Hymenophyllum capillare Desv. f. nanum Hennequin & Dubuisson, 2018

Dubuisson, Jean-Yves, Pechon, Timothée Le, Bauret, Lucie, Rouhan, Germinal, Reeb, Catherine, Boucheron-Dubuisson, Elodie, Selosse, Marc-André, Chaussidon, Cécile, Dajoz, Isabelle, Pynee, Kersley, Grangaud, Edmond, Robert, Yannis, Tamon, Jean- Maurice & Hennequin, Sabine, 2018, Disentangling the diversity and taxonomy of Hymenophyllaceae (Hymenophyllales, Polypodiidae) in the Mascarene archipelago, with ecological implications, Phytotaxa 375 (1), pp. 1-58 : 32

19. Hymenophyllum capillare Desv. f. nanum Hennequin & Dubuisson , f. nov., Fig. 15C, D

Similar to typical Hymenophyllum capillare but with fertile fronds dwarfed, up to 6 cm long (generally less than 4 cm long), with rachis often partly to fully winged (vs. always wingless for the typical form, or only in its upper part).

Type:— LA RÉUNION. Forêt de Bon Accueil, 17 April 2016, J.-Y. Dubuisson & L. Bauret HR 2016-10 (holotype P, P00915544!).

Global distribution—Discovered in the wild on La Réunion in 2016, and represented at P by a few additional specimens from the island. Further studies are needed at other localities ( Mauritius, Madagascar and tropical Africa) to assess the distribution of this dwarf form outside of the island.

Specimens examined— LA RÉUNION. Mare Longue, April 2016, L. Bauret 191 ( P) ; Plaine des Palmistes , June 1957, J. Bosser 11600 ( P01328012 ) ; Mare Longue , October 1972, T. Cadet 3841 ( P01328011 ) .

Note:—In the literature (e.g., Tardieu-Blot 1951, 2008; Kornaś 1993), the range of fertile frond length for typical H. capillare varies from (6–) 7 cm to 40 cm long. We discovered on La Réunion in 2016 colonies of dwarf specimens with fertile fronds from 2.5 cm to 6 cm long, which were firstly suggested to belong to a new distinct species, especially by displaying in many cases winged rachis (vs. wingless in H. capillare ). The sterile fronds of juvenile H. capillare are actually similar with rachis more or less winged. Such atypical colonies seem to correspond to dwarf mature forms keeping a juvenile morphology (progenesis), and personal unpublished molecular data confirm their inclusion in H. capillare . Because this dwarfed form is found in sympatry in the same localities as the typical form, we decide here to propose a forma nova instead of a new variety, the latter one being rather used for designating distinct populations (e.g., as applied here for C. inopinatum var. tamonii and C. minutum var. mascarenense ). Ellison et al. (2014) strongly argued for the abandon of infrageneric taxa as varieties and forma in modern taxonomy in the context of stabilization of names. We support this point of view but keep here this new forma for the following reasons. The dwarf forms are morphologically clearly distinct from the typical specimens and we still have to check if they occur in all populations within the whole distribution of the species or are only restricted to some local populations and/or may be related/ adapted to some micro-habitats. If their presence appears actually normal and widespread, such dwarf forms would have to be finally integrated into the whole variability of a single taxon. If they represent particular, not frequent and likely local cases, their distinction could provide useful information in a context of conservation. For now, this form remains a hypothesis.

Figures

FIGURE 15. A. Hymenophyllum capillare Desv., typical form, as epiphyte (La Réunion, ‘Forêt de Bon Accueil, Les Makes’). B. H. capillare, typical form, detail of fronds. C. H. capillare f. nanum Hennequin & Dubuisson, f. nov., as epiphyte (La Réunion, ‘Pitons Mare à Boue’). D. H. capillare f. nanum, habit, type individual, scale = 0.5 cm (La Réunion, ‘Forêt de Bon Accueil, Les Makes’, J.-Y. Dubuisson & L. Bauret HR 2016-10, P). E. H. digitatum (Sw.) Forsberg, as epiphyte (La Réunion, ‘Forêt de Bon Accueil, Les Makes’). F. H. digitatum, detail of laminar margin showing minute dark trichomes. G. H. digitatum, detail of fronds and sori with tubular bases (s) (photographs.A–B: L. Bauret; C–G: J.-Y. Dubuisson).