Oxyanthus doucetii 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 : 174-176

Oxyanthus doucetii Sonké & O. Lachenaud , spec. nova ( Fig. 1 ).

Typus: CAMEROON. South Region: 16 km on the road from Ebolowa to Minkok, 2°58’N 11°17’E, 28.IV.1975, fl., De Wilde 8207 (holo-: BR!; iso-: P [ P04006788 ]!, WAG [ WAG.1228575 , WAG.1228576 , WAG.1228577 ]!, YA!) .

Oxyanthus doucetii Sonké & O. Lachenaud closely resembles O. unilocularis Hiern in the large and broad asymmetrical leaves with sparsely pilose lower surface, and the manyflowered inflorescences with long corollas. The species differs from O. unilocularis in the minute calyx teeth (<0.3 mm long), the stems with glabrous internodes, the stipules glabrous except sometimes on their margin, the flower buds with a shorter head 8.5-11 mm long, and the corolla lobes not or hardly acute at apex; by contrast, O. unilocularis has well-developed subulate calyx teeth 1.5-10(-14) mm long, densely puberulous stems and stipules, the head of the flower buds 12-25 mm long, and the corolla lobes sharply acute at apex.

Shrub or small tree, 2-10 m tall, with horizontal branches; stems hollow, 7-14 mm thick, glabrous or with short sparse hairs at the nodes only. Stipules 23-48 X 16-25 mm, broadly mitriform with ± acute apex, glabrous or sparsely ciliate, persistent (except at flowering nodes, where often caducous). Leaves with petiole 0.5-1.5 cm, glabrous or with short sparse hairs, and leaf blades 34-60 X 18-30 cm, very broadly elliptic, strongly asymmetrical at base with proximal side rounded to cordate and distal side acute to obtuse inserted 5-12 mm higher, acute or shortly acuminate at apex, glabrous above, sparsely pilose below with ± scabrid erect hairs 0.7-1 mm long; lateral nerves 12-13 pairs, strongly ascending (except the lower ones) and forming loops well away from the margin; tertiary veins laxly reticulate; domatia absent. Inflorescences pseudo-axillary, one per branch and per season, corymbiform, glabrous, with several hundred flowers but usually producing

only few fruits; peduncle 0.6-1.5 cm long, rhachis 5-10.5 cm long, lateral branches 1.5-4 cm long. Bracts minutely triangular, <1(-1.5) mm, glabrous. Flowers 5-merous; pedicels 2-8 mm long, glabrous. Ovary 1.5-2 mm long, glabrous. Calyx with tube 0.7-1 mm long and minute, acutely triangular teeth 0.2-0.3 mm long, entirely glabrous. Corolla tube pale green, very narrowly cylindrical, 12.2-13.7 cm long X c. 0.2 cm wide, glabrous; corolla lobes white, lanceolate, obtuse or only faintly acute at apex, 1.2-1.7 cm long X 0.25-0.3 cm wide, glabrous outside, shortly papillose inside. Anthers exserted, ± patent, inserted at the corolla throat, linear, 3-5 X 0.5 mm including a sterile apical appendage 0.5-1 mm long, glabrous. Style exserted, exceeding corolla throat by 1.5-2 cm, glabrous, with slightly swollen elongated stigma. Fruits green, smooth, ovoid to globose, 1.3-2.7 X 1.4-2 cm when dry, glabrous, with pedicel not markedly accrescent and calyx persistent. Seeds numerous, irregularly polygonal and compressed, c. 9 X 5 mm, the surface folded into numerous closely parallel ridges.

Etymology. – The species name honours Prof. Jean-Louis Doucet, specialist in tropical forestry (Université de Liège, Gembloux Agro-Bio Tech, Belgium), to whom the first author is particularly indebted for his constant support.

Distribution and ecology. – Oxyanthus doucetii is endemic to Cameroon in the Lower Guinea Domain ( Fig. 2 ), and is found mostly in the Central Region around Yaoundé, more rarely in the southwest (Mt. Cameroon) and South Regions (around Bipindi and Ebolowa). The species occurs in primary and secondary forests between 480 and 760 m.

Phenology. – Flower buds in October-November, mature flowers in April; fruits in March, June-July (immature) and from September to December.

