Opegrapha vermicellifera (Kunze) J. R. Laundon

    Thallus crustose, to 8-12 cm across, smooth or minutely rimose, pale grey, grey-whitish, fertile form grey-ochre but most often sterile and then whitish, pale grey, covered with pycnicia, regularly distributed, immersed at first and appearing as black or brown-black ostioles, later as white, pruinose papillae or minute warts; scratches on the thallus induce orange stripes (Trentepohlia). Fertile form rather rare, lirellae confined to restricted areas on the thallus, lirellae to 0.1-0.15 mm wide, isolated, most often simple, but potentially furcate or stellate in small clusters, disc slit-shaped, greyish and pruinose. Prominent pycnidia, brown then black, quickly covered with a whitish jelly containing the pycnidiospores. N.B. This white jelly is excreted in the form of serpentine filaments as from a tube of toothpaste which, on drying, gives an irregular appearance at the top of the pycnidia (appearance of vermicelli hence its name?)! Asci in club 60-75 x 12-15 µm according to our measurements, with 8 spores. Very elongated spores with pointed ends, colorless, 5-septate, 15-25 x 3-4 µm according to the literature; (4)-5-(7) -septate, 18-22 x 3-4 (5) µm according to our measurements. Bacilliform pycnidiospores. Thallus and pycnidia pruine: C-, K-, KC-, P-. Relatively frequent species occurring on rather dry bark of trees in woods and humid forests. The sterile form is the most common and is then very similar to the sterile form of  Lecanactis abietina which has larger pycnidia with a red C + pruine, K- and more still to Lecanactis subabietina with pale yellow C-, K + pycnidia pruine.