Plagiothecium laetum Bruch, Schimp. & W. Gümbel, Bryol. Eur.

Ignatova, E. A., Fedorova, A. V., Kuznetsova, O. I. & Ignatov, M. S., 2019, Taxonomy of the Plagiothecium laetum complex (Plagiotheciaceae, Bryophyta) in Russia, Arctoa 28 (1), pp. 28-45 : 38

Plagiothecium laetum Bruch, Schimp. & W. Gümbel, Bryol. Eur. 5: 185. 495. 1851. Fig. 8 .

Description. Plants medium-sized, in rather dense, soft, lustrous mats. Stems creeping, 1–3 cm long, foliage complanate. Leaves forming 20–70° angle with stem, (1.1–)1.3–1.7 × 0.5–0.7 mm, ovate-oblong, more or less symmetric to slightly asymmetric, slightly concave, tapered to acute apex or shortly apiculate, decurrent in 2– 4(–5) rows of rectangular cells; leaf margins entire, narrowly recurved almost throughout or at lower 3/4, in more asymmetric leaves occasionally recurved on one side; costa short and double; laminal cells 70–140 × 6–8 µm. Gemmae rarely present, of 3–4 short rectangular cells. Autoicous. Sporophytes usually present. Setae 10–15 mm long; capsules erect, straight or inclined, 1.0– 1.5 mm long. Exostome teeth 400 µm long; endostome basal membrane 130 µm high, segments slightly shorter than exostome teeth, cilia 1–2, short, nodose. Spores 11–13 µm.

Distribution and ecology. Contrary to previous publications, plants identical with Central European plants are very rare in Russia. Plagiothecium laetum s. str. was collected twice in the Caucasus, at low and middle elevations. One old specimen from Kaluga Province was also seen in the herbarium; it was collected on sandstone outcrops in a place where some rare species of mosses and vascular plants are found. Data on species ecology is scarce. According to the label information, it grows on tree bases and stumps in pine and broadleaved forests.

Differentiation. Plagiothecium laetum differs from P. rossicum in always narrowly recurved leaf margins, at least partially. Flattened foliage is helpful for separating it from P. curvifolium (see also discussion under that species). From P. svalbardense it differs in larger size of plants; slightly larger leaves (1.3–1.7 × 0.5–0.7 mm in P. laetum vs. 1.2–1.6 × 0.35–0.6 mm in P. svalbardense ); leaves acute or shortly apiculate vs. distinctly attenuate in the apical portion; less variable width of laminal cells (6–8 µm vs. 5–10 µm); and less branched stems on creeping shoots forming flat mats vs. stronger branched stems and loose mats. In addition, these two species have different distributions, never growing in same localities (see comments under both of them). In Eurasia, P. laetum grows in Europe, with a few localities in the Caucasus, while P. svalbardense is mainly a North Asian taxon, distributed in Europe only in its northern regions.

Selected specimens examined: EUROPEAN RUSSIA: Kaluga Province: Chertovo Gorodishche , 3.IX.1924, Zhadovsky s.n. ( MW9055355 ) . CAUCASUS: Republic of Ingushetia: Dzheirakh Distr., Nature Reserve “Erzi”, 42°49’57.2”N, 44°55’06.8”E, 22.IV.2018, Bersanova 18-159 ( MW9090994 ) ; Krasnodar Territory: Nature Reserve “Kavkazsky”, Babuk , Shakhe Creek valley , 43°50’N, 39°50’E, 23.VIII.1990, Ignatov s.n. ( MHA9041733 ) .

AUSTRIA: VII.1914, [coll. unclear] ( LE) . BOSNIA AND HERZEGOVINA: 10. VI .1979, Rusinska s.n. ( LE) . CZECH REPUBLIC: Karlovy Vary, 12.IV.1957, Tyuremnov s.n. ( MW9055294 , MW9055295 ) . FINLAND: 20.VII.1930, Railonsalo s.n. ( LE) . GERMANY: Lorenzer Wald bei Nürnberg , 24. VI .1921 , H. Paul s.n. ( MW9055300 ). HUNGARY: 14.VII.1955, Boros s.n. ( LE) . NORWAY: 25. VI .1972, Frisvoll s.n. ( LE) . POLAND: 20.VII.1972, Flora Silesia exs. 1171, Bedrowski s.n. ( LE) . SWEDEN: 29.VIII.1909, Möller s.n. ( LE) ; 9.VII.1904, Arnell s.n. ( LE) .

Figures

Fig. 8. Plagiothecium laetum (from: 1–3: Czech Republic, Karlovy Vary, 28.VII.2011, Donskov s.n., MHA9055918; 4–6: Russia, Krasnodar Territory, 23.VIII.1999, Ignatov s.n., MHA 9041733; 7–9: Germany, Mosses of Saarlandes 3103, 1.IX.1996, Caspari s.n., SAAR; 10–12: Germany, Mosses of Saarlandes 5382, 24.XII.1995, Caspari s.n., SAAR): 1–2, 4–5, 7–8, 10–11 – leaves; 3, 6. 9, 12 – midleaf cells. Scale bars: 1 mm for 1–2, 4–5, 7–8, 10–11; 100 µm for 3, 6. 9, 12.