Nordic Journal of Botany 27: 305
312, 2009 doi: 10.1111/j.1756-1051.2009.00415.x, # The Authors. Journal compilation # Nordic Journal of Botany 2009 Subject Editor: Ib Friis. Accepted 6 March 2009 Synopsis of the genus Mitriostigma (Rubiaceae) with a new monocaulous species from south Cameroon Bonaventure Sonké, Murielle Simo and Steven Dessein B. Sonke´ and M. Simo, Plant Systematic and Ecology Laboratory, Higher Teacher’s Training College, Univ. of Yaounde I, PO Box 047 Yaounde, Cameroon.
S. Dessein (dessein@br.fgov.be), National Botanic Garden of Belgium, Domein van Bouchout, BE
1860 Meise, Belgium. Mitriostigma monocaule Sonké & Dessein sp. nov., a new Rubiaceae species from south Cameroon is described and illustrated. The novelty is easily separated from the other four Mitriostigma species by its monocaulous growth form, the other species being shrubs, sub-shrubs, or occasionally trees. Another typical character for the species is its supra-axillary inflorescences paired at the nodes. The novelty is related to M. barteri, from which it further differs in the somewhat larger leaves with a more pronounced acumen and a higher number of secondary veins. A first conservation status for the species is given. A synopsis of the genus Mitriostigma with a taxonomic key is also provided. Mitriostigma Hochst. is an exclusively African genus of the family Rubiaceae comprising four species. Mitriostigma axillare Hochst., the type species of the genus, ranges from south Africa to Mozambique and probably also occurs in Tanzania. Mitriostigma usambarense Verdc. and M. greenwayi Bridson are endemic to Tanzania and Kenya, respectively, and M. barteri Hook. f. ex Hiern occurs in Equatorial Guinea (Bioko) and Cameroon (Verdcourt 1987). Mitriostigma was established by Hochstetter in 1842 and its name refers to the mitre-shaped stigma observed in M. axillare. Traditionally, the genus has been included in the tribe Gardenieae (Keay 1958, 1963, Robbrecht 1988), position confirmed by recent molecular studies (Andreasen and Bremer 1996, 2000, Robbrecht and Manen 2006). Mitriostigma is closely related to Oxyanthus from which it mainly differs in the shorter corolla tubes, the apiculate anthers, and the winged stigmatic clubs (though presumably absent in M. usambarense) (Bridson 1979, Verdcourt 1987). During recent botanical inventory work in south Cameroon the first two authors encountered an unusual Mitriostigma: a monocaul, i.e. single-stemmed, dwarf with supra-axillary inflorescences paired at the nodes, and with pinkish ovaries and calyces. This collection did not match any of the species known in the genus (Keay 1963, Verdcourt 1987, Bridson 1988), or any other specimen of Mitriostigma stored in herbaria. Our specimens resemble M. barteri in having campanulate corollas and ovoid to sub-fusiform fruits. However, it differs from all species known in the genus by its monocaul growth form and the supra-axillary inflorescences paired at the nodes. We therefore here propose a new species, i.e. Mitriostigma monocaule. In addition, as there is no document dealing with all the Mitriostigma species, we include a synopsis of the genus. Material and methods Mitriostigma monocaule sp. nov. was collected by B. Sonké and M. Simo in March 2008. Herbarium material of M. axillare, M. barteri, M. greenwayi and M. usambarense was consulted at BR, BRLU, K, SCA, WAG and YA. The distribution maps are based on the specimens of the above herbaria complemented with localities listed in Tropicos (/<www.tropicos.org/>, accessed 30 Jan 2009). Measurements, colours and other details given in the descriptions are based on living material, spirit and herbarium specimens, and data derived from field notes. Phytogeographical considerations follow White (1979, 1983). Descriptive terminology for simple symmetrical plane shapes follows Anonymous (1962). 