Nordic Journal of Botany 27: 305312, 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, BE1860 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 812 (versus 69)
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, 2030 cm tall, internodes very
short. Stems 36 mm in diameter; bark brown, sparsely
hairy when young, becoming glabrous with age. Stipules
persistent, ovatetriangular, at the base protruded into an
awn, 6.512 mm long (including 46 mm long awn),
34 mm wide at the base, puberulous outside. Leaves
decussate; petiole 1318 mm long, sparsely hairy when
young; leaf blades obovate, 11193.56.5 cm; apex
acuminate with acumen (10) 1317 (20) mm long;
base attenuate; leaf surface glabrous above and underneath;
308
mid-vein prominent below, slightly prominent above;
secondary veins brochidodromous, prominent below,
812 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, 210-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, 0.61.2 mm long, very sparsely pubescent
or glabrous. Corolla campanulate, creamwhite with pink
stripes and dots; corolla tube 10.512.0 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 0.10.2 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 59 mm, orange at maturity, glabrous. Seeds 49
per fruit, 5.37.3 3.25 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 1/31/2 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 2225 1416 mm, ellipsoid (Tanzania) ............
............................................................... M. usambarense
4. Inflorescences axillary; corolla tube 914 mm long; fruits
2540815 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
2.04.5
512
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
0.32 (8)
(2) 615
cuneate to rounded
0.31.0
310
cuneate to obtuse
Leaf shape
Leaf length (cm)
Leaf width (cm)
Secondary nerves
Domatia
Stipules (mm)
Inflorescences
elliptic to ovate
317
19
49
tuft domatia
510
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
310-flowered
1218
1/3
(0.3) 0.52
512
narrowly cuneate to
attenuate
narrowly elliptic
1017
35
69
absent
410
axillary at alternate
successive axils
few flowered
914
B1/3
monocaulous dwarf
sparsely hairy when
young
0.2 0.3
1318
attenuate
obovate
6.522
3.010.5
79
absent, sometimes tuft domatia
715
(supra)axillary and paired (sometimes
solitary) at the nodes
few15-flowered
ca 20
1/3
obovate
1119
3.56.5
8 12
absent
6.512
supra-axillary and
paired at the nodes
210-flowered
10.512
B1/3
elliptic to obovateelliptic
4.512
2.55.5
69
tuft domatia
69
slightly supra-axillary, at alternate
successive axils
13-flowered
ca 24
B1/3
included
winged
orange to red
sub-fusiform
2540815
Bioko, Cameroon
partially exserted
winged
green
? globose or pyriform
19251015
southeast Kenya
included
winged
orange
ovoid to sub-fusiform
192859
Cameroon
included
not winged
orange to red
ellipsoid
22251416
northeast Tanzania
20300
tropical rainforest
5160
coastal forest, on coral rock or
limestone outcrops
265
tropical rainforest
15002160
tropical mountain forest
Altitudinal range (m)
Habitat
tips exserted
winged
orange
ellipsoid to sub-globose
1020815
Mozambique to northeast Cape
Province
0450
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. 4142); Dyer (1975, p. 612); Bridson
(1979, p. 113130); Verdcourt (1987, p. 245250).
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
ZanzibarInhambane 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); MabetaMoliwe, 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 KumbaMamfe 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
ZanzibarInhambane 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.
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