Review of Palaeobotany and Palynology 112 (2000) 189–205
www.elsevier.nl/locate/revpalbo
Pollen morphological survey of Pentas (Rubiaceae–
Rubioideae) and its closest allies
Steven Dessein a, *, An Scheltens a, Suzy Huysmans a, Elmar Robbrecht b,
Erik Smets a
a Laboratory of Plant Systematics, Institute of Botany and Microbiology, K.U.Leuven, Kardinaal Mercierlaan 92, B-3001 Leuven, Belgium
b National Botanic Garden, Domein van Bouchout, B-1860 Meise, Belgium
Received 27 January 2000; received in revised form 13 June 2000; accepted for publication 22 June 2000
Abstract
Pollen descriptions, based on LM and SEM observations, are provided for the Pentas complex, an African alliance
within the Hedyotideae–Spermacoceae alliance, which includes the genera Pentas, Otomeria, Batopedina, Parapentas,
and Chamaepentas. The close relationship among these genera is confirmed by pollen morphological data. The
observed variation in pollen character states agrees well with the existing genera and subgenera. The combination of
small spheroidal pollen grains with narrow, acute endocolpi, and a small apocolpium index distinguishes Batopedina
from Otomeria and Pentas. Parapentas differs from Batopedina in having broad endocolpi with vague ends.
Two of the three subgenera of Otomeria, Neotomeria and Volubilis, are supported by pollen data. Pollen of the
subgenus Otomeria, however, is less differentiated and is similar to pollen of Pentas subgenus Pentas. The exact
relationship between the genera Otomeria and Pentas remains unclear, and molecular data are needed to interpret the
observed morphological variation. Chamaepentas has pollen that is nearly identical to that of the Pentas subgenus
Chamaepentadoides and of the subgenus Megapentas, i.e. large spheroidal pollen grains with three, medium-length
colpi and endocolpi with acute ends. Pollen morphology provides additional support for a close affinity between
Pentas subgenus Phyllopentas and Pentas subgenus Vignaldiopsis. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: pollen morphology; Rubiaceae; sexine ornamentation; systematics
1. Introduction
Pentas Bentham comprises 39 species, distributed
in Africa, Arabia, Madagascar and the Comores.
Four other African genera are generally considered
closely related to Pentas: Otomeria Bentham (eight
species, widely distributed in tropical Africa),
Batopedina Verdcourt (three species, restricted to
central and West Africa), Parapentas Bremekamp
* Corresponding author. Tel.: +32-16-321536;
fax: +32-16-321968.
E-mail address: steven.dessein@bio.kuleuven.ac.be (S. Dessein)
(three species, distributed in tropical Africa), and
Chamaepentas Bremekamp (one species, an endemic
from Tanzania). The separation of these genera
from Pentas has been a matter of dispute
(Bremekamp, 1952; Verdcourt, 1953a,b,c, 1976,
1989; Hepper, 1960; Robbrecht, 1981). All these
genera were revised by Verdcourt (1953a,b,c).
From the founding of Otomeria by Bentham
(1849; fide Verdcourt 1953a), the close relationship
between this genus and Pentas has been noticed
by several authors, and some of them even proposed merging it with Pentas (e.g. Elliot, 1896;
and Bär, 1923; fide Verdcourt, 1953a). Verdcourt
0034-6667/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0 0 3 4 -6 6 6 7 ( 0 0 ) 0 0 04 1 - 5
190
S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
(1953a) recognized seven species in his revision of
Otomeria (divided in three subgenera) and confirmed its close affinity with Pentas; he especially
stressed the similarity between the subgenus
Neotomeria and Pentas subgenus Pentas section
Coccineae and between Otomeria micrantha and
Pentas herbacea. However, he maintained
Otomeria because he considered merging it with
Pentas as even less satisfying. The oblong fruit
shape and the spicate infructescence were used as
diagnostic features for Otomeria.
Parapentas and Chamaepentas—both erected
by Bremekamp (1952)—are also closely allied to
Pentas. They share articulate hairs of the external
indumentum, capsular fruits and colleter tipped
fimbriate stipules. They differ mainly from Pentas
in being straggling herbs, while Pentas is a more
or less erect herb or subshrub, often woody at the
base. A third genus, Tapinopentas, erected by
Bremekamp (1952), has been merged with
Otomeria by Hepper (1960).
Verdcourt (1953a) erected Batopedina. This
small genus only differs from Otomeria in its fewflowered inflorescence, depressed fruit apex and its
ericoid growth form. Verdcourt (1989) later
doubted the generic value of Batopedina.
Since the ‘‘Prodromus’’ of de Candolle (1830),
all classifications (Hooker, 1873; Schumann, 1897;
Verdcourt, 1958; Bremekamp, 1966; Robbrecht,
1988) included Pentas and its allies in the tribe
Hedyotideae (called Oldenlandieae by some of
these authors). Bremekamp (1952) stressed that
the knowledge of the Hedyotideae was still too
incomplete to divide it into subtribes, but he was
the first who explicitly recognized the close affinities between the above-mentioned genera of the
Pentas complex.
Verdcourt (1953c) proposed evolutionary
schemes for the members of the Pentas complex,
which are, however, merely based on an intuitive
interpretation of observations.
The homogeneity of the complex was also supported by the cytopalynological study of Lewis
(1965). He made a limited study of the chromosomes and pollen of African Hedyotideae and
recognized three groups, the Oldenlandia group,
the genus Manostachya and the Pentas group. The
latter (including Parapentas, Otomeria and Pentas;
Batopedina and Chamaepentas were not studied )
was characterized by relatively large chromosomes
with chromosome complements based on x=10
and small, 3–4-colporate pollen grains.
Bremer (1987) carried out a cladistic analysis
of morphological data and could confirm Louis’
Oldenlandia (as the ‘Hedyotis group’) and Pentas
group.
A later cladistic analysis by Bremer (1996),
based on rbcL sequences, included Pentas in the
Spermacoceae sensu lato. Her wide concept of the
Spermacoceae merges three former tribes,
Hedyotideae, Knoxieae and Spermacoceae. Within
this larger tribe, Bremer (1996) defined two groups:
a Hedyotis/Spermacoce group (including the
former tribe Spermacoceae sensu stricto) and a
Pentas group (the former tribe Knoxieae and the
genus Otiophora inclusive). The morphological evidence for these two large complexes is rather poor.
