Dear Didier,

I know about three species in the genus Acrosperum: A. compressum, A. graminum and A. pallidulum. All three are hard to determine because they have nearly the same characteristics.
A. compressum usually grows on old stems of Urtica and is 2-4 mm high, with a grey-brown surface. Spores are about 400 µm long.
A. graminum grows on little stems of graminaceen and has smaller fruitbodies (1-2,5 mm). The color is brownish, pale-brown.
A. pallidiulum also has pale-brown fruitbodies but spores around 300µm long and grows on dead stems of "Galium". I just know this species from literature but didn't find it in nature yet.

Differences are flowing into each other and it would not surprise me, if there are only 2 species instead of three or more (I do not know about others...).

Greetings,
Kaz

Merci pour la réponse
Didier