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. 305 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 bar2 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. 306 b a c Figure 2. (a) fruiting stem with supra-axillary infrutescences of Mitriostigma monocaule Sonké & Dessein sp. nov., (b) side view of open flower and successive fruiting nodes, (c) side view of floral bud and fruit. a b c d Figure 3. Pollen and seed morphology of Mitriostigma monocaule Sonké & Dessein sp. nov. (a) polar view of triporate pollen grain in tetrad, (b) detail of apocolpium showing (micro)reticulate sexine, (c) overview of seed, (d) detail of exotesta cells. 307 Figure 4. Distribution of Mitriostigma species (? uncertain locality for M. axillare based on Bisset s. n. (K): Umzumkulu (?  Umzimkulu)). 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
193.5
6.5 cm; apex acuminate with acumen (10
) 13
17 (
20) mm long; base attenuate; leaf surface glabrous above and underneath; 308 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 lobes9elliptic, 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 and9campanulate 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. Key to the species of Mitriostigma 1. Monocaulous (i.e. single-stemmed) dwarf; inflorescences supra-axillary and paired at the nodes (Cameroon) ....... ................................................................. M. monocaule 1. Trees, shrubs or sub-shrubs; inflorescences placed unilaterally at successive nodes on alternate sides, sometimes axillary at the nodes (Africa, also Cameroon) ............. 2 2. Length of corolla lobes 1/3
1/2 of length of corolla tube ............................................................................. 3 2. Length of corolla lobes less than 1/3 of length of corolla tube ............................................................................. 4 3. Leaves elliptic to ovate; inflorescences placed unilaterally at successive nodes on alternate sides ........... M. axillare 3. Leaves obovate; inflorescences usually paired at the nodes ......................................................... M. greenwayi 4. Inflorescences supra-axillary; corolla tube ca 24 mm long; fruits 22
25 14
16 mm, ellipsoid (Tanzania) ............ ............................................................... M. usambarense 4. Inflorescences axillary; corolla tube 9
14 mm long; fruits 25
408
15 mm, sub-fusiform (Cameroon, Bioko) ... ....................................................................... M. barteri 309 310 Table 1. Morphological comparison of the Mitriostigma species, geographic distribution, altitudinal range and habitat. The characters distinguishing M. monocaule (sp. nov.) from the other species are indicated in boldface. M. axillare M. barteri M. greenwayi M. monocaule M. usambarense shrub glabrous 2.0
4.5 5
12 narrowly cuneate to attenuate Habit Indumentum on young twigs and petioles Height (m) Petiole (mm) Leaf base shrub or small tree glabrous shrub glabrous shrub or sub-shrub glabrous 0.3
2 (
8) (2
) 6
15 cuneate to rounded 0.3
1.0 3
10 cuneate to obtuse Leaf shape Leaf length (cm) Leaf width (cm) Secondary nerves Domatia Stipules (mm) Inflorescences elliptic to ovate 3
17 1
9 4
9 tuft domatia 5
10 axillary at alternate successive axils Number of flowers Corolla tube length (mm) Corolla lobe/corolla tube ratio Anthers Style Fruit colour Fruit shape Fruit size (mm) Distribution 3
10-flowered 12
18 1/3 (0.3
) 0.5
2 5
12 narrowly cuneate to attenuate narrowly elliptic 10
17 3
5 6
9 absent 4
10 axillary at alternate successive axils few flowered 9
14 B1/3 monocaulous dwarf sparsely hairy when young 0.2 0.3 13
18 attenuate obovate 6.5
22 3.0