Andersson (1999) considered the recognition of
the Spermacoceae sensu lato as premature and
stated that ‘‘it may prove possible, and more
convenient, to split the heterogeneous Hedyotideae
into monophyletic entities’’ (p. 180). The alliance
of Pentas, Otomeria, Parapentas, Batopedina and
Chamaepentas, further called the ‘Pentas complex’,
can be considered as one of these monophyletic
units.
The purpose of the present paper is to provide
detailed pollen morphological data for the Pentas
complex to (1) test the generic value of Otomeria,
Batopedina, Parapentas and Chamaepentas and (2)
re-evaluate the subgenera and sections within
Pentas and Otomeria.
2. Material and methods
2.1. Methods
Pollen was acetolysed for 10 min in a heating
block at 90°C. Pollen for SEM was mounted on a
stub, air-dried and coated with gold. Observations
were made with a Jeol 6400 SEM. The glycerine
jelly slides have been observed with a Leitz
Dialux 20.
Polar axis (P) and equatorial diameter (E) were
measured in at least 10 mature pollen grains with
S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
LM (magnification 1000). Other measurements
were made on SEM micrographs.
Pollen grains were broken using glass beads, as
described by Huysmans et al. (1993). The terminology follows Punt et al. (1994).
2.2. Material
This study is based on herbarium material (BR)
of 54 specimens of 42 taxa; ‘Congo’ stands for the
Democratic Republic of Congo (ex Zaire). The list
below also indicates which specimens were used
for the illustrations.
Batopedina linearifolia (Bremekamp) Verdcourt var. glabra
Petit: de Witte 06615, Congo (BR 810054); de Witte 03379,
Congo (BR 810140), figs. 5, 12.
Batopedina linearifolia (Bremekamp) Verdcourt: Fanshawe
4143, Zambia (BR 810012).
Batopedina pulvinellata Robbrecht: Duvigneaud 3018R2,
Congo (BR 810097).
Batopedina tenuis (A. Chevalier ex Hutchinson & Dalziel )
Verdcourt: Geerling & Bokdam 1087, Ivory Coast (BR
901718).
Chamaepentas greenwayi Bremekamp var. glabra Bremekamp:
Greenway 6570, Tanzania (BR 810053).
Otomeria cameronica (Bremekamp) Hepper: Carvalho 2732,
Equatorial Guinea (BR 807353), fig. 19; Rodenburg 84,
Ghana (BR 810177).
Otomeria elatior (A. Richard) Verdcourt Dewèvre 494, Congo
(BR 810161); Quarré 2780, Congo (BR 809998), figs. 24,
29–30.
Otomeria guineensis Bentham: Bouharmont 6799, Congo (BR
807311); Pauwels 5899, Congo (BR 807370).
Otomeria lanceolata Hiern: Dacremont 154, Congo (BR
810123).
Otomeria madiensis Oliver: Dewulf 177, Congo (BR 901528),
figs. 3, 25.
Otomeria micrantha K. Schumann: Ankei 79/108I, Congo (BR
807347); de Wilde 7761, Cameroon (BR 810126), fig. 8.
Otomeria oculata S. Moore: Gillet & Faden 18257, Kenya (BR
810154), figs. 17, 26.
Otomeria volubilis ( K. Schumann) Verdcourt: Louis 296, Congo
(BR 810045), figs. 9, 13, 20; Troupin 2445, Congo (BR
810082).
Parapentas battiscombii Verdcourt: Polhill & Verdcourt 283,
Kenya (BR 810020).
Parapentas setigera (Hiern) Verdourt: Louis 8519, Congo (BR
810056); Louis 13330, Congo (BR 810173).
Parapentas silvatica ( K. Schumann) Bremekamp: Mabberley
1435, Tanzania (BR 809992), fig. 7.
Pentas angustifolia (A. Richard) Verdcourt: Bamps, Raimundo
& Matos 4027, Angola (BR 809990).
Pentas arvensis Hiern: Tisserant 3616, Cameroon (BR
810117), fig. 21.
191
Pentas bussei K. Krause: Oxtoby s.n., without locality (BR
810122), figs. 10, 22, 27–28.
Pentas caffensis Chiovenda: de Wilde & de Wilde-Duyfjes 6980,
Ethiopia (BR 809991), fig. 11.
Pentas cleistostoma K. Schumann: Luxen 404, Congo (BR
810058).
Pentas decora S. Moore: Chambon 12, Congo (BR 810010),
figs. 23, 31; Van der Ben 1805, Burundi (BR 810078).
Pentas elata K. Schumann: Richards 24623, Tanzania (BR
810090).
Pentas graniticola E.A. Bruce: Tanner 594, Tanzania (BR
810087).
Pentas herbacea (Hiern) K. Schumann: De Graer 712, Congo
(BR 810057).
Pentas hindsioides K. Schumann: pickled material Dessein 23
from specimen 19981824-18 cultivated at National Botanic
Garden of Belgium (origin Kenya), fig. 2.
Pentas hindsioides K. Schumann var. williamsii Verdcourt:
Shabani 308, Tanzania (BR 810083).
Pentas ionolaena K. Schumann subsp. madagascariensis
Verdcourt: Malcomber, Hutcheon, Razafimanantsoa &
Zjhra 1410, Madagascar (BR 810024).
Pentas lanceolata (Forsskal ) Deflers subsp. cymosa ( Klotzsch)
Verdcourt: Schlieben 11253, Comoro Islands (BR 810022),
figs. 1, 18.
Pentas longiflora Oliver: Greenway & Eggeling 8569, Tanzania
(BR 901618); Schlieben 4311, Tanzania (BR 810124).
Pentas longituba K. Schumann ex Engler: Eggeling 6811,
Tanzania (BR 810153).
Pentas micrantha Baker subsp. wyliei (N.E.Br.) Verdcourt:
Tweedi 1643, Tanzania (BR 810133), figs. 4, 16.
Pentas mussaendoides Baker: Phillipson 1613, Madagascar
(BR 810100).
Pentas nobilis S. Moore: Bidgood, Mbago & Vollesen 2275,
Tanzania (BR 810187).
