Taxonomic studies of lycophytes and ferns from the Pan-Himalaya (I): Hymenophyllum (Hymenophyllaceae)

The present account deals with the filmy fern of genus Hymenophyllum from the Pan-Himalaya, as a precursor of the ongoing “Flora of Pan-Himalaya” project, which aims to update the taxonomy of the vascular plants from the Himalayas and adjacent mountainous regions. More than one hundred specimens of Hymenophyllum especially that grown in the Pan-Himalaya deposited in the China National Herbarium (PE) have been carefully examined, in comparison with the collections from the Himalayas kept in European herbaria (K, BM, and E). The morphology and anatomy of each specimen were studied and photographed. As a result, ten species were recognized and described in detail, key to species, photography illustrations of plant habit, leaf shape and margin, lamina cells and veinlets, and sori are provided to facilitate correct identification of plants. According to the most recent classification system of the filmy ferns, the genus Mecodium is no longer accepted, although the leaf margins of Mecodium are all entire. Hymenophyllum barbatum, H. denticulatum, H. exsertum, H. levingei, and H. simosianum belong to subgenus Hymenophyllum; H. corrugatum, H. longissimum, H. polyanthos belong to subgenus Mecodium; H. badium and H. javanicum belong to subgenus Globosa. It is noticed that Hymenophyllum species often distribute in forests of middle to high altitudes from 2000–3000(4000) m in the Himalayas. Among these, Hymenophyllum corrugatum and H. longissimum are endemic species to China, while H. levingei is globally threatened.


Introduction
The family Hymenophyllaceae Link, or filmy ferns, is one of the largest basal families of leptosporangiate ferns and comprises about 600 species (Iwatsuki, 1990) in tropical, subtropical and temperate regions of both Old and New Worlds.Filmy ferns are mainly constrained in very humid tropical and subtropical mountain forests, since its thin leaf blades consist of one cell thick and lack stomata.They are very sensible to moisture conditions; therefore, its distribution is limited by habitats and can be used as indicator of environment.Traditionally, only two large genera, i.e.Hymenophyllum with bivalved involucres and Trichomanes with tubular involucres were recognized.The most recent classification delivered through molecular phylogenetic analysis recognized nine genera, of which, the genus Hymenophyllum is the largest of the family with about 250 species and is subdivided into ten subgenera, i. e. Hymenophyllum, Mecodium, Globosa, Sphaerocionium, Pleuromanes, Hymenoglossum, Fuciformia, Cardiomanes, Diploophyllum, and Myrmecostylum (Ebihara et al., 2006).
Taxonomic studies of Hymenophyllum in the last century were mainly of floristic accounts.Iwatsuki (1985) reported 22 species from Asia; Mehra and Bir (1964), Chandra (2000), Ghosh et al. (2004), Singh and Panigrahi (2005), and Fraser-Jenkins (2008,2010) studied those of the Indian subcontinent.The numbers of recognized species are five from Nepal (Iwatsuki 1988;Thapa 2002), but four were accepted by Fraser-Jenkins et al. (2015); six species from Bhutan by Fraser-Jenkins et al. (2009); six species from Myanmar by Dickason (1947) and 22 species from China by Liu et al. (2013).In the present revision, the morphological and anatomical characters of Hymenophyllum species from the Pan-Himalaya preserved in the China National Herbarium (PE) at Beijing were carefully studied during the one-year CAS-TWAS scholarship post-doctoral research conducted by the senior author.Altogether 10 species of three subgenera of Hymenophyllum from the Pan-Himalaya were described, illustrated and mapped.This is also a precursor of the ongoing "Flora of Pan-Himalaya" project.

Study area
The Pan-Himalaya (the Himalaya and adjacent regions) forms a natural geographic unit, from the Wakhan Corridor and northeastern Hindu Kush eastwards to the Hengduan Mountains by ways of Karakorum and the Himalayas.This region covers the northern corner of Afghanistan, northern Pakistan, northern India, Nepal, Bhutan, northern Myanmar, and southwest China (S Tibet, SE Qinghai, SE Gansu, W Sichuan, and NW Yunnan) (Fig. 1).

Herbarium Specimens
One hundred and eighteen specimens of Hymenophyllum especially that grown in the Pan-Himalaya deposited in the PE herbaria have been examined.The morphology and anatomy of each specimen were studied and photographed.The morpho-photographs of the plants were taken with a Nikon DXM 1200F camera connected to a stereomicroscope (Nikon SMZ 1000) and computer and measurement was done by D 3.10 (http:// www.nikoninstruments.com).Anatomical study was carried out with Leica DM 4000 B LED (Leica Microsystems, Wetzlar, Germany).The classification system of Hymenophyllum of this paper follows that of Ebihara et al. (2006).brown, slightly zig-zag, densely covered with hairy on lower surface, hairs long, linear, multicellular, reddish brown; broadly winged throughout, wavy or crisped.Lamina bipinnatifid to tripinnatifid, ovate or oblong to oblong-lanceolate, 2-11 × 1-4 cm, membranous and semitransparent, glabrous, base gradually narrowing or broadly cuneate, apex obtuse to acute; pinnae 3-10 pairs, closely spaced, alternate, sessile, widely spreading, oblong or obovate to linear-subdeltoid, 0.5-2 cm × 5-10 mm, base unequally cuneate, apex rounded or acute; ultimate segments linear-oblong, 2-5 × 0.5-2 mm, with 1 or 2 veinlets, margin subentire or serrate or serrulate, apex round, obtuse or slightly emarginate.Lamina cells polygonal, sometimes irregular in shape; cell walls slightly thick and wavy; marginal cell wall slightly wavy.Veins dichotomous, dark brown, free, obviously raised on the surface, densely hairy on Nwe T. Y., Zhang X.-C.Hymenophyllum in Pan-Himalaya the lower surface.Sori on upper part of the frond, terminal on each short segment; involucres ovate to oblong, 0.8-1.8× 0.6-1.4mm, bilabiate almost to the base and hairy at base, lips slightly or deeply serrate and obtuse to rounded at apex; receptacles included (Fig. 2,12A).
Habitat: On moss tree trunks or on wet rocks in subtropical or tropical evergreen forests, and near streams; at 760-1450 m regions.
Habitat: On moss tree trunks or on wet rocks from 1900-2800 m, hilly regions.
Habitat: On rocks near rivers, or on tree trunks in forests in mid to high altitude; at 2500-3000 m region.This is one of the globally threatened species as mentioned by C. R. Fraser-Jenkins (2012).