BFNA Title: Platygyrium
Author: W. D. Reese
Date: April 2008
Edit Level: R
Version: 2a

Bryophyte Flora of North America, Provisional Publication

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XX. PLATYGYRIUM Schimper in P. Bruch, W. P. Schimper & W. T. Gümbel, Bryol. Eur. 5(46--47): 95. 1851, conserved name * [Greek platy, broad, and gyrus, a circle, alluding to the wide annulus]

William D. Reese

Plants glossy, small to medium sized, creeping, yellowish to brownish green or darker. Stems subpinnately branched; branches short to elongate, erect to prostrate, loosely foliated to julaceous, straight to curved at tips; pseudoparaphyllia filamentous or broader; axillary hairs 1--3 per axil, of 3--7 1-seriate cells, the 1--2 proximal cells short and with firm brown walls, the distal cells elongate and colorless. Leaves on stems and branches similar, ascending-imbricate to homomallous, smooth to somewhat plicate, ovate-acuminate, concave, often yellowish across base; margins entire to rarely slightly serrulate distally, somewhat to strongly recurved; apex acute to slenderly acuminate; costa short and double, commonly indistinct; medial cells elongate, smooth; alar cells conspicuously quadrate in vertical rows, conspicuously thick-walled. Specialized asexual reproduction by dehiscent branchlets at branch tips. Sexual condition dioicous. Perigonia gemmiform, axillary along stems. Perichaetia small, inconspicuous, axillary along stems. Seta straight, elongate, smooth, twisted when dry. Capsule erect, long-exserted, cylindric to subcylindric, symmetric or slightly asymmetric, exothecial cells oblong-rectangular, annulus sharply differentiated in 2--3 rows of cells, stomates phaneropore, scanty, proximal on capsule; operculum obliquely rostrate; peristome double, exostome pale yellow-orange, teeth incurved-connivent when dry, erect or incurved when wet, finely papillose to papillose-striolate on outer surface, transversely ridged on inner surface, conspicuously bordered, smooth at tips; endostome segments nearly as long as teeth, brown, narrowly linear, jointed, perforated or intact, sometimes adherent to columella or teeth; cilia lacking. Spores green. Calyptra naked.


Species ca. 8 (2 in the flora): North America, Europe, Asia, Africa.


Platygyrium is generally similar to Homomallium and Pylaisiella. Fruiting specimens of the three genera are easily distinguished: the capsules of Homomallium are curved and asymmetric, often strongly so, in contrast to the erect symmetric capsules of Platygyrium and Pylaisiella. The long, slender rather obliquely rostrate operculum of Platygyrium (especially conspicuous when dry) easily distinguishes it from Pylaisiella, in which the operculum is only conic or with a short blunt rostrum. Platygyrium and Pylaisiella commonly grow on xylic substrates but Homomallium typically occurs on rock. Dehiscent branchlets clustered at branch tips are very common in Platygyrium, particularly in P. repens, but such branchlets also occur in Homomallium, although infrequently. Also, the leaf margins of Platygyrium are usually recurved, sometimes strongly so, while those of Homomallium and Pylaisiella are erect. Recurvature of the leaf margins is not always well defined in P. repens but such is usually conspicuous in P. fuscoluteum.


SELECTED REFERENCES  Crum, H. A. and L. E. Anderson. 1981. Mosses of Eastern North America, vol. 2. New York. Sharp, A. J., H. A. Crum, and P. M. Eckel. 1994. The Moss Flora of Mexico, vol. 2. New York.


1. Leaves mostly 0.8--1.1 mm, ascending to imbricate, not plicate; branches often short, erect, straight or curved; brood branchlets usually common and conspicuous on branch tips         1. Platygyrium repens


1. Leaves mostly 1.3--1.6 mm, usually homomallous, often somewhat plicate; branches elongate, prostrate, straight; brood branchlets rare and inconspicuous on branch tips         2. Platygyrium fuscoluteum


1. Platygyrium repens (Bridel) Schimper in P. Bruch, W. P. Schimper & W. T. Gümbel, Bryol. Eur. 5(46--47): 95. 1851


Pterigynandrum repens Bridel, Musc. Recent. Suppl. 1: 131. 1806; Platygyrium orthoclados Kindberg; P. repens var. ascendens (Schwägrichen) Grout; P. repens var. orthoclados Kindberg


