BFNA Title: Rhynchostegium
Author: M. S. Ignatov
Date: September 27, 2009
Edit Level: R
Version: 1

Bryophyte Flora of North America, Provisional Publication
Missouri Botanical Garden
BFNA Web site: http://www.mobot.org/plantscience/BFNA/bfnamenu.htm

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XX. Rhynchostegium Bruch, Schimper & W. Gümbel, Bryol. Eur. 5: 197. 1852, name proposed for conservation * [Greek rhynchos, rostrum, and stegos, operculum, alluding to rostrate operculum]

Michael S. Ignatov

Platyhypnidium M. Fleischer; Steerecleus H. Robinson

Plants medium-sized to robust, in loose tufts or extensive mats, deep green, becoming brownish or light green to whitish green with age. Stems prostrate, with central strand, terete or moderately densely to subcomplanately foliate, irregularly branched, branches straight, foliage similar to that of stem; axillary hairs 4--7-celled, often brownish throughout; juvenile branch leaves acute or acuminate. Stem leaves imbricate to somewhat spaced, erectopatent or erect and gradually reflexed from an erect base and often twisted in the middle, broadly ovate or suborbicular to ovate-lanceolate, acuminate or ± broadly acute to obtuse, rounded to the base, lacking decurrencies after the leaf is detached, smooth; margin serrate to serrulate, costa reaching 0.5--0.9\x leaf length, ending in an abaxial spine or spine lacking; cells of leaf base somewhat shorter, poorly differentiated in leaf corners, or short-rectangular, thin-walled, indistinctly delimited from neighboring cells; mid leaf cells linear, moderately thick-walled. Branch leaves smaller and sometimes narrower than stem leaves, more strongly serrate, otherwise similar. Sexual condition autoicous, perichaetial leaves abruptly contracted into straight to reflexed acumens. Seta red-brown, smooth. Capsule red-brown to brown, inclined to horizontal; oblong-cylindric, annulus separating, formed by cells with narrow lumen; operculum rostrate; peristome xerocastique, perfect. Calyptra naked. Spores 9--16 \um.

Species 30--60 (2 in the flora): widespread in tropical to north temperate regions on all continents, with a few boreal taxa.

In the present circumscription, Rhynchostegium is one of the largest genera in the Brachytheciaceae, and includes terrestrial, aquatic, and epiphytic groups. Aquatic plants of Rhynchostegium were often segregated as a separate genus Platyhypnidium, which was originally placed in the Amblystegiaceae by M. Fleischer (1923) and later V. F. Brotherus (1925). The latter genus was subsequently accepted by many bryologists, although some authors (e. g. J. Podpěra 1954; N. Takaki 1956; H. Robinson 1962) included it in Rhynchostegium. Phylogenetic analyses of S. Huttunen et al. (2007) and J. Wynns (2006---unpublished M.A. thesis) found that aquatic lineages within Rhynchostegium are polyphyletic; thus the best solution will be the acceptance of Rhynchostegium in a broad sense. K. D. McFarland (1994) expanded Rhynchostegium to include some groups that are here placed with Eurhynchiastrum and Oxyrrhynchium. H. Robinson (1967) correctly recognized the tropical species of the genus as rather distinct from the European R. confertum, the type species of the genus, and segregated tropical taxa into the genus Steerecleus. However, the broader analysis of S. Huttunen et al. (2007) demonstrated that Rhynchostegium is very heterogeneous, but at the same time also very homoplasious, so its subdivisions and acceptance of Steerecleus would cause severe splitting of the genus and enormous nomenclatural problems.

1. Plants terrestrial; leaves long-acute to acuminate . . 1. Rhynchostegium serrulatum

1. Plants aquatic; leaves short-acute to obtuse . . 2. Rhynchostegium aquaticum

1. Rhynchostegium serrulatum (Hedwig) A. Jaeger, Ber. Thätigk. St. Gallischen Naturwiss. Ges. 1876--77: 370 (Sp. Musc. 2). 1878

Hypnum serrulatum Hedwig, Sp. Musc. Frond., 238. 1801; Steerecleus serrulatum (Hedwig) H. Robinson; Brachythecium serrulatum (Hedwig) H. Robinson; Eurhynchium serrulatum (Hedwig) Kindberg

Plants medium-sized to rather robust, in loose tufts, light green to whitish green. Stems to 5 cm, flexuose, terete, occasionally sub-complanately foliate, irregularly pinnate, branches to 7 mm, straight to flexuose, subcomplanately to complanately or, rarely, terete foliate; axillary hairs 3--5-celled. Stem leaves rather densely arranged, erect spreading or erectopatent, 1.3--2 x 0.6--0.8 mm, ovate-lanceolate, gradually acuminate, rounded to base, smooth or only slightly plicate, margin serrulate to serrate, plane distally, recurved at base; costa reaching 0.4--0.7\x the leaf length, lacking a terminal abaxial spine; basal cells in 1--2(--3) rows, short-rectangular, 30--50 x 11--15 \um, moderately thick-walled, not to slightly porose, indistinctly differentiated or sometimes forming a conspicuous pellucid belt across the leaf base; cells adjacent to decurrencies undifferentiated or 1--4 differentiated, or, rarely, slightly larger cells forming an indistinctly differentiated group not reaching the leaf margin, which is always formed by long, narrow cells; laminal cells (50--)80--120(--160) x 7--9(--10) \um, linear-flexuose, quite constant in their shape and size. Seta 20--30 mm. Capsule 1.7--2 mm; operculum with long narrow beak. Spores 9--11 \um.

Soil in forests, especially hardwood, also rotten logs, tree bases, rocks; occasionally in grasslands; 0--2070 m; Ont., Que.; 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.Mex., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.; Mexico; Central America; South America.

