The genus Borassus (Arecaceae) in West Africa, with a description of a new species from Burkina Faso

Borassus akeassii Bayton, Ouédraogo & Guinko sp. nov. (Arecaceae) is described as a new species from western Burkina Faso in West Africa. It has been confused with the widely distributed African species B. aethiopum and more recently with the Asian B. ﬂabellifer . However, it is distinguished by its glaucous, green leaves with weakly armed petioles and a characteristic pattern of lamina venation. The fruits have a pointed apex and are greenish when ripe, and the ﬂowers of the pistillate inﬂorescence are arranged in three spirals. The pollen has a reticulate tectum and distinctive ornamentation. The distribution of B. akeassii is discussed and the status of the varieties of Borassus aethiopum (var. bagamojensis and var. senegalensis ) is examined. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 419–427.


INTRODUCTION
The genus Borassus L. (Coryphoideae: Borasseae) contains between five and seven species of massive dioecious fan palms that are distributed across tropical Africa and Asia. Species delimitation is problematic and several taxa were described from limited herbarium material. These massive palms do not easily conform to conventional collection and storage protocols, and, in addition, a number of type specimens are missing. The most recent taxonomic account of the genus is that of Beccari (1924), who recognized seven species. However, subsequent studies suggest that not all of these can be maintained (Dransfield, 1986a;Bayton, Obunyali & Ranaivojaona, 2003).
The taxonomy of African Borassus has received only limited attention. Beccari (1924) recognized two species (plus two varieties), although these were reduced to a single species (with no varieties) by Dransfield (1986a). Modern treatments follow Dransfield and rec-ognize only B. aethiopum Mart. in mainland Africa (e.g. Tuley, 1995). However, in 1996, a description was published of a West African Borassus palm that differed morphologically from B. aethiopum , and that resembled the Asian B. flabellifer L., as yet unknown in Africa (Aké Assi & Guinko, 1996). Subsequently, more detailed accounts of the morphology of this palm in Burkina Faso were published (Ouédraogo, 1999;Ouédraogo et al ., 2002). In 2004, the present authors participated in a joint expedition in Burkina Faso, the purpose of which was to collect herbarium specimens of the native Borassus palms as part of an ongoing project by the first author to revise the genus Borassus across its geographical range. Inspection of living and herbarium material of the unidentified palm revealed several characters that distinguish it from both B. aethiopum and B. flabellifer , and it is described here as a new species.

MATERIAL AND METHODS
Herbarium material was kindly loaned by the following herbaria: Herbarium Jutlandicum of the Univer-sity of Aarhus (AAU), the Bailey Hortorium of Cornell University (BH), Fairchild Tropical Botanic Garden, Miami (FTG), and the Muséum National d'Histoire Naturelle, Paris (P). In addition, the collections at the Royal Botanic Gardens, Kew (K), and the Université de Ouagadougou (OUA) were consulted. Most of the measurements and other details that follow are based upon inspection of herbarium material. However, living material of Borassus was examined in situ in Burkina Faso and Kenya.
In Burkina Faso, herbarium material was collected from Borassus populations in both the east and the west of the country. Ouédraogo et al . (2002) identified the populations in eastern Burkina Faso as B. aethiopum , while those in western Burkina Faso comprise the unidentified Borassus species attributed to B. flabellifer (Aké Assi & Guinko, 1996;Ouédraogo et al ., 2002). Material was collected at several localities in the eastern provinces of Kompienga and Tapoa, and in the western province of Comoé. Precise collection localities are given in Ouédraogo et al . (2002). Herbarium material was collected according to the protocols of Dransfield (1986b). Ouédraogo et al . (2002) noted the preponderance of branched pistillate inflorescences in the western populations of Borassus in Burkina Faso. This contrasts with the generally spicate pistillate inflorescences found in most eastern populations. Therefore, 100 pistillate palms were examined in the east and west of Burkina Faso, respectively, to determine the relative abundance of this morphological character. Living material of B. aethiopum was examined and collected by the first author at a number of sites in Kenya, and details of these localities can be found in Bayton et al . (2003).
Pollen was extracted from herbarium specimens of staminate palms and was acetolysed according to standard methods (Erdtman, 1969). The acetolysed pollen was placed on the surface of a specimen stub and left to dry. The stub was coated with gold using a sputter-coater, and was examined using a Hitachi S-2400 scanning electron microscope (SEM).