10.5 7
9 absent, sometimes tuft domatia 7
15 (supra)axillary and paired (sometimes solitary) at the nodes few
15-flowered ca 20 1/3 obovate 11
19 3.5
6.5 8 12 absent 6.5
12 supra-axillary and paired at the nodes 2
10-flowered 10.5
12 B1/3 elliptic to obovate
elliptic 4.5
12 2.5
5.5 6
9 tuft domatia 6
9 slightly supra-axillary, at alternate successive axils 1
3-flowered ca 24 B1/3 included winged orange to red sub-fusiform 25
408
15 Bioko, Cameroon partially exserted winged green ? globose or pyriform 19
2510
15 southeast Kenya included winged orange ovoid to sub-fusiform 19
285
9 Cameroon included not winged orange to red ellipsoid 22
2514
16 northeast Tanzania 20
300 tropical rainforest 5
160 coastal forest, on coral rock or limestone outcrops 265 tropical rainforest 1500
2160 tropical mountain forest Altitudinal range (m) Habitat tips exserted winged orange ellipsoid to sub-globose 10
208
15 Mozambique to northeast Cape Province 0
450 coastal or riverine forest, usually close to the sea

Synopsis of other Mitriostigma species
Cultivated at NBGB: Robbrecht 841 (BR); Robbrecht 941 (BR); De Block 1408 (BR). Mitriostigma Hochst. (1842, p. 235 236)
Keay (1958, p. 41
42); Dyer (1975, p. 612); Bridson (1979, p. 113
130); Verdcourt (1987, p. 245
250). Type species: Mitriostigma axillare Hochst. Mitriostigma barteri Hook. f. ex Hiern (1877, p. 111) Based on the same type: Randia barteri (Hook. f. ex Hiern) K. Schum. (1891, p. 75). Mitriostigma axillare Hochst. (1842, p. 235 236) Lectotype (designated here): Equatorial Guinea, Bioko, Mann 234 (lectotype: K000419877 in K, isolectotypes: BR (photo), K, P). Lectotype (designated here): South Africa, Kwazulu Natal Province, Port Natal (Durban), Krauss 144 (holotype: B$, lectotype: K000419875 in K, isolectotypes: BM, BOL, K, TCD). Distribution
Taxonomic synonym: Gardenia citriodora Hook. (1857, Table 4987). Lectotype (designated here): South Africa, Kwazulu Natal Province, Port Natal (Durban), Gueinzius, s. n. (lectotype: K000419871 in K, isolectotypes: K, TCD). Distribution Zanzibar
Inhambane regional mosaic and Tongaland
Pondoland regional mosaic: Mozambique to northeast Cape Province (Fig. 4). Additional specimens examined Mozambique Cabo Delgado Province: Palma District, andados 4 km do Cabo Delgado para palma, Torre and Paiva 12113 (LISC). Inhambane Province: Inhambane, Sim 20871 (K, LISC photo, PRE). South Africa Kwazulu-Natal Province: ‘‘Natal’’, Gerrard 383 (K); ibid., Sutherland s. n. (K); Dukuduku forest, Forest Dept 12 (K); St. Lucia Bay, Evans 3613 (K); Umzumkulu (?  Umzimkulu), Bisset s. n. (K); Inanda, 3 miles north of Umhlanga Rocks, Moll 2994 (K); St Lucia Estuary, Pooley 1730 (K); Umzinto, along road Undoni park, Strey 5972 (K); Durban, Marloth 4186 (K); St Lucia, Lansdell 14 (K); Mapelane (Maphelane), du Toit 1266 (K); Stanger, Hawaan forest, northwest of Umhlanga Rocks, du Toit 1309 (K); Port Shepstone district, Port Edward, Strey 4924 (K). Eastern Cape Province: Port St Johns, Manteku, Dakane location, Strey 10202 (K); Port Saint Johns, Theron 1577 (K); Lusikisiki, Umsikaba (river) mouth, Codd 9724 (K); near Lusikisiki, on the property of Magwa Tea Corporation, Egoso forest above Magwa falls, Balwkill et al. 1916 (K); Lusikisiki, Magwa falls, Strey 6721 (K); Port St Johns, Ntsubane forest, Strey 8597 (K); south bank Msikaba river, 1.5 miles from the coast, Pondoland, Miller 2588 (K); Ntsubane, Fraser falls vallee, Venter 858 (K); Msikaba river mouth, Venter 974 (K); Port St Johns district, Mount Thesiger, Balkwill et al. 1758 (K). Lower Guinean regional subcentre of endemism: Bioko, Cameroon (Fig. 4). Additional specimens examined Cameroon Southwest Province: Bakingili, Akogo 150 (SCA); Upper Boando, Cable 265 (K, SCA); Etinde, Etuge 1242 (K); lower slopes of Mount Etinde (Small Mount Cameroon), ca 10 km west of Limbe, above the village of Batoke, Maurin et al. 6 (K); Moliwe, Hunt 95 (SCA); Dikulu, Jaff 87 (SCA); between Upper Boando and Etome, Kwangue 136 (K, SCA); Mabeta
