Sabicea uniflora Zemagho, O. Lachenaud & Sonké, 2018

Zemagho, Lise, Lachenaud, Olivier & Sonké, Bonaventure, 2018, Four new species of Sabicea (Rubiaceae) from tropical Africa, with additional notes on the genus, Candollea 73 (2), pp. 277-293 : 288-290

Sabicea uniflora Zemagho, O. Lachenaud & Sonké , spec. nova ( Fig. 7 ).

Holotypus: GABON. Ngounié: 27 km on the road Mimongo to Koulamoutou, 4.XII.2000, fl., Wieringa et al. 4567 ( BR [ BR0000024875851 ]! ; isotype: DSM, E, K!, LBV!, MA, MO, WAG!).

Combinatione habitu reptante, inflorescentiis axillaribus 1(‒ 3)-floris, bracteis bracteolisque quam calyce multo minoribus, corolla alba tubo longiore (17‒19.5 mm) ab omnibus congeneribus facile distinguitur.

Creeping herb c. 15 cm high; stems cylindrical, 1‒1.5 mm thick, with a mixed indumentum of straight appressed hairs c. 0.7 mm long and shorter uncinate hairs. Stipules interpetiolar, ovate to elliptic, 3.3‒8 × 2.2‒5 mm, obtuse at apex, curved ± at a right angle from the stem, glabrous to very sparsely pubescent outside, glabrous inside except at the base, persistent. Leaves opposite and ± equal; petiole 1.5‒5 cm long, with same indumentum as the stems; leaf-blade elliptic, 3.3 ‒8 × 2.2‒5.1 cm, acute at base, obtuse or hardly acuminate at apex; papyraceous, green on both sides but markedly paler below, drying olive green to olive brown; both surfaces with minute appressed or half-erect scabrid hairs, intermixed with sparse longer hairs (these mostly on the veins); secondary veins 6‒8 pairs, markedly curved and ascending, forming irregular loops 1‒2.5 mm from the leaf margin; tertiary veins darker and conspicuous below, forming a rather lax reticulum with areolae c. 2 mm in diameter. Inflorescences axillary, solitary, 1(‒3)-flowered, sessile, with a single pair of bracts and one pair of bracteoles per flower; bracts elliptic, 2.5‒3.5 × 0.7‒1.2 mm, shortly connate at base, acute at apex, sparsely pubescent outside and villose at the base inside; bracteoles lanceolate, 4‒ 4.5 × 0.7‒ 1.2 mm, free, otherwise similar to the bracts. Flowers 5-merous; pedicel 1 ‒ 2.5 mm long, villose. Calyx pale pink-green outside and reddish-green inside; tube 1.5‒2 × 1.7‒3.5 mm, sparsely appressed-hairy on both sides; lobes ovate to narrowly triangular, 3‒5 × 1‒2 mm, acute at apex, shortly and sparsely pubescent outside and in the upper half inside, alternating with minute colleters. Corolla white; tube 17‒ 19.5 mm long × 1‒1.2 mm wide at base and 2.5‒3 mm at apex, almost cylindrical, outside sparsely villose with patent silky hairs 1‒1.5 mm long, inside sparsely pubescent in the upper half and with white hairs c. 0.5 mm long in the throat; lobes triangular, 4‒ 4.5 × 1.8‒ 2.5 mm, patent, outside sparsely villose like the tube, inside minutely papillose. Stamens included, inserted around the upper 1/3rd of the corolla tube, subsessile; anthers c. 2.2 × 0.5 mm. Ovary c. 1.5 × 1.5 mm, 5-locular, densely villose with stiff hairs 1.5‒2 mm long. Disk cylindrical, c. 1 mm long, glabrous. Style 5-lobed, exserted, c. 22 mm long including the c. 4.5 mm long linear stigmas, the latter papillose, otherwise glabrous. Fruits and seeds not known.

Etymology. – The species is named after its usually 1-flowered inflorescences, an uncommon (although not unique) character in the genus.

Distribution, ecology and phenology. – Lower Guinea subcentre of endemism (WHITE, 1979). Sabicea uniflora occurs in the Chaillu Massif in south-central Gabon ( Fig. 4 ), and is only known from the type specimen, collected in primary forest at an altitude of 570 m. The area where it is found is a well-known centre of endemism, and has been postulated as a former forest refuge (e.g. SOSEF, 1994; ROBBRECHT, 1996; JANSSENS et al., 2011). Sabicea uniflora is flowering in December.

Conservation status. – Sabicea uniflora is endemic to the Chaillu Massif in south-central Gabon, and has been collected only once. Its extent of occurrence is therefore not calculable, and its area of occupancy is estimated to be 4 km ², within the limit for Critically Endangered under criterion B2. The area where it occurs has no protection status. It does not seem immediately threatened by human activities, and there is no evidence of a decline. However, the species’ very limited range makes it vulnerable to any threats that might arise in the future, e.g. forest clearance for agriculture or mining, and the species qualifies for “Vulnerable” [VU D2] using the IUCN Red List Categories and Criteria ( IUCN, 2012).

Notes. – This is a very distinctive species, easily recognised by the combination of a creeping habit, and 1(‒3)-flowered inflorescences with bracts much shorter than the calyx. It has no obvious close relatives, and in the absence of fruits, its exact affinities within the genus are unclear.

It is not known whether this species is heterostylous; only long-styled flowers are known so far.

Figures

Fig. 7. – Sabicea uniflora Zemagho, O. Lachenaud & Sonké. A. Flowering stem; B. Node and stipules. C. Inflorescence; D. Longitudinal section of flower; E. Inflorescence (corolla removed). [Wieringa et al. 4567, BR] [Drawing: A. Fernandez]

Fig. 4. – Geographic distribution of Sabicea desseinii Zemagho, O. Lachenaud & Sonké (circles), S. ezangae Zemagho, O. Lachenaud & Sonké (squares), S. jongkindii Zemagho, O. Lachenaud & Sonké (triangles) and S. uniflora Zemagho, O. Lachenaud & Sonké (pentagon).