Arthoniaceae Arthonia atra (Opegrapha atra) Growth type crustose Chemical results: Thallus negative. Exciple K+ olive-green. Hymenium I+ blue, epithecium brown or green-brown, K+ olive-green. Common on smooth, shaded trunks, branches and twigs, rare on wood. Forms extensive mosaics on young trees but avoids nutrient-enriched bark. Host to Milospium graphideorum and Muellerella polyspora Short prominent lirellae with central slit on a thin or +/- evanescent whitish or green-tinged thallus. Lirellae variable, simple or branched, forming parellel lines, stellate clumps or densely crowded clusters. Pycnidia rare. Spores are 3-septate, 13-18(-20) x 2.5-4(-5) µm. Micrograph below.

Microscope photograph

Arthonia atra (Opegrapha atra)
On Sycamore. Kimmeridge, Dorset. January 2013