Moliwe, Watts 174 (SCA); ibid., Sunderland 1469 (K), ibid., Tchouto 181 (K, SCA, WAG, YA); Moliwe, Watts 202 (SCA); ibid., Watts 420 (SCA); Mbonge village, Mambo and Thomas 150 (BR, MO, WAG); Etome, Nning 127 (SCA); Mabeta, Etome, Tchouto 1628 (SCA); Bolo forest on Kumba
Mamfe road, 50 km north of Kumba, Thomas and Nemba 5897 (K, MO, YA); mature (old secondary) forest in northeastern corner of Korup National Park, near Baro village, Thomas 3360 (K, MO, YA). Littoral Province: near Ndoktiba, Bafang-Yabassi road, 15 km southsoutheast Nkondjok, Letouzey 11168 (K, P, WAG, YA). Equatorial Guinea Bioko, Carvalho and Casas 3000-2 (K); ibid. Barter s. n. (K, syntype). Mitriostigma greenwayi Bridson (1979, p. 127) Type: Kenya, Kwale District, Jadini, Greenway 9639 (holotype: K, isotypes: EA, PRE). Distribution Zanzibar
Inhambane regional mosaic: southeast Kenya (Fig. 4). Additional specimens examined Kenya Coast Province: Mwarakaya, Brenan, Gillett, Kanuri and Chamba 14669 (BR, K, WAG); ibid., Luke and Robertson 311 2628 (EA, K); Kaya forest, Hawthorne 205, 247 (K); Kambe Kaya near Maereni village, Hawthorne 114 (K); Pangani, crossing of Lwandani stream on Chonyi-Ribe road, R. B. Faden, A. J. Faden, Gillett and Gachathi 77/531 (BR, K, MO, WAG); Monbassa, Kaya Diani, De Block, Muasya, Stieperaere and Bytebier 431 (BR, EA, MO); Kilifi District, Ribe Kaya Forest on Chonyi-Ribe road, R. B. Faden, A. J. Faden, Gillett and Gachathi 77/542 (K); Mleji river, Luke 4702 (K); Kaya Kambe, Robertson and Luke 4788 (EA, K); Kaya Diani, Robertson and Luke 5888 (EA, K); ibid., Robertson and Luke 5935 (EA); ibid., W. R. Q. Luke and P. A. Luke 9019 (EA, UPS). Mitriostigma usambarense Verdc. (1987, p. 245) Type: Tanzania, west Usambara Mountains, Mazumbai, Lovett, J. C. and Lovett, I. M. 171 (holotype: K, isotypes: BR, DSM, MO). Distribution Afromontane regional centre of endemism: mountain ranges of northeast Tanzania (Fig. 4). Additional specimens examined Tanzania Kilimanjaro region: same district, south Pare Mountains, Chome forest reserve, Festo, Gereau and Umila 94 (BR, MO); ibid., Kindeketa, Mwasumbi and Mlangwa 1302, 1302A, 1295 (BR, MO); ibid., Mwangoka, Mwasumbi and Umila 1085 (BR, MO); ibid., Mwangoka and Mwangulango 1104 (BR, K, MO). Tanga region: Balangai West Forest Reserve, southeast slope of Kilimandege, Borhidi, Demissew, Hedren and Iversen 84298 (K, VBI); ibid., Borhidi, Demissew, Hedrén and Iversen 84299 (NHT); west Usambara Mountains, Kwamshunde, Tanner 261 (K); Lushoto, Usambara Mountains, Balslev 302 (MO). Acknowledgements
The Board of Trustees of RBG Kew as well as the curators of BR, BRLU, SCA, WAG, and YA are acknowledged for providing herbarium material. Fieldwork leading to the discovery of Mitriostigma monocaule received the support of National Geographic, through its Committee for Research and Exploration (grant no. 8377-07). We would also like to acknowledge the ‘Sud Expert Plantes’ project under French Ministry of Foreign Affairs, for providing the opportunity for B. Sonké to work with S. Dessein. Professor E. Robbrecht is acknowledged for useful comments and checking the Latin diagnosis. A. Fernandez is acknowledged for preparing the drawing of Mitriostigma monocaule. F. Van Caekenberghe is thanked for operating the SEM. B. Sonké would also like to thank Dr P. Pete Lowry II and 312 other staff at Missouri Botanical Garden for their assistance in Cameroon. References Andreasen, K. and Bremer, B. 1996. Phylogeny of the subfamily Ixoroideae (Rubiaceae).
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