Arthonia helvola (Nyl.) Nyl.

Flora, 50: 330, 1867. Basionym: Arthonia pruinosa var. helvola Nyl. - Prodr. lichenogr. Galliae Algeriae: 165, 1857
Description: Thallus crustose, endosubstratic, smooth, whitish, without a distinct prothallus, the hyphae hyaline, smooth, 1-2 μm thick. Apothecia arthonioid, irregularly maculiform, more or less immersed to erumpent, 0.2-0.8 mm across, 50-90 μm thick, orange-red, epruinose, flat and emarginate. Proper exciple absent; hymenium at least in part orange-red, K+ reddish violet, I-, K/I+ blue; paraphyses branched and anastomosing, 1-1.5 μm thick, the apical cells not swollen. Asci 8-spored, clavate, distinctly stipitate, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, Arthonia-type, (20-)27-42 x 8-11(-12) μm. Ascospores 2-septate, slightly constricted at least at primary septum, hyaline, narrowly ovoid, straight, 2-septate, (8-)20-23(-33) x (2-)3-4 μm, the apical cells of almost equal width, the wall smooth and without epispore. Photobiont trentepohlioid. Thallus K-, C-, KC-, P-. Chemistry: thallus without lichen substances; apothecia with an amorphous hydrophobic pigment reacting K+ violet-red.
Note: on bark of deciduous trees, mostly in shaded places with high humidity, confined to old forests with a long ecological continuity; known from several stations in Central Europe, including Switzerland but certainly very rare. To be looked for in Italy.
Growth form: Crustose
Substrata: bark
Photobiont: Trentepohlia
Reproductive strategy: mainly sexual

pH of the substrata:


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Altitudinal distribution:


Predictive model

P.L. Nimis; Owner: Department of Life Sciences, University of Trieste
Herbarium: TSB (21213)

Source: Redinger, K. (1938) Arthoniaceae, Graphidaceae . In Kryptogamen-Flora von Deutschland, Österreich und der Schweiz. Band 9, 2 Abt., Part 1 (G. L. Rabenhorst, ed.): 181–404. Leipzig: Borntraeger. – Public Domain