Pentas parvifolia Hiern: Gillett 12797, Kenya (BR 810067).
Pentas pseudomagnifica M. Taylor: Schlieben 3379, Tanzania
(BR 901619), fig. 14.
Pentas pubiflora S. Moore: Graham 1105, Kenya (BR);
Reekmans 6450, Burundi (BR 901683).
Pentas purpurea Oliver: Tanner 3889, Tanzania (BR 810034).
Pentas schimperiana (A.Richard ) Vatke: Louis 4877, Congo
(BR 810023); Schlieben 945, Tanzania (BR 901617), fig. 32.
Pentas schimperiana (A. Richard ) Vatke subsp. occidentalis
(Hooker f.) Verdcourt: Alluaud 345, Afrique orientale
(BR 810031).
Pentas schumanniana K. Krause: Milne-Redhead & Taylor
9030, Tanzania (BR 810009), fig. 6.
Pentas zanzibarica ( Klotzsch) Vatke: Haerdi 370/0, Tanzania
(BR 810148); Smith 103, Tanzania (BR 810144).
3. Results: characters and variation
Several pollen characters with systematic value
within the Pentas complex are listed at the specimen level in Table 1.
Taxon
NC
E (mm)
P/E
A.I.
Ectocolpus Endoaperture
ends
Sexine
18–(22.2)–25
28–(29.2)–30
16–(20.2)–24
26–(27.0)–28
1.00–(1.10)–1.16
1.04–(1.08)–1.11
0.3
0.3
Rounded
Rounded
Microreticulate Granules in mesocolpium
Microreticulate Granules in mesocolpium
Fanshawe 4143
de Witte 03379
L
de Witte 06615
3
26–(28.3)–32
20–(24.1)–27
1.08–(1.17)–1.30
0.3
Rounded
Colpus, acute ends
Microreticulate Granules in mesocolpium
L
Duvigneaud 3018R2
3
14–(14.3)–15
13–(14.2)–15
1.00–(1.00)–1,08
0.4
Acute
Colpus, acute ends
Microreticulate Granules around apertures
L
Geerling &
Bokdam 1087
3
24–(26.6)–30
24–(26.6)–30
0.96–(1.00)–1.04
0.3
Rounded
Colpus, acute ends
Microreticulate Granules around apertures
Greenway 6570
3
41–(45.0)–50
38–(42.4)–48
1.00–(1.06)–1.11
0.5–0.7 Acute
Colpus with diffuse
(acute) ends
Microreticulate Granules on whole tectum
Otomeria subg. Neotomeria
Otomeria elatior
I
Dewèvre 494
3
35–(37.3)–40
27–(30.4)–33
1.15–(1.23)–1.30
0.2
Otomeria elatior
I
Quarré 2780
3
28–(32.0)–36
26–(30.0)–32
1.00–(1.07)–1.20
Otomeria madiensis
B
Dewulf 177
3
30–(31.4)–34
25–(26.2)–28
1.11–(1.19)–1.30
Otomeria oculata
L
Gillet & Faden
18257
3
26–(28.8)–30
20–(21.1)–22
1.27–(1.37)–1.45
0.2
Roundedacute
0.2–0.3 Roundedacute
0.4
Rounded
Endocingulum branched
in mesocolpium
Endocingulum branched
in mesocolpium
Endocingulum
Microreticulate Granules around apertures,
muri with irregular margins
Microreticulate Granules around apertures,
muri with irregular margins
Microreticulate Granules around apertures,
to perforate
muri with irregular margins
Microreticulate Granules around apertures,
to perforate
muri with irregular margins
Otomeria subg. Otomeria
Otomeria cameronica
Otomeria cameronica
Otomeria guineensis
Otomeria guineensis
Otomeria micrantha
Otomeria micrantha
Otomeria lanceolata
L
L
B
L
I
I
L
Carvalho 2732
Rodenburg 84
Pauwels 5899
Bouharmont 6799
Ankei 79/108I
de Wilde 7761
Dacremont 154
4
4
3
3
3–4
4
3
30–(31.9)–35
29–(26.8)–25
40–(41.0)–44
31–(32.5)–35
32–(36.1)–38
34–(37.8)–40
30–(31.4)–32
24–(27.7)–29
27–(25.3)–24
36–(38.0)–40
32–(29.4)–26
32–(34.4)–38
32–(33.1)–34
26–(26.7)–28
1.07–(1.15)–1.25
1.00–(1.06)–1.13
1.00–(1.05)–1.19
1.07–(1.11)–1.23
0.94–(1.06)–1.19
1.00–(1.15)–1.24
1.23–(1.17)–1.14
0.7–0.8
0.7
0.7
0.7–0.8
0.5–0.8
0.7–0.8
0.5–0.6
Acute
Rounded
Acute
Acute
Obtuse
Acute
Acute
Endocingulum
Endocingulum
Colpus
Endocingulum (branched)
Colpus, broad ends
Colpus
Endocingulum
Microreticulate
Microreticulate
Microreticulate
Microreticulate
Microreticulate
Microreticulate
Microreticulate
Granules around apertures
Granules around apertures
Granules around apertures
Granules on whole tectum
Granules covering whole tectum
Granules covering whole tectum
Granules around aperture
Otomeria subg. Volubilis
Otomeria volubilis
Otomeria volubilis
I
I
Troupin 2445
Louis 296
3
3
24–(24.3)–26
20–(21.8)–25
24–(24.3)–26
20–(21.3)–24
1.00
0.95–(1.02)–1.05
0.3
0.3
Acute
Acute
Endocingulum
Endocingulum
Reticulate
Reticulate
None
Granules around aperture
Parapentas
Parapentas battiscombii
B
28–(29.7)–32
26–(27.5)–30
0.97–(1.08)– 1.12 0.5
Acute
Broad colpus, vague ends Microreticulate Granules covering whole tectum
L
L
I
Polhill &
Verdcourt 283
Louis 13330
Louis 8519
Mabberley 1435
3
Parapentas setigera
Parapentas setigera
Parapentas silvatica
3
3
3
24–(25.3)–29
24–(24.8)–26
no accurate
measurements
21–(23.0)–25
22–(23.0)–24
no accurate
measurements
1.00–(1.1)–1.16
1.04–(1.08)–1.13
1?
