Physcomitrella Bruch & W. P. Schimper in Bruch & W. P. Schimper, 1849.

The genus Physcomitrella is one of the winter and early spring ephemeral mosses with immersed capsules. As such, Physcomitrella is unlikely to be collected unless it has sporophytes. The ephemeral mosses with immersed capsules are all plants of open, often disturbed soil, but our two species of Physcomitrella are mainly plants of mud laid bare by the retreating water of lakes and ponds. Three Funariaceae mosses of the western United States can have immersed capsules. The capsules of Aphanorrhegma serratum are dehiscent by means of an operculum, have collenchymatous but otherwise thin-walled exothecial cells, and the calyptra is small, mitrate and basally fringed; our two species of Physcomitrella have no operculum, thick-walled but not collenchymatous exothecial cells and the calyptra is small and mitrate but not fringed. Physcomitrium immersum has a well-defined operculum; thin-walled and hexagonal exothecial cells, and the calyptra resembles that of Aphanorrhegma.
see key to Acaulon Etc.

Species included:
Physcomitrella patens (Hedwig) Bruch & W. P. Schimper
Physcomitrella readeri (C. Müller Hal.) Stone & G. A. M. Scott