Leptochilus oblongus Li Bing Zhang, Liang Zhang & Ngan T. Lu, 2015

Leptochilus oblongus Li Bing Zhang, Liang Zhang & Ngan T. Lu View in CoL , sp. nov. ( Figure 1 View FIGURE 1 ).

Type:— VIETNAM. Hoa Binh Province: Lac Son District, Ngoc Son Ngo Luong Nature Reserve , elev. 640–700 m, 20°25’N / 105°20’E, on limestone rocks in disturbed rainforest, 20 November 2013, L.-B. Zhang, L. Zhang & N. T. Lu 6299 (holotype VNMN! GoogleMaps ; isotypes CDBI! GoogleMaps , MO! GoogleMaps ).

Diagnosis: Leptochilus oblongus is most similar to L. ellipticu s in having 1-pinnatifid lamina but the former has oblong sori close to the lobe margins and nearly parallel to costule, while the latter has linear sori located on main lateral veins and forming an angle with costule.

Plants perennial, evergreen. Rhizome creeping, ca. 1.2–1.5 mm in diam., dorsiventrally flattened, densely scaled, with few short roots; cross section of rhizome with 7–10 vascular bundles, irregularly shaped, scattered sclerenchyma strands 25–30; scales peltate, dark brown and more or less shining, clathrate, membranous, margins entire or sparely denticulate, polymorphism: the ones appressed on the rhizome often ovate or elliptic, 0.5–0.7 × 0.4–0.6 mm, the spreading scales lanceolate with twisted apex, 1.0–1.3 × 0.8–1.0 mm. Fronds nearly dimorphic, 0.5 to 3.0 cm distant, (16–) 25–30 cm long; stipe stramineous, (9–) 14–17 cm long, ca. 0.5–1.0 mm in diam. at middle, slightly winged at distal part, less than 1.5 mm wide, with spare scales at base; rachis narrowly winged, 4.0– 4.5 mm wide at the distal part, 2.8–3.5 mm at base. Lamina oblong-ovate or ovate-lanceolate, deeply 1-pinnatifid, (8–)10–13 × 7–9 cm, widest below middle of lamina, thinly papery, dark brown when dry, glabrous, margins of lobes entire and distinctly undulate. Lobes (3–)5–7 pairs, narrowly lanceolate, apex acuminate, 4–6 × 0.4–0.9 cm, basal portions of lobes obviously narrowed, ca. 1/2 to 2/3 as wide as middle portions, slightly ascending upward, basiscopic margins of lobes at angles of 45–80 degree with rachis, distal pairs of lobes slightly narrowed, ca. 3/4 to 4/5 as long as middle ones, opposite or slightly alternate, middle pairs of lobes close to each other, basal pairs of lobes distant with each other in 0.9–1.2 cm. Midribs slightly raised, lateral veins invisible. Sori oblong, (3–)6–12(–15) on each side of lobe, 0.8–1.2 × 1.0– 1.5 mm, close to lobe margins (centers of sori 0.5–1.1 mm from lobe margins, 1.6–3.5 mm from midrib), centers ca. 3.8–5.3 mm apart from one another. Sporangia round, ca. 0.2 mm in diam., dark brown and often separated from stalks when mature, stalks ca. 0.3 mm long. Spores irregularly in shapes, green to pale green, 32 per sporangium. Paraphyses absent.

Geographical distribution:—Currently, Leptochilus oblongus is only found in Lac Son District, Hoa Binh Province and may represent a species endemic to Vietnam.

Ecology:— Leptochilus oblongus was observed to grow in the crevices among limestone rocks in a slightly disturbed rainforests, at elevations between 640 and 700 m.

IUCN Red List category:—Only one population with about 10 individuals of L. oblongus were found. The status of the new species can be classified as Critically Endangered (CR), based on current information and following the IUCN (The International Union for Conservation of Nature and Natural Resources) guidelines ( IUCN, 2011), but more extensive fieldwork in surrounding similar habitats is needed to accurately assess its conservation status. The preservation of the rain forests in Vietnam is the key to conserve this species. Fortunately, the species was discovered in the Ngoc Son Ngo Luong Nature Reserve and all the natural habitats of the nature reserve are protected.

Etymology:—The epithet oblongus refers to the oblong sori which are rare in the genus.

Discussion:—Judging from the habit and the regularly pinnatifid lamina, Leptochilus oblongus is similar to L. ellipticus var. ellipticus . The indistinct lateral veins of the new species are similar to those of L. ellipticus var. longipes (Ching) Nooteboom (2013: 837) . However, L. oblongus differs from L. ellipticus and all the related taxa in having the unique shape and position of sori. The oblong sori of the new species are also reminiscent of Microsorum pteropus (Blume) Copeland (1929: 112) and M. insigne (Blume) Copeland (1929: 112) . The latter two species were transferred into Leptochilus recently ( Fraser-Jenkins, 2008) and are different from the new species and other species of Leptochilus in having irregularly scattered sori. L. oblongus is even similar to species of Phymatosorus (1973: 457) in soral shape and lamina dissection, and Leptochilus and Phymatosorus are closely related ( Kreier et al., 2008), but the habit, rhizome scales, and the cross section of rhizomes of L. oblongus demonstrate that the new species is undoubtedly a member of Leptochilus .

The shapes of sori in Leptochilus are variable from orbicular to elongate [e.g., L. hemionitideus (C.Presl) Nooteboom (1997: 285) ] to linearis [most species assigned to Colysis C. Presl (1849 , 506)], and to partly or completely confluent and developed into acrostichoid fertile leaf ( Leptochilus s.s.). Hetterscheid & Hennipman (1984) indicated those transitions among some species of Leptochilus : one specimen (Sin 20337, B) has two fertile fronds on a single rhizome, one frond with orbicular or elongate sori as L. hemionitideus , another much contracted with acrostichoid sori as L. decurrens Blume (1828: 206) . However, our field observations on the population of L. oblongus showed that the shapes and positions of sori are stable.

The previous molecular work showed that several genera (e.g., Microsorum ) in the microsoroid ferns are not monophyletic, but Leptochilus seems to represent a natural group ( Kreier et al. 2008). Leptochilus is the oldest legitimate generic name in the microsoroid ferns and therefore no matter what nomenclatural changes will be made to the whole microsoroid ferns at generic level, the name of the new species described here will stay.