Opegrapha atra 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) Ám. Micrograph below.

Opegrapha atra

Opegrapha atra

Microscope photograph
Opegrapha atra
Opegrapha atra
On Sycamore. Kimmeridge, Dorset. January 2013

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