Polystichum oligodontum Y. Nong, R. H. Jiang & C. Xiong, 2025

Nong, You, Jiang, Ri-Hong, Xiong, Chi, Wei, Qiu-Jun, Wei, Gui-Yuan, Ji, Xing-Yun, Lai, Ke-Dao & Wei, Hong-Jin, 2025, Polystichum oligodontum (subg. Haplopolystichum, Dryopteridaceae), a new cave fern from Guangxi, China, PhytoKeys 266, pp. 297-310 : 297-310

Polystichum oligodontum Y. Nong, R. H. Jiang & C. Xiong sp. nov.

Figs 3 , 4 , 5

Type.

China. • Guangxi: Bama County, Jiazhuan Town, Renxiang Village , 24°19'36"N, 107°6'19"E, in a limestone cave, elev. 533 m, 29 August 2024, You Nong NY 2024082901 ( holotype: GXMI 051200 !; isotypes: CSH!, GXMI!, IBK!) .

Diagnosis.

Polystichum oligodontum is unique by the pinna acroscopic side with a shallow or nearly free lobe at the base and coarsely toothed along distal margins.

Description.

Plants perennial, evergreen. Rhizome short, erect, densely scaly. Fronds caespitose, 6–20 per rhizome, (6.5 –) 10–19 (– 30) cm long; stipe pale green when dry, (1.5 –) 2–5 (– 8.5) cm long, 0.3–0.5 (– 1) mm in diam. near middle, shallowly grooved adaxially, base densely covered with scales, upward scales sparse; scales ovate to oblong, brown to dark brown, apically attenuate to short-caudate, membranous, 2.5–4 × ca. 1.2 mm, spreading, denticulate on margin. Lamina 1 - pinnate, lanceolate, oblong-lanceolate or oblanceolate, widest at or above middle, (5 –) 7.5–14 (– 23) × (1.4 –) 1.8–2.5 (– 3) cm, slightly narrower towards base, base 0.8–1.7 cm wide, apex acute to short-acuminate; rachis pale green, shallowly grooved adaxially, sparsely covered with loose scales abaxially, scales lanceolate, brown, denticulate on margin. Pinnae 13–22 pairs, usually alternate, occasionally basal 1–2 pairs opposite, remote or proximate, short-stalked, stalks ca. 0.5 mm, abaxially with 1–2 scales similar to those on rachis, upper pinnae spreading to slightly ascending, pinnae below middle usually reflexed; lowest 2–4 pairs of pinnae 2–10 (– 14) mm apart; lowest pair sub-quadrangular, obliquely ovate or broadly flabellate, 0.8–5 (– 7) × 0.4–3 (– 5) mm; middle pinnae (6.3 –) 8.5–12 (– 15) × (3.5 –) 4.5–6 (– 8) mm, oblong, apex obtuse, subobtuse or subacute, with 1 or 2 broad, slightly mucronate teeth, base asymmetrical, acroscopic base wider, somewhat auriculate, auricles partially separated by a deep or shallow sinus, sometimes forming a nearly free lobe, with 4–5 acute, slightly mucronate teeth along upper margin, proximal margins of auricles slightly curved or straight, parallel or subparallel to rachis; upper acroscopic margin of pinnae nearly straight, coarsely toothed, with 2–3 increasingly shallow sinuses between teeth distally, teeth subacute to rounded-apiculate; basiscopic base of pinnae narrowly cuneate; basiscopic margins entire below apex, straight or nearly so, mostly acutely angled to rachis below; distal basiscopic margin curved upwards; adaxial surface glabrous, abaxial surface sparsely covered with lanceolate microscales, scales ca. 0.6–1 mm long, greyish-brown, denticulate on margin. Frond texture papery when dried, green adaxially; venation free, visible abaxially, indistinct adaxially, lateral veins pinnate from base upwards, dichotomous or simple above mid-rib, simple or dichotomous below mid-rib, extending into teeth, but not reaching pinna margin. Sori ca. 1 mm in diam., near pinna margins, terminal on shorter veinlets, 1–6 above mid-rib, often 0–3 below mid-rib, separate or confluent when mature. Indusia small, greyish-brown, membranous, evanescent, margin repand or slightly erose.