0.5
Acute
0.5–0.6 Acute
0.5
Acute
Broad colpus, vague ends Microreticulate Granules around aperture
Broad colpus, vague ends Microreticulate Granules around aperture
Colpus, vague ends
Microreticulate Granules covering whole tectum
3
40–(42.0)–48
38–(41.0)–47
0.98–(1.02)–1.05
0.4–0.5 Rounded
Colpus
3
39–(42.8)–46
35–(40.0)–45
1.00–(1.09)–1.18
0.5
Colpus
Pentas subg. chamaepentadoides
Pentas hindsioides
I
Shabani 308
var. williamsii
Pentas hindsioides
I
Dessein 23
Obtuse
Rounded
Colpus, acute ends
Colpus, acute ends
SE
L
B
Chamaepentas
Chamaepentas greenwayi I 1
var. glabra
3
3
P (mm)
Endocingulum
Microreticulate Granules sparsely covering
whole tectum
Microreticulate Granules sparsely covering
whole tectum
S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
Batopedina
Batopedina linearifolia
Batopedina lineari
linearifolia var. glabra
Batopedina lineari
linearifolia var. glabra
Batopedina lineari
pulvinellata
B. tenuis
FM Collection
192
Table 1
List of species examined, with collecting data, flower morph (FM; B: brevistylous; L: longistylous; I: isostylous), number of colpi (NC ), length of polar axis (P), equatorial
diameter (E ), shape (P/E), apocolpium index (A.I.), colpus ends, endapertures, sexine, and supratectal elements (SE)
Pentas subg. Longiflora
Pentas decora
I
Chambon 12
4
56–(50.7)–58
42–(46.7)–50
1.00–(1.09)–1.17
0.5–0.6 Acute
Pentas decora
I
Van der Ben 1805
4
40–(46.0)–55
35–(42.3)–50
1.02–(1.10)–1.14
0.5
Pentas longiflora
I
3–4
38–(42.6)–46
38–(40.8)–44
1.00–(1.03)–1.10
Pentas longiflora
I
Greenway &
Eggeling 8569
Schlieben 4311
4
40–(42.1)–44
37–(38.3)–40
Tanner 594
3
58–(60.8)–64
Pentas subg. Megapentas
Pentas graniticola
I
0.7–0.8 Acute
Colpus-endocingulum
costa
Colpus-endocingulum
costa
Broad endocingulum
Perforatemicroreticulate
Perforatemicroreticulate
Microreticulate
1.05–(1.1)–1.16
0.7
Broad endocingulum
Microreticulate Granules are sparsely present
52–(56.3)–60
1.07–(1.09)–1.13
Colpus, vague ends
Microreticulate Granules covering whole tectum
Colpus
Colpus
Microreticulate Granules around aperturess
Microreticulate Granules sparsely present
to perforate
Microreticulate Granules covering whole tectum
Acute
Acute
No granules, interwoven muri
Granules sparsely present,
inerwoven muri
Granules around apertures
I
I
Eggeling 6811
Bidgood et al. 2275
3
3
70–(75.0)–80
44–(48.0)–52
60–(65.0)–70
42–(45.0)–50
1.07–(1.15)–1.20
1.00–(1.07)–1.14
Pentas pseudomagnifica
I
Taylor &
Schlieben 3379
3
69–(72.7)–74
60–(64.3)–70
1.07–(1.13)–1.16
Pentas subg. Pentas
Section Coccinea
Pentas bussei
Pentas parvifolia
L
L
Oxtoby s.n.
Gillett 12797
3
3
22–(23.8)–26
24–(25.3)–26
20–(20.6)–22
20–(20.6)–21
1.10–(1.15)–1.18
1.18–(1.23)–1.29
0.3
Rounded
0.4–0.5 Rounded
Colpus+fishtail ends
Colpus-endocingulum
Section Monomorphi
Pentas angustifolia
Pentas herbacea
Pentas micrantha
I
I
I
Bamps et al. 4027
De Graer 712
Tweedi 1643
3
3
3
20–(22.7)–24
22–(23.9)–26
26–(26.9)–28
20–(22.2)–24
20–(20.8)–24
24–(24.9)–26
0.91–(1.10)–1.04
1.08–(1.15)–1.25
1.04–(1.08)–1.13
0.3–0.4 Acute
0.4
Rounded
0.5
Rounded
Endocingulum (branched) Microreticulate Granules around aperture
Endocingulum
Microreticulate Around aperture
Colpus
Microreticulate Granules around apertures
Section Hirtistylus
Pentas caffensis
L
de Wilde & de Wilde
Duyfjes 6980
3–4
34–(36.1)–40
32–(35.5)–40
1.00–(1.02)–1.12
0.7
Acute
Endocingulum (branched) Microreticulate Granules present
Section Pentas
Pentas arvensis
Pentas cleistostoma
Pentas lanceolata
Pentas pubiflora
B
L
L
B
Tisserant 3616
Luxen 404
Schlieben 11253
Reekmans 6450
3
3
3
3
28–(29.1)–30
25–(27.0)–29
25–(25.9)–26
29–(31.6)–34
25–(28.3)–30
22–(23.5)–26
25–(25.3)–26
25–(27.5)–29
1.00–(1.05)–1.12
1.08–(1.15)–1.20
1.00–(1.02)–1.08
1.07–(1.15)–1.28
0.5
0.5–0.6
0.5–0.6
n.a.
Obtuse
Acute
Rounded
Acute
Endocingulum (LM )
Colpus
Endocingulum (branched)
Endocingulum
Pentas pubiflora
L
Granam 1105
3
25–(26.4)–27
22–(22.7)–24
1.13–(1.18)–1.28
Acute
Endocingulum (LM )
Pentas purpurea
Pentas zanzibarica
L
B
Tanner 3889
Haerdi 370/0
3
3
23.5–(26.0)–28.5 20–(21.9)–24.8 1.10–(1.20)–1.29
24–(26.2)–28
20–(21.4)–23
1.18–(1.21)–1.25
0.5–0.6 Obtuse
0.5
Rounded
Pentas zanzibarica
L
Smith 103
3
20–(21.6)–24
16–(19)–20
1.00–(1.14)–1.25
0.4
Malcomber et al.
