Oxyanthus lewisii Sonké & O. Lachenaud, 2016

Sonké, Bonaventure & Lachenaud, Olivier, 2016, Two new species of Oxyanthus DC. (Rubiaceae) from Central Africa, Candollea 71 (2), pp. 173-180 : 176-179

Oxyanthus lewisii Sonké & O. Lachenaud , spec. nova ( Fig. 4 , 5 ).

– Oxyanthus sp. B , sp. C in Sonké (1999: 97, 98).

Typus: CAMEROON. East Region: près Mekomo, 8 km SW confluent Dja et Lobo, [3°14’N 12°22’E], 19.III.1962, fl. buds & fruits, Letouzey 4585 (holo-: BR [ BR880499 ]!; iso-: BR [ BR880466 , BR880508 ]!, P [P 03907844 ]!, YA!) .

Oxyanthus lewisii Sonké & O. Lachenaud has fusiform fruits, resembling those of O. robbrechtianus Sonké , O. dubius De Wild. and O. speciosus DC. subsp. stenocarpus (K.Schum.) Bridson , but more narrowly pointed at apex than in these species. It further differs from O. dubius by the shorter calyx teeth (0.4-0.6 mm, not 3-4 mm) and bracts (1.5-2 mm, not 5-8 mm), from O. speciosus subsp. stenocarpus by the shorter and few-flowered inflorescences, and from both by the nerves sparsely hairy beneath and lacking domatia in their axils. From O. robbrechtianus , it also differs in the glabrous twigs and outside of corolla, the leaves symmetrical at base, and the orange (not red) fruits without longitudinal ridges.

Shrub 1-2 m tall, with horizontal branches; stems glabrous. Stipules 4.5-12 X 2.5-7 mm, narrowly ovate with acute apex, glabrous, persistent. Leaves with petiole 0.4-1.2 cm long, glabrous to sparsely pubescent, and leaf blade 9-18.5 X 1.9- 8.5 cm, elliptic or elliptic-oblong; cuneate at base, acuminate at apex, glabrous above, sparsely and shortly pubescent on the midrib and lateral nerves beneath; lateral nerves 5-9 pairs, moderately to strongly ascending; tertiary veins rather densely reticulate; domatia absent. Inflorescences pseudo-axillary, one per branch and per season, very shortly paniculate, <1.3 cm long, glabrous, with 5-14 flowers (usually producing 1-3 fruits); peduncle absent or very short (to 0.2 cm), rachis <1. 1 c m. Bra cts s u b u l a t e, 1.5 -2 m m l on g, c i l i a t e. Flowers 5-merous; pedicels 1-2 mm long, glabrous. Ovary 1-1.3 mm long, glabrous. Calyx with tube c. 1 mm long and short subulate teeth 0.4-0.6 mm, entirely glabrous. Corolla only known in very young bud stage, c. 0.65 cm long; tube cylindrical, glabrous, c. 0.15 cm long; lobes narrowly lanceolate, c. 0.5 cm long X 0.1 cm wide, glabrous on both sides. Anthers smooth, linear, 3.6-4 X 0.1 mm, including a sterile apical appendage ± 1 mm long. Style ± 6 mm long, with a slightly swollen elongated stigma ± 2 mm long. Fruits orange, fusiform, smooth or slightly rugose but without longitudinal ridges, 3.8-6.5(-8) X 0.5-1.5 cm, gradually pointing into a narrow beak (0.2-0.4 cm wide at apex), glabrous, with accrescent pedicel 0.5-1 cm long and calyx usually deciduous. Seeds numerous, compressed, irregularly ellipsoid, 6-7 X 3-5 mm, the surface folded into numerous closely parallel ridges.

Etymology. – This species is named after Prof. Simon Lewis, internationally renowned British ecologist (Chair in Global Change Science, Department of Geography, University College London) who made several expeditions in Cameroon with the first author, and constantly supported his research.

Distribution and ecology. – Oxyanthus lewisii is sparsely distributed in southern Cameroon, northern Gabon, and eastern D.R. Congo in the Lower Guinea and Congolian Domains ( Fig. 2 ). It appears to have a widely disjunct range, with a gap of over 1,000 km between the D.R. Congo locality and the rest of the range; similar disjunctions are known in other species, e.g. in Chassalia pleuroneura (K. Schum.) O. Lachenaud (Lachenaud & Jongkind, 2010). The species is apparently rare; it occurs in lowland forest between 380 and 750 m, at least sometimes in riverine or periodically inundated formations.

Phenology. – Flower buds in March; fruits from December to April.

Conservation status. – The EOO is estimated at c. 371,433 km 2 and the AOO as 20 km 2. The species is known from five locations, none of which are protected. Logging activities and deforestation for agriculture (and also for mining in the eastern part of its range) represent potential threats to the species. A decline in AOO, extent and quality of habitat, number of locations and number of individuals may therefore be expected, and Oxyanthus lewisii is assigned a preliminary conservation status of “Vulnerable” [VU B2ab(ii,iii,iv,v)].

Notes. – Oxyanthus lewisii is remarkable by the long fusiform fruits, which are somewhat variable in shape, being particularly narrow in Letouzey 4585 and Le Testu 9033, but distinctly broadened in the middle in Dauby et al. 2293. The only other taxa with fruits approaching in shape those of O. lewisii , although not so pointed at apex, are O. robbrechtianus Sonké , O. dubius De Wild. and O. speciosus subsp. stenocarpus (K. Schum.) Bridson. Oxyanthus lewisii differs from O. dubius by the shorter calyx teeth and bracts, from O. speciosus subsp. stenocarpus by the shorter and few-flowered inflorescences, and from O. robbrechtianus , it also differs in the glabrous twigs and outside of corolla, the symmetrical leaf base, and the fruits lacking longitudinal ridges. Oxyanthus speciosus subsp. stenocarpus also has a different habitat (submontane forests, 1000-2000 m).

In the absence of fruits, O. lewisii might be confused with O. gracilis K. Schum. , O. pallidus Hiern and O. subpunctatus (Hiern) Keay , all of which have the leaves fairly similar in size and shape, but either entirely glabrous beneath ( O. pallidus ) or pubescent only on the nerve axils ( O. gracilis and O. subpunctatus ).

Paratypi. – GABON. Estuaire: Parc des Mts de Cristal, région d’Akoga , 0°51’50”N 10°29’53”E, 13.XII.2015, fr., Boupoya & Issembé 1211 ( BRLU) . Ogooué-Ivindo: Concession CEB , N de la Zone de Milolé, 2°14’55”S 12°44’19”E, 12.II.2010, fr., Dauby et al. 2293 ( BRLU) . Woleu-Ntem: Aloum [2°09’N 11°42’E], 14.III.1933, fr., Le Testu 9033 ( BR). D.R . CONGO. South Kivu: Kingulube [2°39’S 28°02’E], fr., 15.IV.1959, A. Léonard 3872 ( BR) .

Figures

Fig. 4. – Oxyanthus lewisii Sonké & O. Lachenaud. A. Fruiting branch; B. Stipules and young inflorescence; C. Detail of lower leaf surface; D. Fruit; E. Seed. [A: Dauby et al. 2293, BRLU; B-E: Letouzey 4585, BR] [Drawing: A. Fernandez]

Fig. 5. – Fruiting stem of Oxyanthus lewisii Sonké & O. Lachenaud. [Photo: GillesDauby]

Fig. 2. – Distribution of Oxyanthus doucetii Sonké & O. Lachenaud (circles) and O. lewisii Sonké & O. Lachenaud (squares).