Didymoglossum beaverianum 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 : 206-209

Didymoglossum beaverianum Senterre & Rouhan , sp. nov., Fig. 2 & 3

Type:— SEYCHELLES. Mahé: Varigault, pentes vers Montagne Posée, - 4.70055°S, 55.50112°E, 372 m, 27/04/2013, B.Senterre & E.Henriette 6577 (holotype P!-02434054, isotype SEY!).

Diagnosis: —Morphologically similar to Didymoglossum beccarianum (Cesati) Senterre & Rouhan from which it differs in having sterile leaves with elongated mid-vein, always more than half the length of the lamina and fertile leaves with a shallow apical notch less than half the length of the sorus. Its ecology is also distinct, being exclusively rupicolous while D. beccarianum is almost always corticolous (observed only twice on rocks, in Seychelles).

Rupicolous herbaceous ferns, forming mats on rocks of understorey stream sides. Rhizomes creeping, filiform, 0.15–0.20 mm in diameter (up to 0.5–0.6 mm with the hairs), abundantly and irregularly branched in all directions, tortuous (especially obvious in young developing parts), hirsutulous, hairs persistent, simple, linear, unicellular, 0.40–0.80 × 0.02 mm, black. Roots absent. Leaves closely set along the rhizome but not overlapping, 0.15–0.25 cm apart, alternate, inclined (not appressed to the substrate), dimorphic (the fertile leaves smaller but with longer petioles), 0.25–0.60 cm (including the petiole), not proliferous. Petioles not inserted on a pulvinus, not articulate, (0.01–)0.04–0.14(–0.16) cm (generally shorter in sterile leaves (0.4–0.7 mm), longer in fertile ones (0.7–1.4 mm )), ca. 0.12–0.15 mm broad, not winged, blackish, moderately to densely pubescent (with same hairs as on the rhizomes). Laminae entire, ovate, elliptic, oblong, or obovate (mostly ovate to elliptic-oblong, never orbicular in sterile leaves, elliptic to obovate in fertile leaves, with rarely some fertile leaves much reduced), 1.5–2.5(–6.0) times longer than petioles (in fertile leaves), to (2–)5–10 times longer than petioles (in sterile leaves), 0.26–0.50 cm (in sterile leaves), or (0.15–) 0.20–0.40 cm (in fertile leaves), 0.1–0.2 cm wide, much longer than broad (occasionally almost as long as broad in some fertile leaves), flat, unbent to slightly bent upwardly; bases of both fertile and sterile leaves attenuate, or rounded, symmetrical to slightly asymmetrical, not decurrent on the petioles; 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 shallow apical notch (in fertile leaves), apical lobes of fertile leaves always shorter than half the length of the sorus, (0.1–)0.3–0.5(–0.9) mm (exceptionally up to 0.9 mm, observed in a leaf where the sorus was 2.2 mm, i.e. apical lobes still less than half the sorus length); venation simple, one-veined, the mid-vein reaching always more than halfway and evanescent towards apex (in sterile leaves), mid-vein reaching the apical notch (in fertile leaves), secondary veins absent, false veins present, pinnate, ascendant, 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, 10–23 pairs per lamina (5–7 false vein-endings per mm at lamina margin), 3–5(–9) rows of cells between false veins, infra-marginal false vein absent, drying folds absent; lamina cells rectangular, tetragonal, or hexagonal, ca. 40–50 × 40 μm, cell walls thick, straight; laminae concolorous, typically dark olive, firm in texture, 1 cell thick, sparsely pubescent (with same hairs as on the rhizome) towards base of mid-vein and first pairs of false veins in a minority of leaves, otherwise most often glabrous. Sori on specialized leaves, solitary in the shallow apical notch, completely exserted, sometimes shortly pedicellate, bending upwards outside of the plane of the lamina (when mature); indusia conical-tubular, bordered by a margin of 1–3 rows of cells on each side (observable when dry but easier to observe on rehumidified specimen), 1.1–1.7(–2.2) × 0.4–0.7(–1) mm, longer than wide to twice as long as wide, apices bilabiate (in mature sori), or enlarged, collar-like (in developing sori), mouth not abruptly enlarged, margins entire, lobes in dorsiventral position, semi-circular, 0.3–0.4 × 0.9–1.0 mm, slightly curved (in mature sori) to ascendant (in developing sori), apices rounded, without lateral veinlets, glabrous; receptacles cylindrical, bristle-like, extruded ca. 0.8–1.3 mm beyond the mouth. Sporangia incorporating a conspicuous equatorial annulus of thick-walled cells. Spores yellow. Gametophyte unknown.

