Anisomeridium polypori  (Ellis & Everh.) M. E. Barr

   Thallus often inconspicuous, effuse, visible as a thin dirty whitish coat, grey or pale grey-green, leaving occasionally the substratum colour visible. Perithecia rarely numerous, black, scattered, 0.15-0.25 mm diam.. Pycnidia numerous, black, 0,1-0,2 mm of two types: 1) conical, 2) conical and ended by peg-like tips or upside-down funnel-shaped appendix, with ellipsoid or ovoid macroconidia, extruded as a white tendril in which the conidia are bound by a gelatinous matrix, ovoid macroconidia: 3.5-4.5 x 1.8-3 µm according to the literature, 2-4 (5) x 2-3 according to our measurements; ellipsoid microconidia: 2-3 x 1-1.3 µm according to the literature. Spores colorless, elliptical, 1-3 septate, 15-20 x 4-5 µm according to the literature; 1-3 (4) septate with a perispore, 12-17 x 3.5-4.5 µm according to our measurements. Photobiont : Trentepohlia.  Chemical spot tests negative. Very small species, understated unless specifically targeted, on rough bark of broad-leaved trees, in humid shaded woodland, seldom on rocks. Easy to determine (magnifying glass) by the observation of some pycnidia apex bearing a long thin appendix.

NB A closely related species has been described from the British Isles which exists in Brittany, in particular not far from the sea: Anisomeridium robustum Orange, Coppins & Aptroot, devoid of perithecia but with a thicker and clearer thallus often  with a blackish prothallus, slightly larger pycnidia and also slightly larger conidia: 5.5-6.5 x 2.5-3 µm, but which seems to be linked to Anisomeridium polypori by intermediaries.