Combretum xylocarpum Dan Liang & R.J. Wang, 2022

Liang, Dan, Yuan, Lang-Xing & Wang, Rui-Jiang, 2022, Combretum xylocarpum (Combretaceae), a new liana species from China, Phytotaxa 564 (3), pp. 288-294 : 289-290

Combretum xylocarpum Dan Liang & R.J. Wang , sp. nov. ( Figures 1 , 2 ).

Type:— CHINA. Hainan: Wenchang City, Long Lou Town, Nearby the Mt. Tongguling , roadsides, 111°0’E, 19°40’N, elevation ca. 4 m, fruiting, 5 August 2018, Dan Liang & Guo-Bin Jiang WP 0569 (holotype IBSC0867967 !; isotype IBSC0867968 !) .

Diagnosis:—This species is similar to C. acuminatum and C. chinense Roxburg ex G. Don (1827: 417) in woody scandent habit, elliptic leaves, axillary or terminal spikes, and infundibuliform calyx shape, but differs from the former by its fewer secondary veins and flowers with pale whitish petals and from the latter by its longer spikes and woody and small ellipsoidal fruit with 4 rounded ridges ( Table 1 ).

Description

Woody liana or large scandent shrub. Young branches densely pubescent and lepidote, ferruginous in buds, becoming glabrous with developing. Leaves simple, opposite or more or less alternate, papery to subcoriaceous, oblanceolate to elliptic, (3.5–)5.8–9.6 × (1.9–) 2.5–4.8 cm, apex acuminate, obtuse or slightly emarginate; base subrounded to cuneate; glabrous and with dome-shaped sessile glands adaxially, glabrous to sparsely lepidote and with orificed pits abaxially; midrib prominent at base and then depressed toward apex, and glabrous or sparsely pubescent adaxially, prominent, glabrous or sparsely pubescent and lepidote abaxially; secondary veins brochidodromous, usually 4–5(–7) on each side; tertiary veins dichotomizing; margin entire. Petioles 0.3–1.0 cm long, puberulous and scaly at surface. Spikes terminal and axillary, cylindrical, 6.0–8.0 cm long; peduncles up to 3.0 cm long, rusty pubescent and scaly, brownish. Flowers bisexual, numerous, sessile, white or yellowish-white, ca. 1.0 cm long, actinomorphic. Hypanthium in two distinct parts, the proximal part tubular in 3.0– 3.5 mm long, the distal one infundibuliform in 2.5–3.0 mm long, with a ring of hairs directed upwards at the mouth, slightly pubescent and densely ferruginous scaly abaxially. Calyx lobes 4, triangular, ca. 2.2 mm long, sparsely ferruginous scaly abaxially. Petals 4, conspicuous, obovate to spatulate, 3.0–3.5 × 1.5–2.2 mm, acute at apex, cuneate at base, pale whitish. Stamens 8, 2-seriate, alternately antesepalous and alternisepalous, exserted; filaments ca. 8.0 mm long, incurved initially and then exceeding the petals; anthers oblong, ca. 0.8 × 0.5 mm, biloculed, dorsifixed, versatile. Style simple, filiform, 6.0–8.0 mm long, for ca. 2/3 free and ca. 1/3 attached to the wall of proximal hypanthium, exserted, glabrous. Stigma punctiform. Ovary inferior, unilocular, with 4 pendulous, anatropous ovules. Fruit a drupe, shortly stipitate, ellipsoidal, ca. 1.5 × 0.7 cm, brownish yellow, muriculate at surface, longitudinally 4 rounded ridges, woody. Seeds ca. 6× 2.5 mm, ellipsoidal, always single, with 4 longitudinally rounded ridges.

Phenology:—Flowering from February to May; Fruiting from June to October.

Etymology:—The species epithet refers to the woody fruit wall of the new species.

Habitat and Distribution:— Combretum xylocarpum is currently known from the coastal region of Hainan Province, China. It grows mainly on litoral thickets of roadsides or sometimes close to mangrove margin, at 4–202 m elev., frequently accompanied by species such as Colubrina asiatica ( Linnaeus 1753: 196) Brongniart (1826: 62) and Secamone elliptica R. Brown (1810: 464) .

Palynology:—The pollen grains of Combretaceae are quite uniform. The basic characters of the family, as well as in Combretum xylocarpum , are monads, isopolar, radially symmetrical, and heterocolpate, which possess two types of colpi, one with endoaperture and one without, forming a grain with three simple apertures alternating with three composite ones ( El Ghazali and Krzywinski 1989, Krachai and Pornpongrungrueng 2015, El Ghazali 2016). The pollen size of C. xylocarpum is 15.85 (13.80–17.70) μm in polar axis (P) and 16.25 (14.57–18.20) μm in equatorial diameters (E), with a P/E value 0.96. According to terminology proposed by Punt et al. (2007), the pollen is medium in size and oblate spheroidal in shape. The exine ornamentation of the pollen is perforate and psilate ( Figure 2 ).

According to the pollen types identified by El Ghazali (2016), the pollen of C. xylocarpum seems to be ascribed to C. primigenium- type because of the 3-angled polar view and without vestibula, but its endoaperture protrusions also make it closed to C. bracteosum- type. Whereas the pollen grains of C. acuminatum was identified to be C. laxum - type for their 6-lobed polar view and absence of endoaperture protrusions, and those of C. chinense belong to C. squamosum - type due to their 3-angled polar view and presence of vestibula at apertures. This may indicate the unique and undescribed characters of the new species in both fruits and pollen grains.

Paratypes:— CHINA. Hainan province: Wenchang City, Longlou Town, Nearby the Mt. Tongguling, roadsides, 111°0’E, 19°40’N, elevation ca. 4 m, young fruit, 4 June 2019, Dan Liang & Guo-Bin Jiang WP0930 ( IBSC!); same locality, flowering, 16 March 2021, Lang-Xing Yuan WP1723 & WP1724 ( IBSC!); Qionghai City, Boao Town, 111°33’E, 19°06’N, elevation ca. 5 m, 27 May 2020, Rui-Jiang Wang & Dan Liang WP1060 ( IBSC!).

Conservation status:—Until now, Combretum xylocarpum is known from several localities in Hainan, including the two subpopulations found by ourselves and two by others. The population at the type locality only includes 2 mature individuals, but at the other sites include more than 100 mature individuals. In addition, the new species grows at road sides and has no any medicinal and ornamental values. According to the related criteria proposed by IUCN Standards and Petitions Subcommittee (2019), the current conservation status of C. xylocarpum can be evaluated as Least Concern (LC).

Figures

FIGURE 1. Combretum xylocarpum Dan Liang & R.J. Wang, sp. nov. A: Habit; B: Adaxial (left) and abaxial (right) side of leaf lamina; C: Dome-shaped sessile glands at adaxial side of leaf; D: Orificed pits at abaxial side of leaf; E: Adaxial side of the midrib, showing the pubescence and scales; F: Spike and flower arrangement (by Lang-Xing Yuan); G: Longitudinally dissected flower, showing the stamens, adnation of style, and a ring of hairs at throat; H: Dissected proximal hypanthium, showing the four pendulous ovules; I: Infructescence, with young fruits; J: Transversely dissected young fruit, showing the 4 rounded ridges; K: Mature fruit (leaf) and seeds (right, by Lei Zhao).

FIGURE 2. Pollen morphology of Combretum xylocarpum under SEM. A: Equatorial view, showing the endoaperture protrusions; B: Polar view, showing the heterocolpate grains; C: Colporate aperture with endoaperture protrusions; D: Perforate and psilate exine. Arrowheads indicate subsidiary colpi.