immersed, effuse or delimited by a brown line, creamy white, pale grey, brown-grey or olive-grey; photobiont trentepohlioid. Apothecia to 1.6 (–2) mm diam., irregularly rounded to bluntly stellate, often disintegrating in places and regenerating to give the appearance of a crowded swarm of punctiform apothecia, black, not pruinose (but often long remaining covered by bark cells), in section 70–95 µm tall; epithecium dark red-brown, K+ green; hymenium 35–50 µm tall, colourless or pale brown (K+ greenish); hypothecium 10–30 µm tall, dark red-brown, K+ dark green; paraphysoids 1–1.5 diam., numerous, the apices obscured by dense pigment. Ascospores (15–) 17–24 (–26) × 7–9.5 (–10.5) µm, obovoid-cylindrical, muriform, with 5–8 transverse septa and (2–) 3 to 7 transverse cells with 1 to 3 longitudinal septa, when old brown and warted. Pycnidia 60–80 µm diam., scarce, the wall red-brown, K+ green; conidia 4–6 × ca
1 µm, bacilliform.

Some forms with white thalli are easily confused Arthonia ilicina in the field, but this lichen has much larger non-muriform spores. Typical material, especially when on Hazel, has a browner thallus with remnants of the bark epidermis remaining over young apothecia a distinctive feature not seen in Arthonia ilicina.

There are some enigmatic specimens from S.W. Ireland which have larger ascospores, 20–31 × 9.5–11.5 µm, with 6–9 (–10) tranverse septa, earlier mentioned by Coppins & James (1979).