Specimens examined (paratypes): — SEYCHELLES. Mahé: Castor (Nord du Chemin Montagne Posée), en allant vers Roche Pilon, 310 m, 25/04/2011, B.Senterre & L.Renguet 6055 ( P, SEY) ; La Drisse, 459 m, 15/03/2016, B.Senterre & L.Chong-Seng 7135 ( SEY) ; La Gogue Réservoir, à l’ouest du sentier allant à Whenshecomes, 385 m, 25/04/2013, B.Senterre 6576 ( SEY) ; Mont du Nord, en venant des hauts de North East Point , 331 m, 11/12/2012, B.Senterre & E.Henriette 6390 ( P, SEY) ; 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. 6043,1 ( P, SEY) ; Morne Blanc, dans un ravin au Nord-Ouest du sommet, 370 m, 24/10/2010, B.Senterre et al. 5904 ( P, SEY) ; Salazie, environ 1 km au sud-est de la route de Sans Souci-Forêt Noire, sur la piste de Salazie, 270 m, 19/06/2011, B.Senterre & L.Chong-Seng 6130 ( P, SEY) ; Trois Frères Nord, pentes descendant vers Le Niol, 450 m, 04/03/2011, B.Senterre & E.Talma 5944 ( P, SEY) ; 385 m, B.Senterre & E.Talma 5946 ( SEY) ; Varigault, 575 m, 10/08/2011, B.Senterre & I.Fabre 6170 ( SEY) . Praslin: Glacis Noir, sur les pentes Sud, dans le ravin principal séparant Glacis Noir de Fond Azore , 350 m, 18/12/2012, B.Senterre & C.Morel 6406 ( SEY) ; Rivière Anse Kerlan, en descendant de Grand Fond (= Upper Zimbabwe ), 241 m, 19/12/2012, B.Senterre & C.Morel 6413 ( SEY) . Silhouette: Grande Rivière, sentier montant vers la forêt de Koko-d-mer, 188 m, 23/05/2015, B.Senterre & E.Henriette 7123 ( P, SEY) ; Jardin Marron, pentes S-O, vers Grand Barbe, près de la crête, 450 m, 20/11/2010, B.Senterre et al. 5908 ( P, SEY) .

Distribution: —Endemic to Seychelles; found on the three main islands: Mahé, Praslin and Silhouette ( Fig. 4 ). The species is relatively common on Mahé and Silhouette, but rare on Praslin where it is known from only two sites.

Ecology: —Narrow ecological range, being restricted to submontane ravines in tropical rain forests (see Senterre & Wagner 2014); it is found typically between 300 and 500 m elevation, although it can also be observed at lower elevations, as low as about 100 m, but there it is less abundant and occurs closer to streams.

Etymology: —The species epithet honours Katy Beaver, who has dedicated her life to the conservation and study of plants in Seychelles, including small ferns and mosses. She played an important educational role in popularizing the results of scientific research (e.g. in the journal Kapisen) or in developing educational material for children and young researchers.

Vernacular name: —We propose to name this fern ‘Fouzer Kati’ in Creole, meaning ‘Katy’s fern’, for the same reasons given above.

Morphological similarities: —Among the species having been included in the Didymoglossum motleyi complex (see previous section), only two species have been described with mid-veins extending more than half the length of adult sterile leaves, i.e. Trichomanes minutissimum (from Ambon, Indonesia) and T. cultratum (from Fiji). Nevertheless, we agree with the detailed revision of Copeland (1933) and consider T. minutissimum as conspecific with T. beccarianum . The shape of leaves described in the diagnosis of T. minutissimum corresponds typically to the variability of T. beccarianum and we believe that the supposedly elongated mid-veins of T. minutissimum (“ costa apicem versus sensim evanescente ”) are either a misinterpretation of false veins in the prolongation of the true costa or an observation made on fertile leaves at an early developmental stage (see Copeland 1933: plate 29, 4). Didymoglossum beccarianum , as circumscribed here and discussed hereafter, differs from D. beaverianum in having the mid-vein of sterile leaves not elongated towards the apex and the apical notch of fertile leaves deeper (more than half the length of the sorus). On the other hand, based on Copeland’s drawings (1933) and the original description, T. cultratum seemed to be morphologically more similar to Didymoglossum beaverianum . Nevertheless, after reviewing the high resolution scan of the type, it appears that the mid-vein of sterile leaves is typically of the T. beccarianum type (mid-way to the apex, not further) and the apical notch in fertile leaves is deep rather than shallow. For these reasons, Trichomanes cultratum is clearly distinct from D. beaverianum .

Figures

FIGURE 2. Illustration of Didymoglossum beaverianum (Fouzer Kati). A. Micro-habitat, rupicolous in wet ravines; B. Mat, showing the inclined position of leaves, not appressed on the substrate; C. Tortuous rhizome irregularly branching; D. Shape of fertile and sterile leaves; E. Developing sori in shallow apical notches; F. Developing sorus (early stage); G. Developing sorus (intermediate stage); H. Winged margins of the sorus; I. Rhizome showing unicellular hairs.

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 4. Distribution map for Didymoglossum beccarianum and D. beaverianum in Seychelles.