Didymoglossum beccarianum (Cesati) Senterre & Rouhan, 2017

Senterre, Bruno, Rouhan, Germinal, Morel, Charles & Dubuisson, Jean-Yves, 2017, New species and records in the fern genus Didymoglossum for the flora of Seychelles, with notes on the Southeast Asian D. motleyi complex (Hymenophyllaceae), Phytotaxa 292 (3), pp. 201-217 : 210-211

Didymoglossum beccarianum (Cesati) Senterre & Rouhan , comb. nov., Fig. 3 & 5

Basionym:— Trichomanes beccarianum Cesati (1876: 8) . Type:— MALAYSIA. Sarawak: Beccari s.n. (holotype: FI!-013661, isotypes: K!-000362070, K!-000362071, BM!-001019579).

Homotypic synonym:— Microgonium beccarianum (Cesati) Copeland (1938: 63) .

Heterotypic synonyms:— Trichomanes cognatum Cesati (1877: 24 , 28), nom. illeg. Trichomanes motleyi var. cognatum (Cesati) Christensen (1906: 637) . Type:— PAPUA NEW GUINEA. East Sepik Province: Andai, Terra d. Papuas, Beccari s.n. (FI!-013662). Trichomanes sayeri Baker (1891: 195) . Type:— AUSTRALIA. Queensland: Trinity Bay, W. Sayer s.n. (holotype: BRI!-AQ0024778, isotypes: BM!-001067987, K!-001090237, MEL!-19411). Trichomanes minutissimum van Alderwerelt van Rosenburgh (1916: 102 pl. 5 f. 1). Type:— INDONESIA. Maluku Province, Ambon [Amboina], Soja, Robinson 1944 p.p. (holotype?: BO, isotype: L).

Corticolous herbaceous ferns, forming mats at the base of tree trunks. Rhizomes creeping, slender, 0.18–0.25 mm in diameter (0.70–1.00 mm in diameter including the coat of hairs), abundantly and regularly branched, most branches at a 45° angle, alternate (visible at extremities of the mat), moderately to densely hirsutulous, hairs curled or bent distally, persistent, simple, linear, unicellular, 0.5–0.8 × 0.02 mm, black (brown on younger parts). Roots absent. Leaves closely set along the rhizome but not overlapping, 0.15–0.26 cm apart, inclined (not appressed to the substrate or at most appressed on the basal third), dimorphic, 0.3–0.5(–0.6) cm (including the petiole), not proliferous. Petioles not inserted on a pulvinus, not articulate, 0.00–0.10(–0.14) cm (0.00 mm in sterile leaves, 0.05–0.10(–0.14) cm in fertile leaves), ca. 0.2 mm broad, not winged, black, moderately to densely pubescent (same hairs as on the rhizome). Laminae entire, orbicular, obovate or oblanceolate (orbicular in sterile leaves, oblanceolate to obovate in fertile leaves), (2–)3–5 times as long as the petiole (in fertile leaves), (0.22–)0.30–0.50 × (0.20–) 0.24–0.40 cm, as long as broad or up to 1.3 longer than broad, flat, unbent to slightly bent upwardly; bases rounded to cordate in sterile leaves, or attenuate in fertile leaves (rarely cordate-truncate on some fertile leaves), symmetrical, not decurrent on the petiole (in sterile leaves) or decurrent (in some fertile leaves); margins entire, not sclerose, with 1 row of specialized marginal cells (with thicker cell walls, not elongated as compared to non-specialized lamina cells but larger), glabrous (rarely some leaves with marginal hairs similar to rhizome hairs towards base), flat (not crisped); apices rounded (in sterile leaves), or with a deep apical notch (in fertile leaves), apical lobes of fertile leaves longer than half the length of the sorus, (0.45–) 1.20– 1.70 mm; venation simple, one-veined, the mid-vein distinct to a third up to halfway to apex (in sterile leaves), 0.6–1.4 mm, mid-vein reaching the apical notch (in fertile leaves), secondary veins absent, false veins present, radiating from the mid-vein, almost always reaching the margin (rarely a few false veins are free basally or distally, or reduced to segments in only a few leaves), straight, never branching, 5–10(–16) pairs per lamina (4–5 false vein-endings per mm at lamina margin), 5–11 rows of cells between false veins, infra-marginal false vein absent, drying folds absent; lamina cells rectangular, tetragonal or hexagonal, ca. 30–50 × 30 μm, 1.0–1.6 times as long as broad, cell walls thick, straight; laminae concolorous, pale to middle green, membranous, 1 cell thick, densely pubescent (with same hairs as on the rhizome) below the mid-vein, and below the false veins (mostly near the base but some to close to the margin, especially in basal false veins), or glabrescent on older leaves. Sori on specialized leaves, solitary in the deep apical notch, completely exserted, sometimes shortly pedicellate, bending upwards outside of the plane of the lamina (when mature); indusia conical-tubular, with a dark vein running on each lateral side, not bordered by a row of cells (or rarely so), reaching the mouth base, (1.1–)1.4–1.75(–2) × 0.4–0.7(–1) mm, slightly longer than wide to twice as long as wide, apex distinctly bilabiate (in mature sori), or enlarged (in developing sori), mouth much enlarged 0.9–1.2(–1.5) mm wide, margin entire, lobes in dorsiventral position, semi-circular, 0.4–0.5(–0.8) × 0.9–1.2(–1.5) mm, strongly curved (in mature sori) to ascendant (in developing sori), apex rounded, without lateral veinlets, glabrous; receptacle cylindrical, bristle-like, extruded ca. 0.5 mm beyond the mouth, totally covered by sporangia. Sporangia ca. 20–25 per sorus, extruding, incorporating a conspicuous equatorial annulus of thick-walled cells. Spores yellow. Gametophyte unknown.

