Rhynchostegium serrulatum
Rhynchostegium aquaticum
leaves erect-spreading or erectopatent, ovate-lanceolate, often twisted mid leaf, 1.3–2 × 0.6–0.8 mm;
margins serrulate to serrate;
apex gradually acuminate;
costa to 40–70% leaf length, terminal abaxial spine absent;
alar cells undifferentiated, or 1–4 cells slightly differentiated, or, rarely, slightly larger cells forming indistinctly differentiated group not reaching margins;
laminal cells (50–)80–120(–160) × 7–9(–10) µm;
basal cells short-rectangular, region in 1–2(–3) rows, indistinctly differentiated or sometimes forming conspicuous pellucid belt across base.
leaves erect or gradually reflexed from erect base, broadly ovate or suborbicular to ovate-lanceolate, 1.5–2.5 × 0.9–1.8 mm;
margins serrate;
apex ± broadly acute, extreme apices acute to blunt;
costa to 70–85% leaf length, terminal abaxial spine inconspicuous, occasionally absent;
alar cells somewhat enlarged, to 12–15 µm wide, region indistinct;
laminal cells 40–90 × 5–7 µm;
basal cells shorter, region in 1–2(–3) rows, indistinctly delimited from adjacent cells.
2–3 cm.
1–2.3 cm.
1.7–2 mm;
operculum narrowly long-rostrate.
1–2 mm;
operculum stout-rostrate.
9–11 µm.
13–16 µm.
Rhynchostegium serrulatum
Rhynchostegium aquaticum
Rhynchostegium serrulatum can be recognized in the field by its light green color and somewhat complanate foliage. The long leaves with long laminal cells allow rather easy separation from Oxyrrhynchium hians. Sterile collections are sometimes confused with Sciuro-hypnum curtum, but that species usually has conspicuous groups of enlarged alar cells; in R. serrulatum laminal cells are rather homogeneous across the whole base as they are somewhat enlarged and have a specific appearance rather characteristic for this genus. This pattern of leaf base areolation distinguishes R. serrulatum 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, the axillary hairs of R. serrulatum are 3–5-celled, often brownish, while in B. asperrimum they are 2- or 3-celled, short, and pellucid. Rhynchostegium scariosum (Taylor) A. Jaeger from Mexico differs mainly due to its small plants with stem leaves 0.9–1.3 mm; it might be expected in the southeastern United States.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
North American aquatic species of Rhynchostegium were long attributed to R. riparioides (Hedwig) Cardot. The molecular phylogenetic analyses by S. Huttunen et al. (2006) and Huttunen and M. S. Ignatov (2010) demonstrated that European-African populations and American-Asian-Australian populations of R. riparioides represent two groups of differing origins. Rhynchostegium aquaticum is widespread in North America. Some species of Hygrohypnum, for example, 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. Donrichardsia pringlei is also similar, differing in smaller plant size, and stouter costa that reaches 0.85–0.95 leaf length, but is shorter in R. aquaticum. Leaves of R. aquaticum are usually characteristically obtuse, better observed in young leaves. The most reliable difference is the axillary hairs composed of short cells with length to width ratio 1.5–2.5:1 in Donrichardsia, while in Rhynchostegium cells of axillary hairs are long, with length to width ratio 3–6:1 (Huttunen and Ignatov).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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