Rhizomnium (Mitten ex Brotherus) T. Koponen, 1968.

Key to Rhizomnium Etc.

Mosses included in this section are broad-leaved acrocarps with rounded leaf apices. Cells are isodiametric or short-hexagonal, and there is a strong limbidium of elongate and thick-walled cells. The leaf margin is absolutely entire to the apex.

Species included in this key are all in Mniaceae:
Mnium blyttii Bruch & W. P. Schimper
Rhizomnium glabrescens (Kindberg) T. Koponen
Rhizomnium gracile T. Koponen, not known from CA
Rhizomnium magnifolium (Horikawa) T. Koponen
Rhizomnium nudum (E. Britton & R. S. Williams) T. Koponen, not known from CA
Rhizomnium pseudopunctatum (Bruch & W. P. Schimper) T. Koponen
Rhizomnium punctatum (Hedwig) T. Koponen

One of the major taxonomic subdivisions of the genus Mnium concerns the presence or absence of micronemata. In general, a plant with micronemata has a dense coating of rhizoids over the entire stem up to the bases of the youngest leaves. In general, a plant without micronemata has dense rhizoids restricted to clusters arising from some of the more proximal portions of the stem. Mosses have several types of rhizoid origin. Often rhizoids originate from cells at the ab- or adaxial leaf insertion. Sometimes rhizoids originate from cells of the leaf, most often the abaxial face of the costa or lamina. In the Bryales and some other groups there are special types of rhizoids based upon patterns of origination on the stem. Macronemata form a halo around a bud primordium, and these are supposedly homologous to the pseudoparaphyllia of many pleurocarpous mosses. Micronemata come from cells scattered, or arranged in longitudinal rows, on the stem surface. Even when the rhizoids are absent, the cells from which micronemata arise will show as somewhat enlarged cells without chloroplasts. It should be noted here that the macronemata are not necessarily bigger than micronemata.

2008 Key, P. Wilson

A. Micronemata present (younger or poorly developed stems may lack the micronemata but the initials will still be present and visible in a stem whole-mount) .....B
A. Rhizoids restricted to base of plant and around buds .....D

B. Leaves and plant small with leaves seldom exceeding 4 mm long; costa short, ending at about 3/4; leaf border unistratose .....Rhizomnium gracile not known from CA
B. Leaves and plant larger; costa percurrent or ending within a few cells of apex; leaf border one or more cells thick .....C

C. Costa confluent with the limbidium or ending within 2-3 cells of the limbidium; dioicous; leaves on at least the well developed stems more than 7 mm long .....Rhizomnium magnifolium
C. Costa ending several cells below the limbidium; synoicous; leaves smaller .....Rhizomnium pseudopunctatum

D. Limbidium obvious and extending to the apex; costa mostly confluent with the limbidium and apiculate .....E
D. Limbidium narrow near base and seldom extending to apex; costa of most leaves subpercurrent with apex hardly apiculate .....F

E. Laminal cells mostly about 2:1 with walls evenly thickened; stems reddish .....Rhizomnium punctatum
E. Laminal cells mostly shorter with end walls having small corner thickenings; stems brownish .....Rhizomnium glabrescens

F. Leaves nearly orbicular; limbidium mostly not reaching leaf apex or sometimes one cell wide; plant green after drying; leaves not decurrent .....Rhizomnium nudum not known from CA
F. Leaves obovate, mostly more than 1.5:1; limbidium several cells wide at apex with costa percurrent or nearly so; dried older plant with a blue-green blush; leaves decurrent by means of a 1-2 cell wide strip that reaches nearly to base of next lower leaf .....Mnium: M. blyttii