1410
Phillipson 1613
4
19–(21.1)–22
17–(18.6)–20
1.10–(1.13)–1.20
0.5–0.6 Acute
Endocingulum (branched) Microreticulate Granules sparsely present
4–(5)
28–(29.5)–32
24–(27.0)–30
1.20–(1.11)–1.07
0.5
Endocingulum (branched) Microreticulate Granules sparsely present,
abundantly around apertures
Endocingulum (branched) Microreticulate Granules around apertures
Endocingulum (LM )
Endocingulum (branched)
Endocingulum
Endocingulum (branched)
Pentas subg. Phyllopentas
Pentas ionolaena subsp. L
madagascariensis
Pentas mussaendoides
L?
Rounded
Pentas schumanniana
L
Milne-Redhead
& Taylor 9030
4
26–(27.3)–30
22–(23.6)–25
1.12–(1.15)–1.20
Somewhat
rounded
0.5–0.6 Acute
Pentas subg. Vignaldiopsis
Pentas elata
Pentas schimperiana
Pentas schimperiana
Pentas schimperiana
subsp. occidentalis
L
B
L
L
Richards 24623
Louis 4877
Schlieben 945
Alluaud 345
4
3–4
3–4
4
32–(34.1)–36
24–(24.7)–27
26–(27.5)–30
27–(29.6)–32
30–(31.8)–34
22–(24.4)–27
26–(27.2)–30
25–(27)–28
1.13–(1.08)–1.13
0.96–(1.01)–1.09
1.00–(1.02)–1.08
1.00–(1.1)–1.20
0.7
0.5–0.6
0.5–0.6
0.5–0.7
Acute
Obtuse
Rounded
Obtuse
Colpus
Perforate
Granules around apertures
Microreticulate Granules around apertures
to perforate
Microreticulate
Microreticulate
Perforate
Microreticulate
perforate
Microreticulate
to perforate
Perforate
Microreticulate
Granules
Granules
Granules
Granules
throughout tectum
around aperture
around apertures
around apertures
Granules around apertures
Endocingulum
Granules around apertures
EndocingulumGranules covering whole tectum
colpus+fish tail
Endocolpus-endocingulum Microreticulate Granules around apertures
Microreticulate
Microreticulate
Microreticulate
Microreticulate
S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
Pentas longituba
Pentas nobilis
0.3–0.4 Acuterounded
n.a.m. Acute
0.4–0.5 Rounded
or blunt
0.5–0.6 Rounded
Granules covering whole tectum
Granules covering whole tectum
Granules around apertures
Granules around apertures
193
194
PLATE I
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S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
3.1. Polarity and symmetry
All species investigated have isopolar and radially symmetrical pollen.
195
or sub-quadrangular (Plate I, 3, 6), due to the
sunken colpi. All species investigated are
angulaperturate.
3.4. Apertures
3.2. Size
Pollen size in the Pentas complex varies from
small to large. P ranges from 14 to 80 mm respectively in Batopedina pulvinellata and Pentas longituba. E values are generally slightly smaller and
range from 13 to 70 mm. Most species have P and
E values in the range between 25 and 35 mm. In
some species, we noticed a considerable (more
than 10%) size variation within a given species. In
some cases, this variation could be correlated with
the infraspecific classification (e.g. Pentas schimperiana subsp. schimperiana: P=27.5; P. schimperiana
subsp. occidentalis: P=29.6) or with flower dimorphism (e.g. Otomeria guineensis, longi: 32.5; O.
guineensis, brevi: 41.0). In other cases (e.g. Pentas
decora Chambon 12: P=50.7; P. decora Van der
Ben 1805: P=46.0), the observed size variation
could only be explained in terms of a natural size
variation, which is probably correlated with
growth conditions. Such variation detracts from
the systematic value of these characters.
3.3. Shape
The majority of species has spheroidal pollen
(0.88<P/E>1.14); however, subprolate grains
(1.14<P/E>1.33) are also common. A remarkable infraspecific variation is observed in Otomeria
elatior, having both spheroidal and subprolate
pollen grains.
In a polar view, pollen of the Pentas complex
is circular (Plate I, 1–2, 4–5) or slightly triangular
The apertures in the Pentas complex are compound, i.e. consisting of openings on top of each
other in different layers of the wall.
Ectoaperture: All specimens investigated have
ectocolpi; the length and width vary considerably.
Most species have narrow ectocolpi, often not
exceeding 1 mm in width (Plate III, 13, 16–18).
Ectocolpus ends are mostly acute (Plate I, 3;
Plate III, 16) or rounded (Plate I, 1, 5; Plate III,
17, 18); in some species, both acute and rounded
ends are present. Rounded ectocolpus ends tend
to appear as acute when pollen grains are not
fully hydrated; this makes it difficult to distinguish
both character states.
Endoaperture: Endoapertures have an important
systematic value within Rubiaceae. In the Pentas
complex, two major types were observed: an
endocolpus and an endocingulum. The endocolpus ends are acute (Plate V, 27), blunt, only
vaguely delimited or with two diverging tips ( like
a fish tail, Plate V, 28, 32).
Small pointed extensions on the lateral sides of
the endocolpi were also observed (e.g. Pentas
bussei, Plate V, 27). In some species, endocolpi as
well as endocinguli were observed, suggesting a
trend from distinct endocolpi towards an endocingulum by fusion of the colpi (e.g. P. bussei, Plate V,
28). Endocinguli sometimes have extensions in the
mesocolpium (e.g. Otomeria elatior, Plate V, 29,
30). Fig. 1 illustrates the different endoapertures
of the Pentas complex and indicates the hypothesized relations between the character states. Only
PLATE I
Polar views (SEM ).