Additional note. The dried leaves of B. akeassii exhibit a rather distinctive venation pattern compared with other Borassus species (see Figs 8-10). The spacing between the commissural veins is greater in B. akeassii (5-7 veins cm −1 ) than in either B. aethiopum (10-15 veins cm −1 ) or B. flabellifer (11-18 veins cm −1 ). Consequently, less support is provided to the surface layers of the leaf, and upon drying they collapse into the lacunae, producing a characteristic pattern of rectangular sunken pits. In comparison, the dried leaves of both B. aethiopum and B. flabellifer have a smooth texture and the lacunae are barely visible.
Distribution. Restricted to West Africa (Fig. 11). Aké Assi & Guinko (1996) reported that the palm is present in Benin, Burkina Faso, Côte d'Ivoire, Mali, Niger and Nigeria. Borassus sp. aff. flabellifer described by Arbonnier (2004) can be assigned to B. akeassii based on the description and photographs. In addition to the previous countries, it was recorded in Senegal and the Central African Republic. The range of B. akeassii may be much wider than suggested owing to misidentification of the palm as B. aethiopum. A fragmentary specimen at K that includes only a single mature leaflet (Liben 2822) has the same venation pattern as leaves of B. akeassii, but this specimen was collected in south-eastern Congo (Zaire), a considerable distance from the known localities for B. akeassii. In Burkina Faso, the major populations are in Comoé and Léraba Provinces in the south-west. However, a few specimens are found outside this area, including in the major cities of Ouagadougou and Bobo-Dioulasso.
Habitat and ecology. Sudanian and Sudano-Guinean savannas with 800-1100 mm annual precipitation, often in close proximity to human settlements. In Burkina Faso it is the common palm in the western provinces. Vast areas of land are populated with B. akeassii, with food and cash crops (particularly cotton) growing beneath the palms. Borassus akeassii is exploited in several ways, though wine production is the most significant of these. The apical bud is hollowed out and the resulting sap is collected and fermented. Endosperm from the seeds of immature fruits is harvested and consumed, and the leaves are collected for thatch and basketry. Borassus akeassii shows a preference for well-drained soils and is fireresistant (Arbonnier, 2004). Little is known about its ecology outside the sphere of human influence.
Conservation status. Borassus akeassii exists in close proximity to human settlements, and provides a large number of useful products. Therefore, there is a risk of over-exploitation, although in Burkina Faso the species is abundant. Populations of B. aethiopum (?) are threatened by over-exploitation in Senegal (Sambou, Lawesson & Barfod, 1992) and Guinea (Sambou et al., 2002). There is a lack of accurate information regarding the population size and geographical range of B. akeassii. Therefore, the Red List Category of Data Deficient (DD) is selected. With the identification of B. akeassii as a new species, it is hoped that more information on its distribution and frequency will be collected, allowing a re-assessment.
Etymology. This species is named in honour of Professor Laurent Aké Assi of the Université d'Abidjan in Côte d'Ivoire. Professor Aké Assi (together with SG) provided the first description of the palm which is named here (Aké Assi & Guinko, 1996). In addition, his overall contribution to West African botany cannot be overstated.

DISCUSSION
Borassus akeassii (Fig. 1) most closely resembles the Asian B. flabellifer, and has been likened to this species by previous authors (Aké Assi & Guinko, 1996; Ouédraogo et al., 2002). However, B. flabellifer is absent from Africa. Both taxa appear to be rather smaller in overall stature than B. aethiopum. In West Africa, this difference in stature has been attributed to the intensive harvesting of sap for wine production (Portères, 1964). Palm wine production in Burkina Faso entails hollowing out the apical bud, which causes massive damage to the palm.   Fig. 8. B. aethiopum (Bayton & Ouédraogo 59), Burkina Faso. Fig. 9. B. akeassii (Bayton et al. 69), Burkina Faso. Fig. 10 flower-bearing portion. The flowers are arranged in two spirals. Branching in the pistillate inflorescence is common in populations of B. akeassii. In a sample of 100 pistillate trees in western Burkina Faso, 47% of trees and 31% of inflorescences showed some degree of inflorescence branching, whereas in eastern Burkina Faso, inflorescence branching was not detected in B. aethiopum. This character has been cited as being of taxonomic significance (Ouédraogo et al., 2002). However, given the destructive nature of the tapping procedure used in Burkina Faso, it is possible that the branching occurs due to damage of the apical bud, and so this character should be treated with caution. It is worth noting that the illustration selected as lectotype for B. flabellifer (see Moore & Dransfield, 1979) shows a branched pistillate inflorescence, and branching also occurs rarely in pistillate inflorescences of B. aethiopum (Dransfield, 1986a). Borassus aethiopum is a common species in West Africa including Burkina Faso, where it is found in the eastern part of the country. It differs from B. akeassii in that its petioles are yellow, with large spines on the margins, even in mature plants. The lamina of B. akeassii can be distinguished from that of B. aethiopum even in herbarium material, as the dried leaves have a characteristic pattern of large sunken lacunae (see Figs 8-10). This character is found in all material of B. akeassii, but has also been found in one immature specimen of B. aethiopum collected in Kenya (Dransfield 4810). It seems unlikely that B. akeassii is present in East Africa, and mature leaves collected at the same locality can be readily assigned to B. aethiopum.
