Zwackhia sorediifera (P. James) Ertz
Opegrapha sorediifera P. James
Thallus crustose, 2-5 cm in its greatest length, slender, smooth to a little cracked, hydrophilic, in all shades of brown: reddish-brown, olive-brown, greyish-brown in dry weather, more greyish in humid weather, sometimes with orange spots or areas; covering itself with punctate to spherical soralia 0.5-1 mm in diameter, yellow, yellowish, yellow-orange, at first isolated and rare, then in small tufts which by meeting can form large soredia giving an almost continuous crust on the whole thallus, poorly defined margin but sometimes a brownish or blackish prothallus. Scanty apothecia scattered throughout the thallus or gathered in cracks in the bark, in short, squat lirella 0.5-1.5 x 0.2-0.5 mm, black, slit often deformed and inconspicuous. Utriform asci with 8 spores arranged in a spiral; 75-100 x 18-20 µm according to our measurements. Colorless spores, very long spindle shaped, with a hyaline perispore, 8-14 times septate and giving the appearance of rectangular cells in strings, 30-50 (55) x 4-6 µm according to the literature; 50-60 x 6-7 µm according to our measurements. Frequent black pycnidia. Photobiont: Trentepohlia. Thallus: C-, K-, KC-, P-; soralia: C + fleeting pink-red, KC- pink-red a little fleeting orange. Fairly common but localized species occurring on the bark of deciduous trees in very humid oceanic forests.
N.B. Species quite variable depending on whether the bark is smooth or rough.