1. Pentas lanceolata, 3-colporate grain showing perforate tectum and ectocolpi with rounded ends.
2. Pentas hindsioides, 3-colporate grain showing microreticulate sexine and ectocolpi with acute ends.
3. Otomeria madiensis, 3-colporate grain showing perforate to microreticulate tectum and long ectocolpi with acute ends.
4. Pentas micrantha, 3-colporate grain showing irregularly shaped lumina and fastigium, ectocolpi with rounded ends.
5. Batopedina linearifolia, 3-colporate grain showing microreticulate sexine and ectocolpi with rounded ends.
6. Pentas schumanniana, 4-colporate grain showing microreticulate sexine, ectocolpi with acute ends.
196
PLATE II
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S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
Fig. 1. Hypothetical scheme (theoretical 2-aperturate grains,
broken line=ectoaperture) illustrating the different endoapertures within the Pentas complex. Arrows indicate the hypothesized relations between the observed types. A—endocolpi with
acute ends; B—endocolpi, the left one with a small extension;
C—endocolpi, the left one with a fish-tail end; D—endocingulum with fish-tail extensions in mesocolpium; E—endocolpus
with vague ends; F—endocingulum; G—endocingulum with
extensions in the mesocolpium.
Pentas decora (Plate V, 31) is characterized by the
presence of a costa (as defined by Huysmans
et al., 1998).
Where the ectocolpus and the endocolpus or
endocingulum overlap, a gap is present in the
pollen wall (in acetolysed grains). We did not
consider this gap as a mesoaperture because it
results from the wall thinnings of both the ectoand endoaperture. The gap is sometimes rounded,
being porus (Plate III, 15), or elongated (with the
length/breadth ratio exceeding 2) and lolongate
(Plate III, 14).
In general, pollen grains of the Pentas complex
are 3-colporate (Plate I, 1–5), though 4-colporate
grains are also present (Plate I, 6). Some species
(e.g. Otomeria micrantha, Pentas longiflora) have
both 3- and 4-colporate grains, mostly in equal
197
quantities. In Pentas mussaendoides, a small
number of 5-colporate grains were observed.
The colpus membrane always has granular elements, mostly small (Plate III, 13–14, 16), but
occasionally large and cube-shaped (Plate III, 15).
The apocolpium index (an indirect measure of
the length of the colpi) ranges from 0.2 (e.g.
Otomeria madiensis, Plate I, 3) to 0.6–0.8 (e.g.
Chamaepentas, Plate I, 2). There is limited infraspecific variation, which makes the character very
interesting from a systematic point of view.
A fastigium is present in Pentas micrantha
(Plate I, 4; Plate III, 16). In the apertural region,
the inner part of the exine is separated from the
domed sexine. In some species, there is a distinct
thickening of the colpus membrane around the
gap; this character, however, is only sporadically
met, even within one species; except for Otomeria
volubilis where an indistinct thickening is invariably present (Plate II, 9; Plate III, 13).
3.5. Sexine
The most common sexine pattern in the Pentas
complex is microreticulate (Plate IV, 19). In some
taxa (e.g. Otomeria subgenus Neotomeria, Pentas
subgenus Pentas section Pentas), there is a tendency towards a perforate tectum (Plate I, 1, 3;
Plate IV, 22, 25). The distinction between these
two states is often difficult to determine. Moreover,
the sexine pattern can differ between the apocolpium and mesocolpium region, resulting in pollen
grains that are both microreticulate and perforate.
Perforations or lumina vary from 0.1 to 1.5 mm in
diameter; the lumina are rounded or roundedangular. Reticulate pollen is only present in
Otomeria volubilis (Plate II, 9; Plate IV, 20). In
PLATE II
Equatorial views (SEM ).
7.
Parapentas silvatica, 3-colporate grain showing microreticulate sexine with granulate muri and lolongate porus.
8.
Otomeria micrantha, 3-colporate grain showing microreticulate sexine with smooth muri.
9.
Otomeria volubis, 3-colporate grain with reticulate sexine, narrow ectocolpus and indistinct annulus around porus.
10.
Pentas bussei, 3-colporate grain with mesocolpium showing variably sized lumina.
11.
Pentas caffensis, 4-colporate grain, mesocolpium showing elongated lumina, endocingulum visible through exine.
12.
Batopedina linearifolia, 3-colporate grain showing microreticulate sexine.
198
PLATE III
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S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
Pentas caffensis the lumina are more elongated
(Plate II, 11). Large lumina are often intermingled
with smaller lumina or with perforations (Plate II,
10; Plate IV, 19–20).
Two variations of the sexine pattern were
observed: (1) in Pentas decora, the muri are slightly
interwoven, resulting in a microreticulate to perforate tectum (Plate IV, 23); (2) in Otomeria subgenus Neotomeria, the muri (or perforations) have
irregular margins (Plate IV, 24–26).
In almost all species, the muri around the
apertures have minute granules. In some species,
the muri are granular throughout (Plate II, 7;
Plate IV, 21).
3.6. Columellae and nexine
All species are simplicolumellate, i.e. one row
of columellae is supporting the muri. The inner
nexine is always granular (Plate V, 32).
4. Discussion
4.1. Pollen morphology and heterostyly
Heterostyly is very common in the Pentas complex. It occurs in all genera, but not in all species
(e.g. heterostyly absent in members of Pentas subgenus Megapentas, and of subgenus longiflora, and
Otomeria elatior). Most heterostylous species are
distylous, but tristyly has been reported for Pentas
lanceolata (Bahadur, 1970). The taxonomic value
of heterostyly is rather poor, as stated by Verdcourt
(1958, p. 227): ‘‘complete heterostyly occurs only
in the Rubioideae but it is a character of limited
value. It may have arisen on numerous occasions,
independently which destroys the value of any
character’’.
199
Both morphs of Batopedina linearifolia var.
glabra, Otomeria guineensis, Pentas pubiflora,
Pentas schimperiana and Pentas zanzibarica were
investigated here. Pollen from the two morphs
only differs in pollen size and sexine ornamentation
( Table 1). According to Ganders (1979), pollen of
the brevistylous morph is generally larger than
pollen of the longistylous flowers. This was confirmed for B. linearifolia, O. guineensis, P. pubiflora, and P. zanzibarica. In P. schimperiana, pollen
of the brevistylous morph is slightly smaller than
that of the longistylous morph. It is very difficult
to draw any conclusion from these measurements,
since pollen size may also be influenced by the
growth conditions. In Otomeria volubilis, an isostylous species, the pollen size differs between the two
investigated specimens by more than 10%. In
assessing pollen differences associated with heterostyly, the two morphs should come from the same
population. Because of the known polyploidy of
the Hedyotideae (see Lewis, 1965), such a study
should also include chromosome counts.
