Arthoniaceae Arthonia didyma Growth type crustose. Photobiont Trentepohlia. Red-brown pigment K+ grey or olive. Epithecium yellow-brown to red-brown, K +/- purple-violet. Hymenium colourless to spotty pale orange-brown, pigmented parts K+ purple-violet. Both K- and K+ apothecia can occur on same thallus Smooth bark of shrubs and trees (often Corylus) in unpolluted woodlands Thallus +/- inconspicuous, pale brownish or olive-grey. The small, flat, brown or black apothecia are irregular in shape, rounded or shortly linear. Spores 1-septate, 14-17 x 4.5-7 µm, colourless then becoming brownish and warted. Pycnidia are rare. Micrographs below. Can be separated from Arthonia vinosa by the flat, shallower apothecia. Arthonia spadicea has larger apothecia, smaller spores. A. muscigena (now Bryostigma lapidicola), photobiont chlorococcoid, narrower spores

Micrographs





Arthonia didyma
Brands Bay, Broadstone and Kingston Lacy, Dorset. January 2013, March 2020, January 2024, March 2025