Mitriostigma monocaule Sonké & Dessein, 2009

Sonke´, Bonaventure, Simo, Murielle & Dessein, Steven, 2009, Synopsis of the genus Mitriostigma (Rubiaceae) with a new monocaulous species from south Cameroon, Nordic Journal of Botany 27, pp. 305-312 : 305-309

Mitriostigma monocaule Sonké & Dessein sp. nov. ( Fig. 1 - 4)

Mitriostigmati barteri Hook. f. ex Hiern affinis sed ab illa habitu monocauli (versus frutescenti et ramoso), nervorum secundariorum numero ab utroque latere 8 - 12 (versus 6 - 9) atque inflorescentiis supra-axillaribus in paribus oppositis (versus pseudo-axillaribus) differt.

Type: Cameroon, south province, Elephant Mountains , 18 Mar 2008 (fl., fr.), Sonké and Simo 4742 (holotype: BR, isotypes: BR, BRLU, K, MO, P, WAG, YA) .

Woody monocaul dwarf, 20-30 cm tall, internodes very short. Stems 3-6 mm in diameter; bark brown, sparsely hairy when young, becoming glabrous with age. Stipules persistent, ovate -triangular, at the base protruded into an awn, 6.5-12 mm long (including 4-6 mm long awn), 3-4 mm wide at the base, puberulous outside. Leaves decussate; petiole 13-18 mm long, sparsely hairy when young; leaf blades obovate, 11-19 × 3.5-6.5 cm; apex acuminate with acumen (10-) 13-17 (-20) mm long; base attenuate; leaf surface glabrous above and underneath; mid-vein prominent below, slightly prominent above; secondary veins brochidodromous, prominent below, 8-12 on each side of the midrib, ascending, straight to slightly curved at the base, strongly curved at the margin to join with the adjacent vein; intersecondary veins forming a dense reticulate network; domatia absent. Inflorescences supra-axillary and paired at the nodes, shifted ca 2 mm above the nodes, 2-10-flowered, peduncle sparsely pubescent. Flowers 5-merous, pedicels often pink, 2.5 mm long; bracts and bracteoles often pink, triangular to narrowly triangular, sparsely pubescent particularly at the margin. Corolla buds pink with contorted aestivation to the left. Calyx pink; calyx tube ca 0.3 mm; calyx lobes subequal, triangular, 0.6-1.2 mm long, very sparsely pubescent or glabrous. Corolla campanulate, cream -white with pink stripes and dots; corolla tube 10.5-12.0 mm long, glabrous outside, with a zone of hairs of ca 2 mm just below the enlarged part of the tube inside; corolla lobes 9 elliptic, 3.5 × 2.9 mm. Anthers entirely included, basifixed, attached ca 7 mm from the base of the corolla tube, ca 4.5 mm long including a 0.1-0.2 mm long sterile appendix. Pollen 3-porate, dispersed as tetrads; sexine (micro-)reticulate. Ovary 2-locular, glabrous or sparsely pubescent; disc cylindrical, surrounding the base of the style. Style exserted, ca 15 mm long; stigma lobes ca 0.6 mm long. Fruits ovoid to sub-fusiform, crowned by persistent calyx lobes, 19- 28 × 5-9 mm, orange at maturity, glabrous. Seeds 4-9 per fruit, 5.3-7.3 × 3.2-5 mm; seed-coat with fine narrow reticulations.

Distribution, habitat and ecology

Mitriostigma monocaule occurs in the Lower Guinean subcentre of endemism ( White 1979), and is restricted to the region of south Cameroon ( Fig. 4 ). The area from which M. monocaule is known, supports a closed-canopy evergreen forest with many epiphytes and a rich herb layer, which can be classified as Biafran evergreen forests, rich in Caesalpiniaceae ( Letouzey 1985) .

Phenology

Flowering and fruiting in March.

Conservation status

Data sparse. On the evidence available, M. monocaule is highly localized, being known only from the southern edge of the Elephant Mountains. So far, it is known from just one site and one single population. This is despite the fact that the species is highly conspicuous in flower and fruit and the fact that two of us have made several lengthy visits to the Elephant Mountains over recent years. We hope that more populations and new sites for M. monocaule will be located in the future. Given its apparent rarity, M. monocaule may be a suitable subject for a propagation and reintroduction scheme.

Etymology

The epithet ‘ monocaule ’ refers to the monocaulous life-form of the plant (below).

Diagnostic characters and relationships

Floral and seed morphology clearly place the novelty within Mitriostigma . As in the other Mitriostigma species , the corolla is relatively short and 9 campanulate or cylindrical and widened at the apex, and the seeds are rounded to angular but not strongly compressed. This is in contrast with the species of the sister genus Oxyanthus , which are characterized by long cylindrical corollas and strongly compressed seeds. Mitriostigma monocaule takes a rather isolated position within the genus as it has a unique combination of character states ( Table 1 ). With M. usambarense , the new species shares the supra-axillary position of the inflorescences, the relatively short corolla lobes compared to the length of the corolla tube, and the included anthers. However, the leaves of M. usambarense are much smaller, the inflorescences are solitary at the nodes, the corolla tube is longer and only widened at the throat, and the fruits are ellipsoid not ovoid to sub-fusiform.

With M. greenwayi it shares the large leaves and the axillary inflorescences paired at the nodes. The corolla of M. monocaule , however, is much shorter, the stipules are narrower, the anthers are included, and the inflorescences are supra-axillary (although we occasionally observed supraaxillary inflorescences in M. greenwayi, De Block et al. 431).

With M. barteri , to which we think M. monocaule is most closely related, the novelty shares the included anthers and the very similar flowers and fruits. The habit of the two species is different, however. Mitriostigma barteri is a branched shrub up to 2 m tall, whereas the novelty is a small unbranched woody plant up to 30 cm tall.

Robbrecht (1988) has tentatively proposed the term ‘monocaul dwarfs’ for this kind of life-form, and the term has since then been used regularly in Rubiaceae systematics. In older literature this life-form has been reported under vague terms such as sub-shrub, treelet, or by means of intricate circumscriptions, e.g. ‘‘small pachycaul treelet’’ ( Ridsdale et al. 1972) or ‘‘unbranched understorey treelet’’ ( Ridsdale 1975). Apart from the monocaulous growth form and the supra-axillary inflorescences paired at the nodes, M. monocaule differs from M. barteri in the somewhat larger leaves with more numerous secondary veins and a more pronounced acumen ( Table 1 ).

The finding of M. monocaule confirms that the genus Mitriostigma is morphologically heterogeneous, which might support the suggestion of Bridson (1979) that several subgenera could be recognized.

Figures

Figure 1. Mitriostigma monocaule Sonké & Dessein sp. nov. (a) fruiting stem with supra-axillary infrutescences, scale bar=1 cm, (b) stipule and part of the stem showing the supra-axillary position of the inflorescence, scale bar=1 mm, (c) detail of inflorescence, scale bar=2 mm, (d) calyx, corolla and exserted stigma, scale bar=2 mm, (e) corolla opened out, scale bar=2 mm, (f) style, scale bar= 2 mm, (g) longitudinal section of ovary, scale bar=1 mm, (h) fruit, scale bar=5 mm, (i) seed, scale bar=1 mm.

Figure 4. Distribution of Mitriostigma species (?=uncertain locality for M. axillare based on Bisset s. n. (K): Umzumkulu (?= Umzimkulu)).