Conservation status. – The EOO is estimated as c. 35,435 km 2 and AOO as 44 km 2. The species is known from nine locations. None of these locations are protected, and deforestation for agriculture (and also for urban extension in the periphery of Yaoundé) represents a clear threat to the species. A decline in AOO, extent and quality of habitat, number of locations and number of individuals is therefore expected, and O. doucetii is assigned a preliminary conservation status of “Vulnerable” [VU B2ab(i,ii,iii,iv)].

Notes. – As mentioned above, O. doucetii has previously been confused with the morphologically similar, and presumably closely related, O. unilocularis . It differs from the latter in having minute calyx teeth (<0.3 mm long), the stems with glabrous internodes, the stipules glabrous except sometimes on their margin, the flower buds with a shorter head 8.5- 11 mm long, and the corolla lobes not or hardly acute at apex (compare Fig. 1 and 3 ). Additionally, O. doucetii tends to have larger and broader stipules (23-48 X 16-25 mm) than O. unilocularis (7-35 X 4-22 mm), but this character shows significant overlap between the two species. The stipules of O. doucetii are the largest in the genus.

Two sterile collections from Mt Kolodom near Yaoundé, Sonké 120 & 121 (BR), are intermediate between O. doucetii and O. unilocularis in vegetative characters. The internodes are very sparsely pubescent, often only towards the apex, and the stipules are pubescent only at the base and margins. In the absence of fertile material, these collections cannot be referred with certainty to either species; they may represent hybrids, since both species are found in the region.

Paratypi. – CAMEROON. Central Region: Yaoundé, NE, 1939, fl. buds, Jacques-Félix 4799 ( BR); Nkolakié, 3°57’N 11°21’E, VII.2010, fr. imm., Kayo 1 ( BR); ibid. loc., 4.V.2003, fl. (fallen), Nguembou & Djuikouo 665 ( BRLU); ibid. loc., 8.VI.2003, fl., Nguembou & Djuikouo 796 ( BR, BRLU); Mont Kala , 3°53’N 11°30’E, 5.V.2004, fl. buds, Nguembou et al. 1202 ( BR, BRLU); Mont Ngoa Ekele [3°51’N 11°24’E], 4.XII.1986, fr., Sonké 43 ( BR, YA); Mt Akondoué [3°50’N 11°29’E], 23.IV.1987, fl. buds, Sonké 95 ( BR, K, YA); ibid. loc., 29.IX.1987, fr., Sonké 116 ( BR, WAG); ibid.loc., 31.X.1987, fl. buds & fr., Sonké 117 ( BR); Mt Kala , 14.XI.1987, fr., Sonké 123 ( BR, YA); ibid. loc., same date, Sonké 124 ( BR, YA); Mt Kolodom , [3°51’N 11°22’E], 23.IV.1988, fl., Sonké 141 ( BR, YA); Nkolfep , Massif du Mbaminkom , 3°58’N 11°23’E, 3.III.2002, fr., Sonké, Nguembou & Djuikouo 2758 ( BR, BRLU); Ndjamtchourou (Ngoro) , 4°57’N 11°20’E, 20.VII.2009, fr. imm., Sonké & Simo 5297 ( BR) . South Region : Bipindi , 3°5’N 10°24’E, 26.XI.2004, fl. buds & fr., Sonké & Nguembou 3598 ( BR) . Southwest Region: env. du village Efolofo , pentes septentrionales du Mt Cameroun , 4°21’N 9°7’E, 1.VI.1976, fr. imm., Satabié 300 ( BR, P, YA) .

Figures

Fig. 1. – Oxyanthus doucetii Sonké & O. Lachenaud. A. Inflorescence; B. Apex of twig with one leaf and the base of the opposite leaf (pubescence shown on a portion of the leaf); C. Apex of twig with stipules; D. Calyx; E. Detail of the apex of a flower bud; F. Detail of opened flower showing the lobes and upper part of tube; G. Fruit. [A, D-F:J.J. de Wilde 8207, BR; B, G: Sonké 124, BR; C: Sonké et al. 2758, BR] [Drawing: A. Fernandez]

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

Fig. 3. – Oxyanthus unilocularis Hiern. A. Stipules and fragment of twig; B. Calyx; C. Apex of flower bud. [Wieringa et al. 2479, BR] [Drawing: A. Fernandez]