Pleocnemia siamensis X.G.Xu & Li Bing Zhang, 2016

Xu, Xiao-Gang & Zhang, Li-Bing, 2016, Pleocnemia siamensis (Dryopteridaceae), a new fern species from southern Thailand, Phytotaxa 289 (1), pp. 88-92 : 88-90

Pleocnemia siamensis X.G.Xu & Li Bing Zhang , sp. nov. ( Fig. 1 )

Type:— THAILAND. Yala Province: Than To District, Bang Lang National Park , near headquarters, 06°12’N, 101°10’30’’E, 80 m, at base of limestone cliffs, 8 February 2004, D.J. Middleton 2787 (holotype A! , isotypes E-00674306 ! , BKF? ).

Diagnosis:— Pleocnemia siamensis is most similar to P. macrodonta ( Fée 1852: 267) Holttum (1974: 348) in being relatively small with regularly pinnatifid laminae and indistinct sinuses, but the former has sori in one row on each side of costules and located at the ends of the veinlets (rarely on the back of the veinlets), and not confluent below sinuses, while the latter has sori in two rows on each side of costules and located on the back of veinlets, and often somewhat confluent below sinuses.

Plants perennial, evergreen. Rhizomes erect, ca. 1.1–1.3 cm in diam., dorsiventrally flattened, with few short roots; cross section of rhizome with 9–14 vascular bundles, irregularly shaped, scattered sclerenchyma strands 19–26; Scales polymorphic, the ones on upper rhizome, lanceolate with twisted apex, 9–15 × 0.3–2.5 mm, blackish brown, shining, clathrate, membranous, margins entire or sparsely denticulate, appressed ones on the basal stipes often lanceolate with twisted apex, 0.5–22 × 0. 2– 0.9 mm, dark brown, more or less shining, upper spreading ones linear, 1.0–22 × 0.3–0.6 mm. Fronds nearly dimorphic, 0.7 to 3.5 cm distant, (18–) 26–35 cm long. Stipes stramineous (pale brown), (40–) 44– 57 cm long, ca. 3–6 mm diam. at middle, with sparse scales at base, slightly winged at distal part, wings less than 2 mm wide. Rachises slightly winged, wings 1.8–2.5 mm wide at base of rachis, 0.6–1.2 mm wide at the distal part, Laminae elliptic, deeply bipinnatifid, (10–)16–25 × 9–15 cm, the widest part below middle of lamina, thinly papery, pale brown when dry, bearing densely short hairs. Pinnae ca. 4 pairs, lower ones each with 1 pair of petiolate pinnules, otherwise pinnules sessile or fully adnate lobes, 6–8 pairs per pinna, elliptic to lanceolate, apex acuminate. Pinnules ovate-deltoid 7–9 × 3–5 cm with petioles 0.8–1.2 cm long; pinnules and larger lobes with 5–7 pairs of oblong lobules 2–4 × 0.4–0.8 cm, free pinnules sometimes with one free lobule ca. 24 × 9 mm at basiscopic side, basiscopic margins of lobules at angles of 40–60 degrees with costules, distal pairs of lobules slightly narrowed, ca. 3/4–4/5 as long as middle ones, opposite or slightly alternate, margins of lobules entire or slightly undulate, hairy. Sinuses 0.2–0.3 mm wide, sometimes with a short rounded tooth bearing hairs. Veins forming two narrow areoles along costae, midribs, and costules between two costules and two midribs, in addition to 3–5 areoles on each side of costules, midribs, and costae below sinuses; free veins rarely present below sinuses but present on distal areas of lobules; midribs and costules raised, lateral veins slightly raised. Lower surface of rachis and costae bearing dense short hairs, surface of costules and veins bearing red glands like those in the sori. Sori rounded, indusiate, regularly on each end of vein forks or few on the back of veins, 0.3–0.5×1.0– 1.2 mm. Indusia distinct, rounded and thin, dark brown. Sporangia rounded, ca. 0.2 mm in diam., dark brown and often separated from stalks when mature, stalks ca. 0.1 mm long.

Geographical distribution: —Currently, Pleocnemia siamensis is only found in Than To District, Yala Province based on our current knowledge, and may represent a species endemic to Thailand.

Ecology: — Pleocnemia siamensis was observed to grow in the crevices among limestone rocks in a slightly disturbed rainforest, at elevations between 60 and 100 m.

Etymology: —The epithet siamensis refers to Thailand, the Southeast Asian country, from which the species is discovered.

IUCN Red List category: —Only one population is known from the type locality. It was observed to grow on limestone cliffs, generally very fragile habitats. Based on the current knowledge, the status of the new species clearly should be classified as CR-Critically Endangered category based on current information and following IUCN (the International Union for Conservation of Nature and Natural Resources) guidelines ( IUCN 2015), although the type locality is currently in a protected national park.

Discussion: —The habit, the dissection of leaves, the rhizome scales, and the morphology of the cross section of rhizomes of Pleocnemia siamensis clearly demonstrate that the new species is a member of Pleocnemia . Judging from the small size, the regularly pinnatifid lamina, and the indistinct sinuses, P. siamensis is most similar to P. macrodonta in the genus. However, P. siamensis differs from the latter in the distribution of sori (see above). In addition, the two species have different morphology of sinus teeth: the former has sinus teeth highly hairy and with rounded apex, while the latter has sinus teeth nearly glabrous and with triangular apex. In fact, P. siamensis has the most hairy sinuses and margins around them in the genus. Pleocnemia macrodonta has (0–)1–4 additional sori close to the costule, which is not observed in P. siamensis . Judging from the pattern of venation, P. siamensis is similar to P. brongniartii ( Bory, 1828: 263) Holttum (1974: 348) , but the latter has sori all near the margins of the lobules of pinnules or pinnae in the lamina apex. Pleocnemia siamensis is one of the few species in the genus growing in limestone habitats.

The previous morphological work identified two groups in the genus, the Pleocnemia group and the Dictyopteris group ( Holttum 1991). Based on the pattern of venation and less distinctive sinus teeth, Pleocnemia siamensis belongs to the Dictyopteris group with bipinnatifid basal pinnae and numerous additional areoles below sinuses between lobules. However, its phylogenetic relationships await confirmation by molecular study.

The examined material of Pleocnemia siamensis has only immature sori, which could indicate that the frond was not wholly expanded. Therefore, the true size of the new species may be somewhat larger than what we have observed.

Figures

FIGURE 1. Pleocnemia siamensis. —A. Habit. —B. Venation and position of attachment sori on lobes. —C. Appressed scale from stipe. —D. Cross section of rhizome, stippled: vascular tissue; black: sclerenchyma strands. —E. Part of venation and sinus tooth with margin hairs. (Drawn by Xiao-Gang Xu and Hai-Tian Wu, based on the holotype).