Ptychostomum pseudotriquetrum

Ptychostomum pseudotriquetrum, commonly known as marsh bryum, is a species of moss belonging to the family Bryaceae. It is distinguished by its strongly decurrent leaves that extend down the stem, central leaf stalks which may extend slightly beyond the tip of the leaf, dioicy, and long stems densely matted with rhizoids. It is found worldwide, excluding the tropics.

Description
Ptychstomum pseudotriquem is a medium to large moss with an acrocarpous growth form. It is a yellow-green moss that becomes red-brown with age. The stems occur in dense turfs. The stems usually grow to lengths of 2 – but can be as short as 1 cm and long as 6 cm. The stems may be weakly comose (i.e. crowned with a tuft of soft leaves) or evenly foliate. The lower part of the stem is matted with papillose red-brown rhizoids. The stems branch sparingly by means of subfloral innovations, which are elongate and evenly foliate.

The leaves typically measure 2 – in length, but may as short as 1 mm or as long as 4 mm. The leaves are glossy and green, red-green, or yellow-green when young. They become dull brown-red or brick-red with age. They are twisted and contorted when dry. When moist, they are erect spreading, oblong‑lanceolate to ovate‑lanceolate, and flat to weakly concave. This means that the leaves are widest near the base and taper to a point. The leaves’ cell walls are firm to incrassate and sometimes porose. The reddish basal cells are rectangular. The distal cells are rhomboidal-hexagonal to hexagonal, with a length of 20–40 µm and a width of 12–20 µm. The leaves are bordered by 2–3 rows of narrow, thick-walled, usually red cells. The leaf margins are revolute (i.e. rolled downwards) from the base of the blade to the middle of the leaf or further. Near the apex, the leaf margins are denticulate (i.e. having fine teeth). The stout red-brown costa, or central stalk of the leaf, is percurrent to slightly excurrent, meaning it extends beyond the end of the apex, ending in a smooth awn. The leaf base is narrow and strongly decurrent, extending far down the stem.

Some populations in the Northern Hemisphere can reproduce asexually by means of brown gemmae in the leaf axils. The plants are dioicous, having male and female sexual organs on separate plants. The seta, or capsule stalk, is usually 1 – long, though infrequently as long as 4 cm. The seta is red, becoming browner with age. The capsule, which contains the spores, is 3 – long and brown but with a yellow mouth. The capsule is thickened at its distal end, with its shape being variously described as “elongate-ovate” (i.e. having an elongated egg shape), “cylindrical to clavate” (i.e. club-shaped), and “broadly pyriform” (i.e. pear-shaped). It is either pendulous or inclined. The diplolepidous peristome is composed of two sets of teeth, an inner endostome and an outer exostome, which ring the mouth of the capsule. The exostome teeth are orange- or yellow-brown to pale yellow and papillose at the tips. The yellowish hyaline (i.e. translucent) endostome teeth are lightly papillose, have broad perforations, and have a membrane at the base that is half the height of the exostome teeth. Attached to the endostome cells are 2–3 long, filiform (i.e. threadlike), appendiculate cilia. The operculum (the cap of the capsule) is 0.5 – long and conic-apiculate, ending in a sharp point. The pale yellow or green spores measure 12–18 μm in diameter and are finely papillose. The capsules mature in the summer or fall, in the Northern Hemisphere from April to October.

Taxonomy
Bryum pseudotriquetrum is a synonym of Ptychostomum pseudotriquetrum.

Ptychostomum bimum has historically been treated as a synoicous form of Ptychostomum pseudotriquetrum. P. bimum has twice as many chromosomes as and probably evolved from P. pseudotriquetrum As of June 2024, World Flora Online treats the two species as separate. Compared to P. pseudotriquetrum, P. bimum has a shorter stem and significantly shorter leaf decurrencies.

Distribution and habitat
P. pseudotriquetrum has a bipolar distribution and is additionally found in temperate regions in the Southern Hemisphere. It is found on all continents, including Antarctica, but is largely absent from the tropics, subtropics, and islands in the central Pacific Ocean. It grows on rocks and wet soil by streams and lakes and in fens, wet heaths, and marshes. at elevations of 0–4000 m.