Plants cladocarpous or rarely acrocarpous. Stem creeping, ascending, to erect, central strand absent. Leaves erect or spreading, straight or curved, sometimes recurved when wet, lanceolate to oblong-lanceolate, less often elliptic, ovate to lingulate, keeled to canaliculate-concave; margins recurved to revolute, entire or erose-dentate, serrate or cristate at the apex, costa rarely spurred or forked distally, sometimes ending in mid leaf, subpercurrent or excurrent, smooth or papillose, in transverse-section reniform to elliptical, sometimes semiterete or strongly flattened, with (2–) 3–15 adaxial cells near base, much larger than abaxial cells, often excurrent as an awn, awn smooth, or toothed or papillose or both; laminal cells smooth, pseudopapillose, or papillose; basal-cells rectangular to linear, nodulose-porose, usually thick-walled, always with spiral thickenings forming a colored strip along the insertion; mid leaf cells quadrate to elongate, mostly strongly sinuose-nodulose. Specialized asexual reproduction very rare by gemmae arising from the base of the costa on the abaxial side. Sexual condition dioicous. Seta usually long, straight or rarely slightly arcuate, smooth or papillose, one to several per perichaetium; vaginula with sinuose-nodulose epidermal-cells. Capsule erect, exserted, symmetric, ovoid, obloid to cylindric, usually smooth or obscurely striate; stomates present; annulus present, deciduous; operculum long-rostrate; peristome mostly with basal membrane and preperistome, equally thickened and weakly trabeculate both adaxially and abaxially, irregularly split into 2–3 branches to the middle or regularly divided into two filaments nearly to the base. Calyptra conic-mitrate, not plicate, often papillose at the apex, covering operculum to 1/2 of capsule.

Distribution

Worldwide

Discussion

Genera 4, species ca. 75 (4 genera, 28 species in the flora).

Subfamily Racomitrioideae is characterized by a Racomitrium-type peristome, consistently sinuose-nodulose walls of the laminal cells and epidermal cells of the vaginula, absence of stem central strand, non-plicate calyptrae, and cladocarpous arrangement of the perichaetia. Taxa belonging to this subfamily have sometimes been associated with Ptychomitrium and Campylostelium, and placed in the subfamily Ptychomitrioideae in the Grimmiaceae. Despite their overall morphological similarity, these taxa seem to be only remotely related. In Ptychomitrium the laminal cell walls are straight or weakly sinuose, the calyptrae deeply plicate, and the plants acrocarpous. In addition, the preperistome is absent, and the peristome teeth lack trabeculae and have a characteristic air gap at their base. Moreover, Ptychomitrium is cryptoicous because the male branches are small and arise from the base of vaginula inside the perichaetial leaf circle. The Racomitrioideae consists of four genera that are segregates from the large and heterogeneous Racomitrium in the broad sense.

Selected References

Allen, B. H. 1994b. The Grimmiaceae (Musci) in Maine. III. Racomitrium. Evansia 11: 41–54.

Frye, T. C. 1917–1918. The rhacomitriums of western North America. Bryologist 20: 91–98; 21: 1–16.

Lawton, E. 1972. The genus Rhacomitrium in America and Japan. J. Hattori Bot. Lab. 35: 252–262.

Lower Taxa

Bucklandiella, Codriophorus, Racomitrium

Key

1	Laminal cells smooth or pseudopapillose; peristome teeth short, divided to the middle, rarely deeper, into 2-3 irregular prongs.	Bucklandiella
1	Laminal cells papillose; peristome teeth long, split at least to the middle into 2(-3) filiform, ± regular filaments	> 2
2	Laminal cells with tall, conical papillae situated over the lumina; alar cells hyaline or yellowish hyaline, thin-walled, forming prominent, decurrent auricles.	Niphotrichum
2	Laminal cells with large, flat papillae situated over the longitudinal walls; alar cells absent or distinct, brown to yellowish orange, not hyaline, thick-walled	> 3
3	Hyaline hair-point always present, usually long, strongly papillose eroso-dentate, long decurrent down the leaf margins; seta papillose, sinistrorse when dry; costa percurrent, unbranched; capsule slightly ventricose at base; calyptra smooth or minutely roughened.	Racomitrium
3	Hyaline hair-point absent or present, rarely long, smooth to denticulate, never papillose or decurrent; seta smooth, dextrorse when dry (in C. fascicularis and C. corrugatus only once twisted to the left immediately below the capsule and proximally twisted to the left); costa ending well before the apex, often branched and spurred distally; capsule never bulging at base; calyptra distinctly verrucose to papillose.	Codriophorus