Other pollen morphological differences, which
are often associated with heterostyly of Rubiaceae,
such as larger lumina in pollen from longistylous
flowers (Robbrecht and Puff, 1989), and scabrate
tecta in brevistylous flowers versus glabrous muri
in longistylous flowers, were not observed except
for Pentas schimperiana, where the brevistylous
morph had scabrate muri throughout, while in the
longistylous morph, scabrate muri are restricted
to the aperture region.
4.2. Taxonomic discussion of pollen data
Verdcourt (1958) and Lewis (1965) provided
the only pollen data available for the Pentas complex. Both studies are based on LM observations
PLATE III
Variation in apertures (SEM ).
13.
Otomeria volubilis, showing thickening of ectocolpus membrane around aperture, colpus membrane finely granular.
14.
Pentas pseudomagnifica, lolongate aperture.
15.
Chamaepentas greenwayi, porus, large cubic granules on colpus membrane.
16.
Pentas micrantha, fastigium around porus, ectocolpus with acute ends.
17.
Pentas oculata, ectocolpus with vague to rounded ends.
18.
Pentas lanceolata, ectocolpus with rounded ends.
200
PLATE IV
S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
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S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
Table 2
Summary of important character states of the Pentas complexa
Genus/subgenus
NC
PSi
P/E
A.I.
Sexine
Endoaperture
Batopedina
Chamaepentas
Otomeria subg. Neotomeria
Otomeria subg. Otomeria
Otomeria subg. Volubilis
Parapentas
Pentas subg. Chamaepentatoides
Pentas subg. Longiflora
Pentas subg. Megapentas
Pentas subg. Pentas
Pentas subg. Phyllopentas
Pentas subg. Vignaldiopsis
3
3
3
3
3
3
3
3
3
3
4
3
S
L
S-M
M
S
S
L
L
L
S-(M )
S
S-(M )
S-(Sp)
S
(S)–Sp
S-Sp
S
S
S
S
S-(Sp)
S-Sp
S-(Sp)
S
S
L
S
L
S
M
M
M-L
S-M
S-M
M
M-L
Microreticulate
Microreticulate
Microreticulate-perforate
Microreticulate
Reticulate
Microreticulate
Microreticulate
Microreticulate-interwoven muri
Microreticulate
Microreticulate-perforate
Microreticulate
Microreticulate
Colpus, acute ends
Colpus, acute ends
Endocingulum
Colpus or endocingulum
Endocingulum
Broad colpus, vague ends
Colpus, acute ends
Colpus to/or endocingulum
Colpus, acute ends
Colpus to/or endocingulum
Endocingulum
Endocingulum
or 4
and 4
(4)
and 4
a NC: number of colpi; Psi: pollen size (S<30 mm; 30<M>40 mm; L>40 mm); P/E: pollen shape (S: spheroidal; Sp: subprolate);
A.I.: apocolpium index (S<0.4; 0.4<M>0.6; L>0.6).
and treat only few species. Lewis (1965) confirmed
the relationship between Otomeria, Parapentas and
Pentas, as suggested by Verdcourt (1953b,c). The
results of the present study further corroborate
the close relationship between the genera of the
Pentas complex. Pollen grains of Batopedina,
Chamaepentas, Otomeria, Parapentas, and Pentas
are isopolar, radially symmetrical, 3- or
4-colporate; spheroidal to subprolate; microreticulate to perforate (rarely reticulate) and granules
are almost always present around the aperture.
The variation observed in size, number of colpi,
A.I., and endoaperture type coincides, to a large
extent, with the different genera and subgenera
( Table 2) recognized by Verdcourt (1953a,b,c).
Pollen grains of Batopedina and Parapentas can
be distinguished from those of Pentas: Batopedina
by the small pollen grains with a small A.I. and a
narrow endocolpus with acute ends (Plate I, 5),
and Parapentas (Plate II, 7) by its small pollen
grains with a broad endocolpus with vague ends.
In Otomeria, two subgenera have distinct pollen
types, which agree with Verdcourt’s subgenera
Neotomeria and Volubilis. Otomeria subgenus
Neotomeria has medium-sized 3-colporate pollen
grains. The ectocolpi are very long and relatively
broad with rounded ends. The lumina or perforations have ragged margins; the endoaperture is a
branched or unbranched endocingulum (Plate I, 3;
Plate IV, 24–26).
Otomeria volubilis, the only species of subgenus
Volubilis, has small, spheroidal, 3-colporate pollen
grains. The ectocolpi are long and small with acute
ends. The tectum is reticulate (Plate II, 9); the
endoaperture is an endocingulum.
Pollen of subgenus Otomeria is less distinctive,
medium-sized, and 3- or 4-colporate. The ectocolpus is relatively short, and the ends are acute. The
PLATE IV
Variation in sexine pattern (SEM ).
19.
Otomeria cameronica, microreticulate sexine with rounded-angular lumina, muri smooth.
20.
Otomeria volubilis, reticulate sexine.
21.
Pentas arvensis, microreticulate sexine with granulate muri.
22.
Pentas bussei, perforate tectum.
23.
Pentas decora, interwoven muri, lumina slit-like narrowed.
24.
Otomeria elatior, microreticulate to reticulate tectum with minute granules on the margins of the lumina.
25.
Otomeria madiensis, perforate tectum with mintue granules on the margins of the perforations.
26.
Otomeria oculata, microreticulate to perforate tectum, lumina irregularly shaped, margins of the muri ragged.
202
PLATE V
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S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
tectum is microreticulate, and the endoaperture is
a short colpus or an endocingulum. Otomeria
cameronica (recognized as Tapinopentas by
Bremekamp in 1952) differs from the other species
included in having four colpi. Otomeria micrantha
has both 3- and 4-colporate pollen grains. Pollen
of Otomeria micrantha, Otomeria guinensis and
Otomeria lanceolata is difficult to distinguish from
some pollen grains of Pentas subgenus Pentas
section Monomorphi, but they tend to be larger.
This palynological resemblance confirms the morphological evidence mentioned by Verdcourt
(1953c). However, the close affinity between
Otomeria subgenus Neotomeria and Pentas subgenus Pentas section Coccineae, suggested by habit
and flower morphology, is not further supported
by our data (cf. below).
