Mnium Hedwig, 1801


Key to Mnium

Mosses included in this key are acrocarps with broad leaves, isodiametric to shortly elliptical smooth cells, and with a geminate-toothed limbidium. Such a moss could be confused with Atrichum but that genus has photosynthetic lamellae along the abaxial surface of the costa.

Species included are all in Mniaceae:
Mnium ambiguum H. L. H. Müller, not known from CA
Mnium arizonicum Amann
Mnium blyttii Bruch & W. P. Schimper
Mnium marginatum (Withering) Palisot de Beauvois
Mnium spinulosum Bruch & W. P. Schimper
Mnium thomsonii W. P. Schimper

Several untraditional characters are emphasized in Mnium. We speak of "corner thickenings" as areas of thickened cell walls at the intersection of three adjacent cells. In such an area, the corner thickening is visible as a triangular area of wall material. The description of corner thickening in Mnium is independent of a description of the degree of thickening of the walls. Some species such as M. spinulosum and M. thomsonii are often rather thick-walled without the often glistening triangular corners. Corner thickening is more frequently emphasized in liverworts where the word "trigone" is used.

Many bryophytes have teeth on the abaxial face of the distal portion of the costa. Such teeth are best seen by turning an excised leaf so that the abaxial face is in view. Focusing up and down on that costa should reveal any teeth that may be present. Sometimes abaxial teeth are even more clearly seen when an entire leafy branch is mounted on the slide. Under such conditions one may be able to see a leaf with its costa in profile with the teeth therefore protruding out from the costa.

Mnium blyttii may be difficult to place in Mnium because its marginal teeth are often abortive or wanting. With complete lack of the marginal teeth, one might place Mnium blyttii in Rhizomnium. Mnium blyttii might also be keyed into Plagiomnium on the basis of the occasional abortive marginal teeth being singly placed. The keys to both Plagiomnium and Rhizomnium reflect this ambiguity.

2008 KEY, P. Wilson

A. Leaf cells elongate in radiating rows from costa to margin; limbidium greatly weakened proximally and marginal teeth inconspicuous proximally .....Mnium arizonicum
A. Leaf cells not in radiating rows or the rows parallel to the costa; in many species limbidium strong even proximally, marginal teeth various .....B

B. Cells with somewhat angular lumina due to the equal thickening of the cell walls .....C
B. Cells with more or less rounded or elliptic lumina due to the trigone-like corner thickenings .....E

C. Many mid-laminal cells >25 µm in longest diameter; distal limbidium in cross-section with very thick-walled cells (stereids), >2 cells thick; costa confluent with limbidium at apex; paroicous .....Mnium spinulosum
C. Few mid-laminal cells >20 µm in longest diameter; limbidium cells only slightly thickend with lumen:wall ~1:1, rarely >2 cells thick; costa in many leaves weakening before apex; dioicous .....D

D. Leaf margins mostly entire or with a few poorly developed teeth, only occasionally with those teeth geminate; dried older plants with an iridescent (gasoline) cast or bluish-green blush (suggestive of the color of cyanobacteria); limbidium 1-2 cells wide and 1-2 cells thick; costa not confluent with limbidium; leaves generally shorter though variable depending on position (perichaetial bracts to 4.5 mm, oblong to oblong-lanceolate, perigonial bracts ovate) .....Mnium blyttii, I am not sure if the iridescent cast fades or if the ones that are bluish-green like cyanobacteria are just sick.
D. Leaf margins prominently toothed from at least mid-leaf to apex and teeth geminate; dried plant a purer green turning pink in death; limbidium mostly wider and thicker at least in well-developed leaves; costa various but often with a weak connection to the limbidium; leaves generally longer (perichaetial bracts to 6 mm linear lanceolate; perigonial bracts ovate-lanceolate) .....Mnium thomsonii

E. Median leaf cells mostly >20 µm in diameter; costa seldom with abaxial teeth; many leaves long acuminate at apex; marginal teeth blunt .....Mnium marginatum
E. Median leaf cells mostly <20 µm in diameter; costa with abaxial teeth; leaves short acute at apex; marginal teeth sharply acute .....Mnium ambiguum not known from CA