Slideshow image
Arthrosporum populorum A. Massal.
Family: Ramalinaceae
[Arthrosporum accline (Flot.) A. Massal.,  more]
Arthrosporum populorum image
Life habit: lichenized (mutualistic with algal photobionts) thallus: crustose (crustaceous) – endosubstratal, inconspicuous, immersed | crustose (crustaceous) – episubstratal – unspecified [th] upper surface: white(ish) [th marginal and upper surface] specific structures: absent ascomata: absent | present ascoma: apothecial, apothecioid – hymenial ascoma [mm]: (median) 0.7 ascoma: sessile, superficial [ascm, if apoth] disc, mazaedium: black(ish) [ascm, if apoth] margin surface; [if perith] periostiolar area, ostiole, involucrellum: black(ish) [ascm, if apoth] subhymenial layers, hypothecium; [if perith] basal excipulum: brown(ish) (if pale: fawn, tan; if mid: cinnamon) | hyaline, colourless | red(dish) brown (if pale: orange brown) [ascm] paraphyses/-oids: present [ascm] epihymenium, epithecium: red(dish) blue (violet) | blue(ish) green (turquoise) | brown(ish) green (olivaceous, olive green) | green(ish) brown (olive brown) asci: lecanoralean [asc] tholus: thickened [asc] tholus amyloidity (iodine reaction): present [asc] tholus amyloidity pattern: amyloid with acuate axial body towards the apex (= Bacidia-, Buellia-, Ramalina-types etc) ascospores: (low) 8.0 (high) 16.0 [asp] shape: [cur] – curved, unciform, falcate, sickle-shaped [asp] length [µm]: (low) 10.0 (high) 18.0 [asp] width [µm]: (low) 4.0 (high) 5.0 [asp] septa: present [asp] septa: transversely septate [asp] transversal septa: (median) 3.0 (max) 5.0 [asp] pigmentation: hyaline, colourless [asp] perispore, epispore: not apparent secondary metabolites: absent primary photobiont: present secondary photobionts (eg in cephalodia): absent primary photobiont: chlorophytaceous – trebouxiaceous, chlorococcoid.

substrate: bark, cork, plant surface – trunks, branches, twigs.
Thompson, J., 1997. American Arctic Lichens: The Microlichens.
Thallus thin to dispersed or lacking, of tiny verrucules, ashy or ashy white. Apothecia 0.3-0.7 mm, broadly attached, black; disk flat; margin thin, black, more or less persistent; exciple reddish brown to reddish violet, reddish violet in K; the hyphae more or less radiate and thick-walled; hypothecium red to pale red-brown; epihymenium blue or bluish brown; hymenium 55-60 µm, upper part blue, 1+ blue; paraphyses lax, 1.5 µm, tips 4-5 µm; asci broadly clavate; spores 8-16 in ascus, straight or curved, oblong, tips rounded, 3-5-septate, 10-18 x 4-5 µm.

Reactions: exciple and hypothecium reddish violet in K, epihymenium turning violet with HN03.

This species grows on a variety of broadleaved plants, especially on Populus and Salix. It is known from Europe and North America, where it is eastern, from New England to Nebraska and Saskatchewan, but with one collection from Coppermine in the Northwest Territories.