Bacidina caligans (Nyl.) Llop & Hladun

Fl. Liquenol. Ibérica, 3: 109, 2002. Basionym: Lecidea caligans Nyl. - Flora, 57: 10, 1874
Synonyms: Bacidia caligans (Nyl.) A.L. Sm.; Woessia caligans (Nyl.) Sérus. & Diederich
Description: Thallus crustose, thinly episubstratic, dull green, grey or pale brown, with granulose, soredia-like, 20-50 μm wide goniocysts which are often gathered in rather discrete, orbicular, soralia-like aggregates. Apothecia lecideine, 0.2-0.6(-1) mm across, with a flat to irregularly convex, red-brown to brown-black disc, and a usually darker, finally often excluded proper margin. Proper exciple red-brown and K+ purple in outer edge, colourless within, the cell lumina up to 7 μm in diam. towards the edge and in lower part; epithecium poorly differentiated, colourless or or pale purple-brown, the pigmented parts K+ intensifying purple; hymenium colourless, 45-55 μm high; paraphyses mostly simple, 1-2 μm thick at base, the apical cells up to 5 μm wide; hypothecium colourless. Asci 8-spored, cylindrical-clavate, approaching the Bacidia-type, the ocular chamber wide, the axial body never penetrating through the entire d-layer, surrounded by a narrow, darker amyloid layer. Ascospores 3-7-septate, hyaline, acicular, 24-48(-63) x 1.5-2(–2.5) μm. Pycnidia immersed, white or pink-brown around the ostiole. Conidia thread-like, curved, 29-35(-45) x c. 1 μm. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: thallus without lichen substances.
Note: a mild-temperate to Mediterranean, mainly calcicolous lichen, sometimes occurring also on mosses and mortar, more rarely on base-rich bark (e.g. of Sambucus, Salix, etc.), found in shaded and humid situations at relatively low elevations; probably overlooked, being often sterile, and to be looked for in Italy.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)

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Predictive model

Photo uploaded by Paul Cannon – CC BY-SA NC – Source:
United Kingdom, VC30 Bedfordshire.
thallus and apothecia on an old wooden plank