Let's start with the differences: although both groups have drooping capsules, Bryums have capsules with distinct 'necks':
|Bryum caespiticium capsule|
the neck is the top smaller 'bulge' before the stem.
|Plagiomnium cuspidatum capsule (with calyptra)|
note that although the capsule is drooping, there is no 'neck' to it
|Bryum caespiticium leaf|
Mnium leaves tend to be large-ish, rounded, oblong, elliptical or obovate and often have obvious teeth (excepting the Rhizomnium group, which we'll talk about later).
|Mnium spinulosum leaf|
|Plagiomnium ciliare leaf|
Although both groups have leaves that are bordered with long cells, the border is usually quite strong in the Mniums and weaker in Bryums. The interior cells are also different, with Mnium cells being generally round or hexagonal and Bryums having long rectangular or diamond-shaped (rhomboidal) cells.
|This drawing is a section from the edge of Plagiomnium ciliare,|
note the rounded interior cells bordered by several rows of long cells
and the long, multicellular teeth
|Interior rhomboidal cells from Rhodobryum ontariense|
So, now you know how to tell a Bryum from a Mnium...
next post we'll talk about the main groups of Mnium: Rhizomnium, Plagiomnium & Mnium