Specimens examined: — SEYCHELLES. Mahé: Mont Le Niol (= Mount Simpson), dans la vallée séparant le Mont Le Niol du Mont Cotton , 250 m, 13/04/2011, B.Senterre et al. 6043,2 ( P, SEY) ; Montagne Planneau (= Mont Harrison), sur les pentes vers Cascade, 460 m, 11/10/2010, B.Senterre et al. 5899 ( P, SEY) ; Morne Blanc, dans un ravin au Nord-Ouest du sommet, 270 m, 24/10/2010, B.Senterre et al. 5901 ( P, SEY) ; 300 m, B.Senterre et al. 5903 ( P, SEY) ; Roche Pilon, en remontant vers le Nord, vers Montagne Planneau, 380 m, 25/04/2011, B.Senterre & L.Renguet 6064 ( P, SEY) ; Varigault, pentes vers Montagne Posée, 372 m, 27/04/2013, B.Senterre & E.Henriette 6578 ( P, SEY) .

Praslin: Vallée De Mai , dans un ravin montant vers Mont Takamaka, 234 m, 20/12/2012, B.Senterre & C.Morel 6415 ( P, SEY). Silhouette: Grande Rivière, sentier montant vers la forêt de Koko-d-mer, 315 m, 23/05/2015, B.Senterre & E.Henriette 7124 ( SEY) ; Rivière Quatre Cent, dans les hauts, en remontant vers Mont Plaisir, 410 m, 23/07/2010, B. Senterre et al. 5864 ( P, SEY) ; 375 m, B. Senterre et al. 5865 ( P, SEY) ; Silhouette, 355 m, B. Senterre et al. 5866 ( P, SEY) ; pentes S-O de Jardin Marron, dans la vallée de la rivière Quatre Cent, 350 m, 20/11/2010, B. Senterre et al. 5909 ( P, SEY) ; 350 m, B. Senterre et al. 5910 ( P, SEY) ; 350 m, B. Senterre et al. 5911 ( P, SEY) ; 300 m, B. Senterre et al. 5913 ( P, SEY) .

Distribution: —This is a new record for the flora of Seychelles, where it is rarely found on Mahé, Praslin and Silhouette. Widely distributed from Seychelles to Sri Lanka, the Andaman islands ( Chandra et al. 2008), Christmas Island, Burma, Thailand, southern China, the Ryukyu Islands, Taiwan, the Philippines, Peninsular Malaysia, Java, Borneo ( Sarawak), Indonesia ( Maluku Islands), New Guinea, the Solomon islands, Queensland, and the Caroline Islands ( Fosberg 1950; Fosberg & Sachet 1981).

Ecology: —The ecological range in Seychelles is narrow, restricted to submontane ravines of tropical rain forests. It is found typically between 300 and 500 m elevation and does not transgress towards lower elevations as much as D. beaverianum (lowest observation is 234 m).

Vernacular name: —We propose to name this fern ‘Pti fouzer ron’ in Creole, meaning ‘small round fern’, as a reminder of the main diagnostic character (small orbicular sterile leaves).

Morphological similarities: —This species has been largely confused with Didymoglossum motleyi , mostly due to inappropriate consideration of diagnostic characters related to leaf dimorphism (fertile vs. sterile). The most detailed revision of the group of species generally included in the D. motleyi complex has been done by Copeland (1933), who considered D. motleyi as a rare species known from just one or two specimens. After careful revision of reference specimens and specimens from Seychelles, we fully agree with Copeland’s treatment. Didymoglossum beccarianum differs from D. motleyi in having the mid-vein of sterile leaves a third to half the length of the lamina (never constantly almost null), the petioles of fertile leaves are elongate and slender at least in some leaves (never constantly short and stout), and the base of fertile leaves are only rarely cordate-truncate on some leaves (never constantly cordate). In addition, D. beccarianum has leaves that are appressed to the substrate only towards the base, while D. motleyi is almost entirely appressed to the substrate (as in D. tahitense and D. hildebrandtii ). Finally, the false veins of sterile leaves are radiating from the base and are branched distally in D. motleyi (according to Copeland 1933), a character also shared with D. tahitense and D. hildebrandtii . In contrast the false veins of D. beccarianum are always simple.

Figures

FIGURE 3. Illustration of the range of within specimen variability in the lamina shape and petiole length and pubescence in fertile leaves of the Seychellois population of Didymoglossum beccarianum (A, B, C) and D. beaverianum (D, E, F).

FIGURE 5. Illustration of Didymoglossum beccarianum (Pti fouzer ron). A. Micro-habitat, corticolous at the base of endemic palm Phoenicophorium borsigianum; B. Mat, showing leaves slightly appressed to the substrate; C. Rhizome showing regular branching; D. Shape of fertile and sterile leaves; E. Developing sorus in deep apical notch; F. Un-winged margins of the sorus; G. Leaf base showing unicellular hairs below the false veins.