Spore morphology.

Perispore with coarse, irregularly inflated, partially interconnected folds and both cristate and echinulate microstructure, the latter on the spore surface and on the cirstae (Fig. 6 ).

Geographical distribution.

Polystichum oligodontum is found so far only in three limestone caves in Guangxi Zhuang Autonomous Region, south China. Two of them are located in Xinglong Village, Fengcheng Town, Fengshan County and one of them in Renxiang Village, Jiazhuan Town, Bama County (Fig. 7 ).

Ecology.

Polystichum oligodontum was discovered in karst caves at an elevation of 530–770 m, growing on limestone rocks with sizes less than 0.1 m 3 or on ground covered with thin sandy soil, at a distance of about 50–100 m to the cavern mouths.

IUCN Red List category.

Three populations with approximately 40 individuals were found respectively at three caves located in a karst region without any protection. The distance between the two closest caves is about 150 m. The third cave is about 17 km away from them. There are many villages in this region. The closest village is about 400 m (in a straight line) away from the cave, the type locality. However, this cave is easily visited and disturbed by the nearby villagers and some domestic animals, such as cattle and goats. The new species could disappear at any time if the habitat was seriously damaged. The status of the new species should be CR – Critically Endangered category, based on current information and following IUCN (the International Union for Conservation of Nature and Natural Resources) guidelines ( IUCN 2022).

Etymology.

The specific epithet is derived from the Latin, oligodontum, meaning “few teeth”, and referring to the number of the teeth on pinna margins.

Chinese name.

稀齿耳蕨 (xī chǐ ěr jué).

Paratypes.

China • Guangxi: Fengshan County, Fengcheng Town, Xinglong Village , in a karst cave, elev. 769 m, 18 July 2024, Chi Xiong & Xiao-Ying Fu DXH 240718-28 ( IBK!, CSH!) ; • ibid., elev. 736 m, 22 March 2025, Chi Xiong DXGJ 250322-52 ( CSH!) & DXGJ 250322-54 ( CSH!) .

Taxonomic notes.

Polystichum oligodontum has 1 - pinnate lamina and lanceolate stipe and rachis scales and is clearly a member of sect. Haplopolystichum within P. subg. Haplopolystichum. Morphologically, P. oligodontum can be readily distinguished from any species in the section in terms of pinna serration pattern, namely, the pinnae proximally exhibit a broad and 2–4 (or more) toothed auriculate lobe which is sometimes partially separated by a deep or shallow incision, distally followed with some coarse teeth along the acroscopic margin. This pinna serration pattern can be shown from some precocious Polystichum species such as P. normale Ching ex P. S. Wang & Li Bing Zhang in sect. Crucifilix Tagawa and P. bifidum Ching and P. pseudolanceolatum Ching ex P. S. Wang in sect. Sphaenopolystichum Ching ex W. M. Chu & Z. R. He. However, the new species can be easily recognised by its pinnae in having fewer teeth and shallower incisions on pinna margin, as well as shorter size.

Figures

Figure 3. Polystichum oligodontum (based on paratypeDXH 240718-28 in CSH). A. Habit; B. Portion of rachis with pinna. Illustrated by Hong-Jin Wei.

Figure 4. Polystichum oligodontum (from Bama county). A. Inside view of the cave where the new species was discovered; B. Lower portions of stipes showing scales; C, D. Habits; E. Portions of laminae (adaxially); F. Portions of laminae (abaxially); G. Pinna showing sori; H. Portion of pinna showing indusia.

Figure 5. Polystichum oligodontum (from Fengshan County). A. Inside view of the cave where the new species was discovered; B, C. Habit; D. Portions of laminae (adaxially); E. Scales from stipes base (bottom), pinna base (middle) and pinna surface (top); F. Portions of lamina (abaxially).

Figure 6. Scanning electron micrographs of spore of Polystichum oligodontum (from type specimen GXMI 051200). Left image depicting proximal view of spore, while right image showing distal view of spore surface.

Figure 7. Distributions of Polystichum oligodontum (red stars) in Guangxi, China.