Rhynchostegium serrulatum is common in the East. The most western of known localities for the species are in New Mexico, Dona Ana Co., W side of Organ Mountains, at 1980 and 2070 m elevevation. This species can be recognized in the field by the light green color and somewhat complanate foliage. The long leaves with long mid leaf cells allows rather easy separation from Oxyrrhynchium hians. Sterile collections are sometimes confused with Sciuro-hypnum curtum, but that species usually has conspicuous groups of enlarged cells adjacent to decurrencies, whereas in R. serrulatum leaf cells are rather homogeneous across the whole base---they are somewhat enlarged, and have a specific appearance rather characteristic for this genus. This pattern of leaf base areolation distinguishes it from Brachythecium asperrimum, which is sometimes superficially similar; this Pacific coast species has a small but rather well-differentiated alar group of pellucid cells; in addition, axillary hairs of R. serrulatum are 3--5-celled, often brownish, while in B. asperrimum they are 2--3 celled, short and pellucid. Rhynchostegium scariosum from Mexico differs mainly in having small plants with stem leaves 0.9--1.3 mm long. It might be expected in the southeastern United States.

2. Rhynchostegium aquaticum A. Jaeger, Beri. Thätigk. St. Gallischen Naturwiss. Ges. 1876--77: 378. 1878

Platyhypnidium aquaticum (A. Jaeger) M. Fleischer; Rhynchostegium subrusciforme (Müller Hal.) A. Jaeger; R. obtusifolium (Mitten) A. Jaeger

Plants robust, in loose tufts or extensive mats, deep green, becoming brownish with age. Stems moderately densely terete foliate, irregularly branch foliage similar to that of stem; axillary hairs 4--7-celled. Stem leaves erect or gradually reflexed from an erect base, often twisted in the middle, broadly ovate or suborbicular to ovate-lanceolate, ± broadly acute, with acute to blunt apices, smooth, rarely indistinctly plicate; margin serrate, costa reaching 0.7--0.85\x the leaf length, ending in an inconspicuous abaxial spine or spine occasionally lacking; basal cells shorter in 1--2(--3) rows, indistinctly delimited from adjacent cells; cells adjacent to decurrency thin-walled and somewhat enlarged to 12--15 \um wide, usually forming a small indistinct group; mid leaf cells linear, 40--90 x 5--7 \um, moderately thick-walled. Seta 10--23 mm. Capsule 1--2 mm; operculum rostrate, with relatively stout beak. Spores 13--16 \um.

Rocks, running water of small streams and springs, beds of waterfalls, seepy cliffs, especially in limestone areas; 0--2000 m; B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., Que.; Ala., Ariz., Ark., Calif., Conn., Del., Ga., Idaho, Ill., Ind., Ky., Maine, Md., Mass., Mich., Minn., Mo., Mont., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Oreg., Pa., S.C., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis.; Mexico; Central America; South America; e Asia; Pacific Islands; Australia.

For a long period, all North American Rhynchostegium species were attributed to R. riparioides. This is not surprising considering a huge variation of this species, as well as in most of aquatic mosses. The molecular phylogenetic analysis of S. Huttunen et al. (2007) demonstrated that the European-African populations and the American-Asian-Australian populations represent two independent groups of different origins. The latter group is probably somewhat heterogeneous, but until a thorough revision of the main haplotypes is completed, a broad circumscription of Rhynchostegium aquaticum is preferable. This is a widespread moss in North America, absent only in northern boreal and Arctic regions. As the diversity of mosses growing in and beside running water is limited, R. aquaticum is usually an easily recognizable species in the field. Some species of Hygrohypnum, e.g., H. duriusculum, are rather similar, but their double costa distinguishes them. Oxyrrhynchium hians sometimes is similar, but the consistently spinulose abaxial end of the costa in branch leaves is a reliable character of that species, while the costa is smooth or ends in a small, inconspicuous projection in R. aquaticum; in addition, R. aquaticum is autoicous and often has setae that are smooth, not rough as in Oxyrrhynchium. Donrichardsia pringlei is also similar, differing in smaller plant size, and a more stout costa reaching 0.85--0.95 the leaf length, but shorter in R. aquaticum. Leaves of R. aquaticum are characteristically obtuse, this better observed in young leaves.

OTHER REFERENCES

Brotherus, V. F. 1925. Musci. In: A. Engler and K. Prantl, eds. Die Natürlichen Pflanzenfamilien, ed. 2. 11: 1--522. Leipzig,

Fleischer, M. 1923. Die Musci der Flora von Buitenzorg 4: 1104--1729. Leiden.

Huttunen, S., A. A. Gardiner & M. S. Ignatov. 2007. Advances in the knowledge of the Brachytheciaceae (Bryophyta). In: A. E. Newton & R. S. Tangney, eds. Pleurocarpous Mosses: Systematics and Evolution. Systematics Association Special Volume 71: 117--143.

McFarland, K. D. 1994. Rhynchostegium. In: A. J. Sharp, H. A. Crum and P. M. Eckel, eds. Moss Flora of Mexico. Mem. New York Bot. Gard. 69: 932--941.

Podpěra, J. 1954. Conspectus Muscorum Europaeorum. Nakladatelstvi Ceskoslovenske Akademie Ved.

Robinson, H. 1962 [1963]. Generic revisions of North American Brachytheciaceae. Bryologist 65: 73--146.

Robinson, H. 1987. Notes on generic concepts in the Brachytheciaceae and the new genus Steerecleus. Mem. New York Bot. Gard. 45: 678--681.

Takaki, N. 1956. Researches on the Brachytheciaceae of Japan and its adjacent areas III. J. Hattori Bot. Lab. 16: 1--71.