Plants green to bronze or darker, often blackish-green, slender to moderately robust. Branches commonly short but sometimes elongate, ascending, often curved at tips or straight, loosely foliated or sometimes julaceous; brood branchlets usually abundant and conspicuous on branch tips. Leaves ascending to imbricate, non-plicate, mostly 0.8--1.1 mm; margins in proximal half inconspicuously to strongly recurved; apex acute to acuminate; medial cells slenderly elongate, mostly 44--57 x 5--6 \um. Seta reddish brown, 12--20 mm. Capsule 1--2.5 mm, brown, subcylindric, broadest proximally, symmetric or slightly asymmetric, somewhat strumose when dry, operculum slenderly and mostly obliquely long-rostrate. Calyptra 1.5--2 mm. Spores 13--18 \um, finely granular.


Capsules mature August--April, June, perhaps all year. Shady, often moist forests, logs, stumps, tree trunks and bases, old wood structures, less commonly on shaded rock and soil; 0--1980 m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine,, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., W.Va., Wisc., Wyo.; Europe; Asia; Africa.


Platygyrium repens is frequent and common throughout most of its remarkably wide range in the flora area. It is one of the most ubiquitous pleurocarpous mosses in eastern North America, and in the north it ranges westward to British Columbia and Northwest Territories. It fruits rather infrequently in the northern reaches of its range, and not at all in the southern portions. Brood branches are usually present, and are rarely much elongate and in such abundance that the plants dissolve into seemingly amorphous masses of slender branchlets. Young colonies comprise straggly loosely foliated plants, sometimes Fabronia-like in appearance and quite different from the more compact mature forms with more appressed leaves. Compact plants with elongate turgid, julaceous branches occur sporadically throughout the range of P. repens, but most populations have loosely foliated branches. The julaceous morphology is most frequent on rock substrates. A common colony form is compact mats with numerous very short, erect branches bearing abundant brood branches at their tips. The characteristically bronzy dark-green or blackish green color makes the colonies recognizable from a distance. Although most specimens of P. repens can be identified easily and with confidence, occasional sterile specimens lacking brood branchlets may defy certain identification. Homomallium adnatum sometimes has brood branchlets at its branch tips, and some sterile specimens may not be confidently distinguished from sterile specimens of P. repens. However, in H. adnatum the medial leaf cells of are generally much shorter (rhombic or to 3--5:1) than those of P. repens, and foliose paraphyllia are sometimes present in the former but lacking in P. repens. Erect branches bearing brood branchlets at their tips are also common in Leskeella nervosa, a more slender dull, brownish plant with narrow costate leaves, which co-occurs with P. repens in the northern reaches of its range.


2. Platygyrium fuscoluteum Cardot, Rev. Bryol. 37: 49. 1910


Regmatodon fuscoluteus Schimper ex Bescherelle, Mem. Soc. Sci. Nat. Cherbourg 16: 232. 1872


Plants yellowish brown, relatively robust. Branches mostly elongate, prostrate, straight, loosely foliated, never julaceus; brood branchlets usually inconspicuous, on branch tips. Leaves usually homomallous, often somewhat plicate, mostly 1.3--1.6 mm; margins in proximal half usually conspicuously recurved; apex rather abruptly slenderly acuminate; medial cells slenderly elongate, mostly 50--65 x 5--6 \um. Seta reddish, 10--13 mm. Capsule 2--2.5 mm, brown, cylindric, nearly symmetric to distinctly asymmetric, somewhat strumose when dry, operculum slenderly and mostly obliquely long-rostrate. Calyptra 2--2.5 mm. Spores 18 \um, finely granular.


Capsules mature February. Montane forests on old logs, stumps, tree trunks, damp boulders, rock faces, and shaded banks, 1300--2744 m; Ariz., N.Mex., Tex.; Mexico.


The glossy yellowish-brown creeping plants bearing usually homomallous leaves with long slender apices on elongate branches distinguish Platygyrium fuscoluteum. Under the microscope the often somewhat plicate leaves with recurved margins and quadrate alar cells make the species easy to recognize. It is not sympatric with P. repens. Brood branchlets are much less conspicuous and much less common than in P. repens, but can often be found by careful search; they are rather effectively concealed in leaf axils at the branch tips. Homomallium mexicanum often co-occurs with P. fuscoluteum and could be mistaken for it. The capsule of the Homomallium is, however,  curved, and constricted below the mouth, and the leaves are imbricate (not homomallous), have erect margins, and are not plicate, and brood branchlets are lacking.