Plagiomnium is a genus of leafy mosses belonging to the family Mniaceae (placed in Plagiomniaceae by some authorities) within the order Bryales, class Bryopsida, and phylum Bryophyta. The genus was established in 1968 when Finnish bryologist Timo Juhani Koponen split the previously broad genus Mnium into several smaller, more precisely defined genera; Plagiomnium was among those segregates.
Members of the genus are characterised by their distinctively toothed leaf margins — a defining feature known as singly placed marginal teeth — and by leaves that are wide-spreading and glistening when moist, becoming shrivelled and dull upon drying. Plants are typically small to medium in stature, with shoots ranging from around 1.5 cm in compact species to 8 cm or more in larger ones such as Plagiomnium insigne (badge moss or coastal leafy moss), which is the largest mnium. Sterile stems in several species arch in a manner reminiscent of strawberry runners.
The genus has a predominantly Northern Hemisphere distribution, occurring widely across North America, Europe, and Asia, with individual species extending into montane, boreal, and temperate zones. Several species favour shaded, moisture-rich microhabitats: stream banks, seeps, springs, wet meadows, forested peatlands, and the bases of trees or rotting logs on humus-rich soil. Plagiomnium insigne is endemic to western North America, while Plagiomnium cuspidatum (toothed thyme-moss) has expanded beyond its North American origin to Europe, Africa, and parts of Asia. Plagiomnium medium, an alpine species of the Northern Hemisphere, is notable for being a natural allopolyploid hybrid between P. ellipticum and P. insigne — evidence that polyploidisation, long considered rare in bryophytes, does occur within this genus.
GBIF recognises 7 accepted species in the genus. Several species are ecologically significant: P. cuspidatum serves as nesting material for songbirds and as an egg-attachment substrate for four-toed salamanders (Hemidactylium scutatum), and is also cultivated in bioactive terrariums for its antimicrobial properties.
Etymology
The name Plagiomnium combines the Greek plagios (“oblique” or “sideways”) with Mnium, the older and broader moss genus from which it was separated. The genus was formally delimited by Finnish bryologist Timo Juhani Koponen in 1968.
Distribution
Plagiomnium is distributed predominantly across the Northern Hemisphere, with species occurring in North America (where P. cuspidatum is native and P. insigne is endemic to the west coast from Alaska to California), Europe, and Asia. Montane species such as P. medium reach into alpine zones, while lowland species occupy temperate forests and peatlands. P. cuspidatum has also naturalised in parts of Middle America, Africa, and southern Asia.
Ecology
Species of Plagiomnium favour shaded, moisture-rich environments — stream banks, seeps, springs, wet meadows, rich peatlands, and the bases of trees or rotten logs — typically on humus-rich or base-rich soils. Plagiomnium cuspidatum is known to provide nesting material for songbirds and to serve as an egg-attachment substrate for four-toed salamanders. Some species, including P. cuspidatum, can grow under artificial light in otherwise lightless environments such as caves.
Taxonomy Notes
Plagiomnium was segregated from the broadly circumscribed genus Mnium in 1968 by Finnish bryologist Timo Juhani Koponen, who justified splitting Mnium into several smaller genera. Family placement differs between authorities: Wikipedia places the genus in Mniaceae, while GBIF places it in Plagiomniaceae (a segregate family). Plagiomnium medium is a naturally occurring allopolyploid hybrid of P. ellipticum × P. insigne, an unusual finding given that polyploidy was long considered rare in bryophytes.
