Collema subnigrescens Degel.

    Leaf-like thallus more or less arranged in a rosette, quite large: 5-12 cm in diameter, firmly attached to its substrate, formed of lobes 1-1.5 cm wide, translucent and membranous in appearance, especially towards the periphery which is lobed with lobes with a smooth, uncut margin. The entire surface is strongly covered with irregular bubble folds arranged radially. The general coloration is olive-brown to greenish-brown in wet weather and blackish-brown in dry weather. Apothecia usually very numerous, especially located in the center of the thallus, first pustular then sessile, 0.5-1 mm in diameter, disc concave to plane but sometimes a little convex, reddish brown to blackish brown (like the thallus!) , very thin rim more or less persistent. Spores hyaline, long elliptical and almost acicular, sometimes slightly curved, 4-5-(6) -septate, (35) 40-75 x 5-6 (7) µm according to literature, 40-55 x 4-6 µm according to our measurements. Photobiont: Nostoc. No significant color reactions. Species growing on the bark of deciduous trees or conifers in very humid forest.

N.B. Very similar to Collema nigrescens but the latter species is smaller, has pustular isidia in places without apothecia and its spores are less broad and 5-8 (9-10)-septate.

N.B. Collema furfuraceum has much fewer apothecia and its thallus is abundantly covered with fine coralloid isidia.