Verdcourt (1953c) recognized six subgenera in
Pentas. The observed palynological variation confirms the value of at least some of these subgenera.
Pentas subgenus Megapentas has large
3-colporate spheroidal grains, large or mediumsized colpi with acute or rounded ends, and an
endocolpus with acute ends. The same pollen (but
slightly smaller) is found in Pentas subgenus
Chamaepentadoides, confirming the close relationship between these two subgenera, as proposed by
Verdcourt (1953c). This type of pollen also occurs
in Chamaepentas (Plate I, 2). Bremekamp (1952)
erected this monospecific genus based on its straggling habit, conspicuous colleters and ebracteate
inflorescences. Verdcourt (1953c, 1976) doubted
the generic value of the genus and pointed to an
affinity with Pentas hindsioides. Pollen features
further support the close relationship between
Chamaepentas and Pentas hindsioides. Although
there is morphological evidence for merging
203
Chamaepentas with Pentas, we will reconsider the
genus when our molecular data are available for
this group.
Pollen grains of Pentas subgenus Longiflora are
large and spheroidal. The two species investigated
differ remarkably: Pentas decora has an endocolpus
with a thick costa around the mesoaperture
(Plate V, 31); sometimes the endocolpi are fused
and form an endocingulum. Pentas longiflora has
an endocingulum with extensions into the mesocolpium. Pollen of this subgenus is most similar to
pollen of Pentas subgenus Megapentas. It differs
from the latter in the presence of four apertures,
shorter colpi (A.I. larger), and the fusion of the
endocolpi into an endocingulum.
Pentas subgenus Phyllopentas also has characteristic pollen grains. The grains are small and
4-colporate, and the endocolpi are fused to form
a branched endocingulum (Plate I, 6). This subgenus is characterized by enlarged, membranous
calyx lobes and is distributed in Madagascar and
East Africa, indicating that this genus could have
emerged relatively early in evolution, at a time
when Madagascar was still a part of Africa. Pollen
of Pentas subgenus Vignaldiopsis (widely distributed on the African continent) resembles the pollen
of subgenus Phyllopentas, but the grains are both
3- and 4-colporate (50/50) or exclusively
4-colporate (Pentas schimperiana subsp. occidentalis). Verdcourt (1953c; p. 248) stated that ‘‘It is
likely that Vignaldiopsis and Phyllopentas are
closely related despite the strong calyx differences
and the fact that only at the northern end of Lake
Nyasa do their distributions overlap’’. The two
subgenera also share the essentially montane distribution of the species.
Pollen grains of subgenus Pentas have no shared
PLATE V
Variation in endoapertures shown on broken pollen grains (SEM ). Two small triangles indicate the equatorial orientation of the
endoaperture.
27.
Pentas bussei, half pollen grain showing endocolpus with acute ends, small lateral extensions present (arrow).
28.
Pentas bussei, broken pollen grain showing fusion of an endocolpus with acute end with an endocolpus with fishtail ends.
29.
Otomeria elatior, broken grain showing endocingulum with lateral extensions in mesocolpium.
30.
Otomeria elatior, pollen grain showing endocingulum with regularly shaped, large extensions in mesocolpium.
31.
Pentas decora, costa around endoaperture.
32.
Pentas schimperiana, view of granular nexine and endocolpus with fishtail ends.
204
S. Dessein et al. / Review of Palaeobotany and Palynology 112 (2000) 189–205
features that distinguish it from the other subgenera. However, differences correspond very well
with the sections proposed by Verdcourt (1953c).
Section Coccineae has small, subprolate, microreticulate to perforate pollen with a variable endoaperture: ranging from a small endocolpus with acute
ends to a large endocolpus having the shape of a
fish tail and almost fused. No true endocingulum
was observed in Pentas bussei (Plate II, 10; Plate V,
27–28). Pollen grains of section Coccineae are
similar to pollen of subgenus Neotomeria, differing
only in their small size and the absence of muri
with irregular margins; thus, the close relationship
between the two taxa, as proposed by Verdcourt
(1953a), is not fully confirmed. Section
Monomorphi is characterized by small, spheroidal
3-colporate pollen grains with an endocingulum
(Pentas angustifolia, Pentas herbacea) or an endocolpus (Pentas micrantha). Pentas caffensis, the
only species from section Hirtistylus, has medium
sized, 3–4-colporate, microreticulate pollen
(Plate II, 11). The endocingulum is broad and
branched in the mesocolpium. Section Pentas has
small, subprolate (or more seldom spheroidal )
pollen with a tendency towards a perforate tectum
(Pentas lanceolata, Pentas pubiflora). The endoaperture is a branched endocingulum (Plate V, 29–
30) or a small endocolpus (Pentas cleistostoma).
Pollen of this section agrees well with that of
section Coccineae. Pollen of section Papillopilosa
was not investigated.
Phyllopentas, Megapentas, Longiflora, Hindsioides,
and Vignaldiopsis, have more or less characteristic
pollen. Pollen of subgenera Megapentas and
Chamaepentadoides, on the one hand, and
Phyllopentas and Vignaldiopsis, on the other hand,
strongly resemble each other and support the
relationships between these subgenera as proposed
by Verdcourt (1953c). Pollen of the genus
Chamaepentas is almost identical to that of Pentas
subgenus Chamaepentadoides and supports
Verdcourt’s (1976) view that the two genera probably should be merged.
Pentas subgenus Pentas and Otomeria subgenus
Otomeria have less differentiated pollen; pollen
features appear to have a scattered distribution
over these taxa. This indicates the close relationship between these two subgenera, but additional
data, both molecular and morphological, are
needed to support merging Otomeria subgenus
Otomeria with Pentas.
Acknowledgements
Technical support by Anja Vandeperre and
Marcel Verhaegen is gratefully acknowledged. We
are also grateful to Dr. W. Punt and an anonymous
reviewer for their detailed comments. This study
was supported by the National Fund for Scientific
Research—Flanders (F.W.O.)—project numbers
2.0038.91 and G.0143.95 and by a grant form the
Research Council of the K.U.Leuven (OT/97/23).
5. Conclusion
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