Arthopyrenia salicis  

Thallus inconspicuous to whitish, often of scruffy appearance, not well delimited; photobiont, a trentepohlioid alga, observed only in sections, but usually present.
Anamorph: conidiomata infrequent and inconspicuous, scattered on the thallus around the ascomata, occurring singly, pycnidial, dark brown to almost black, to 60 μm diam., and producing only one type of conidium. Conidia short, narrowly ellipsoidal or bacilliform, 2–3 (–4) × ca 1 μm, aseptate, analogous to the ‘microconidia’ in species of this and similar genera which produce two types of conidia.
Teleomorph: ascomata perithecial, scattered, 100–150 (–250) μm diam., oblate, covered by a dark brown or almost black involucrellum, semi-immersed in the bark, sometimes becoming detached and then leaving black scars on the thallus; ostiolate, with the ostiole situated in the middle and slightly sunken. In vertical section dimidiate, beneath an involucrellum to 30 μm thick, incorporating elements of bark cells, composed of thick walled hyphae ca 5 μm diam., forming a tissue intermediate between epidermoid and of interwined hyphae, becoming darker in KOH, extending into the thallus to form a short fringe to 40 μm broad. Ascomatal wall (exciple) 7–15 μm thick, hyaline, consisting of a few layers of isodiametric cells. Interascal tissue absent, with a few periphysoids surrounding the ostiolar region. Hymenial contents gelatinized, sometimes pale blue in Lugol’s iodine. Asci 30–40 (–50) × 15–20 μm, obpyriform, saccate, slightly geniculate at the base, fissitunicate, the inner layer abruptly narrowing at the apex, 8-spored. Ascospores arranged irregularly, (12–) 15–18·5 (–22) × (3·5–) 4·5–6 μm diam., narrowly soleiform, hyaline, 1(–3)-septate, constricted at the septa, cell(s) in the lower half of the spore narrower, smooth, guttulate with distinct vacuoles, perispore, gelatinous sheath and appendages absent.