The fruits of B. aethiopum are large, yellow-orange at maturity and with a flattened apex. As in B. flabellifer, the pistillate inflorescence has a short rachis, and the flower-bearing portion is long. The pistillate flowers are arranged in two spirals. The staminate flowers of all Borassus species are arranged in cincinni, which are sunken in pits on the rachillae formed from imbricate rachilla bracts. When the staminate rachillae of B. akeassii were examined in the field, it appeared as though several male flowers were emerging from their pits simultaneously. This differs from the situation in other Borassus species where flowers emerge singly. However, this character is difficult to examine in herbarium material as the staminate flowers detach rapidly upon collection of the rachillae.
Aké Assi & Guinko (1996) noted that the stem of B. akeassii is more slender, and there are fewer leaves in the crown, when compared with B. aethiopum. Examination of living material has shown that the stems of both species become ventricose. However, most specimens of B. akeassii have a stem pitted with scars, revealing a long-term history of exploitation for wine production (Fig. 1). The overall appearance of the crown of leaves in B. akeassii is different to that in B. aethiopum, and indeed many of the herbarium specimens include mature leaves of rather small dimensions. However, with the harvesting of both leaves and sap widespread within populations of B. akeassii, these characters may be anthropogenic.
The pollen of tribe Borasseae was reviewed in 1986 but only two species of Borassus (B. aethiopum and B. flabellifer) were included (Ferguson, Havard & Dransfield, 1986). Although there appeared to be some slight differences in overall size and in the size of the gemmae, the small sample size made it impossible to assess the extent of variation within the two species. The present study demonstrates that pollen of both B. aethiopum and B. flabellifer (Figs 2,3,respectively) are rather similar, in terms of both size and ornamentation. However, B. akeassii (Figs 4-7) has distinctive pollen. The covering of gemmae on the tectum is so dense as to make the tectum itself almost invisible, whereas in the other two species, the gemmae are widely spaced. Close examination reveals that the tectum is reticulate in B. akeassii rather than finely perforate as in B. aethiopum and B. flabellifer. The diagnostic characters of B. akeassii are contrasted with those of B. aethiopum and B. flabellifer in Table 1.
The specimens used to typify the names of several Borassus species are missing. The type of B. aethiopum (Thonning 258) was collected in modernday Ghana, but is now lost (Hepper, 1976). This absence of type material could create a nomenclatural dilemma in the light of the recognition of two Borassus species in West Africa. However, modern herbarium material collected close to the type locality of B. aethiopum can be confidently assigned to that species.
The varieties of B. aethiopum (var. bagamojensis Becc. and var. senegalensis Becc.) were not recognized by Dransfield (1986a), on the basis that the pyrene characters which were used to define them vary considerably within the pyrenes from an individual tree. Numerous fruits develop on each infructescence, and each fruit may contain between one and three pyrenes. The number of pyrenes in a fruit and the position on the infructescence has a formative effect on the size and shape of the pyrenes, so these characters are insufficient to delimit taxa. In the absence of vegetative material, it is impossible to identify African Borassus pyrenes to species with confidence, and although it is appropriate that the varieties should be reduced to synonymy, the diagnostic characters are not sufficient to determine with which species they should be placed in synonymy (either B. akeassii or B. aethiopum). A third variety was described by Chevalier (in Chevalier & Dubois, 1938), although the absence of a Latin diagnosis renders the name invalid. This variety (var. domestica) had yellow-green fruits at maturity and could conceivably be synonymous with B. akeassii. The designated type was collected in Mali and comprises pyrenes only. Without leaf material, it is impossible to determine the true identity of this specimen.
Species delimitation in Borassus is problematic in large part owing to the difficulties associated with collecting material of these massive palms. In addition, many characters of taxonomic significance in the genus (and in palms in general) are of greater value in the field when examining living material than when restricted to herbarium specimens. The combination of pollen and gross morphological characters presented in Table 1 is sufficient to distinguish the Borassus species in West Africa. Molecular data not presented here also confirm the distinctiveness of this species and suggest affinities with species other than B. aethiopum or B. flabellifer (R. P. Bayton, W. J. Baker & C. E. Lewis, unpubl. data). The recognition of B. akeassii may stimulate future research, which could explore the distribution and conservation status of this extremely useful palm.