Revision of the genus Sphenostylis ( Fabaceae : Phaseoleae ) in South Africa and Swaziland

A taxonomic revision of the genus Sphenostylis E.Mey. (tribe Phaseoleae) in southern Africa is presented. The genus is distinguishable by its characteristic wedge-shaped, dorsiventrally flattened style tip. Of the seven known species in the genus, only two, S. angustifolia and S. marginata , occur in South Africa and Swaziland; and the former is endemic to these countries. The two species differ in the size, shape, and venation of the leaflets; as well as the length of the petiole, peduncle, and pod. A key to the two species is provided and the correct nomenclature, typification, and known geographical distributions are given.


INTRODUCTION
Sphenostylis E.Mey.belongs to the subtribe Phaseolinae of tribe Phaseoleae (Fabaceae).It is named for the wedge-shaped, dorsiventrally flattened style (from the Greek word spheno = wedge).It is a small genus comprising seven species, all occurring in the tropics and southern parts of the African continent (Gillett et al. 1971;Potter 1992;Potter & Doyle 1994;Schrire 2005).The Indian species S. bracteata (Baker) Gillett, transferred from Dolichos L. to Sphenostylis by Gillett (1966), was moved to Nesphostylis Verdc.by Potter & Doyle (1994) based on a cladistic analysis of morphological data.Three species of Sphenostylis are used as food sources in Africa.Sphenostylis stenocarpa (Hochst.ex A.Rich.) Harms is known as African yam bean or girigiri (Burkhill 1995), and cultivars of this tropical African species are grown for their seeds and tubers in tropical and West Africa.The leaves, flowers, pods, and seeds of S. schweinfurthii Harms and S. erecta Hutch.ex Baker f. are wild-harvested.The roots of S. erecta are also used as fish poison (Potter 1992).

Sphenostylis appears to have a close relationship with
Nesphostylis (Potter & Doyle 1994) with which it shares the dorsiventrally flattened style tip; but it differs by features of the calyx (Lackey 1981), the standard petals, and the stamens (Potter 1992).In Nesphostylis, the inner surface of the calyx is pubescent, standard appendages are present, the base of the vexillary stamen is hooked, and the stamen apices are dilated.None of these characters are present in Sphenostylis.Based mainly on the narrow pods, several botanists including Harvey (1862), Bentham (1865) and Taubert (1894), relegated Sphenostylis into synonymy with the genus Vigna Savi (Gillett 1966).Harms (1899) reinstated the genus and expanded it to include related species previously placed in Dolichos and Vigna.Sphenostylis differs from these two genera by its distinctive style tip.Furthermore, Vigna generally has peltate stipules (stipules not peltate in Sphenostylis).In addition, a phylogeny based on molecular data shows that Vigna and Sphenostylis are not as closely related as previously thought, and that Sphenostylis is rather closer to Dolichos and Macrotyloma (Wight & Arn.) Verdc.(Wojciechowski et al. 2004).Sphenostylis can be distinguished from Dolichos, Macrotyloma, and Vigna by the absence of appendages on the standard petals (appendages present on the standard petals in the latter three genera).
Sphenostylis is represented in South Africa and Swaziland by two species, namely S. angustifolia Sond.and S. marginata E.Mey.; the former being endemic to the two countries.

MATERIALS AND METHODS
Plant material was studied mainly from herbarium specimens housed in JRAU, NH, NU, and PRE (acronyms after Holmgren et al. 1990); and also in the field.Habit affinities are described according to Mucina & Rutherford (2006).

Key to species of
Note: the Zeyher specimen in the Sonder Herbarium in S is chosen as lectotype because this is probably the specimen that Sonder used in his description.The twig on the left is chosen because it bears a flower and some immature fruits.Curiously Potter (in sched.)chose the Zeyher specimen in K as lectotype but this choice was apparently never published.
Diagnostic characters: Based on Potter & Doyle's cladistic analysis (1994), Sphenostylis angustifolia appears to have close affinities with S. zimbabweensis R.Mithen, which is restricted to the Highlands of Zimbabwe.The two species share the deciduous bracts and the persistent bracteoles.Sphenostylis zimbabweensis is, however, differentiated by the more prostrate growth form (as opposed to the erect growth form of S. angustifolia), and the broader, ovate to elliptic leaflets (leaflets narrower and oblong to linear-lanceolate in S. angustifolia).Sphenostylis angustifolia can be distinguished from S. marginata by the narrower leaflets and the much shorter petioles, peduncles, and pods (dimensions are given in the key).Note: although the P specimen which bears an original Drège label with locality details corresponding exactly to those given in the prognosis as well as Meyer's handwriting ('mihi') would have been more appropriately selected as lectotype, Potter & Doyle's (1994) designation of the K specimen as the holotype constitutes effective lectotypification (Art.9.8).
Diagnostic characters: Sphenostylis marginata differs from S. angustifolia in having broader leaflets and longer petioles, peduncles, and pods (see key for dimensions).Verdcourt (1970) divided what he called the Sphenostylis marginata complex, which included the two central and East African species S. erecta (Baker f.) Baker f. and S. obtusifolia Harms, into three subspecies, namely subsp.marginata (occurring only in South Africa and Swaziland), subsp.obtusifolia (Harms) Verdc., and subsp erecta (Baker f.) Verdc.However, based on morphological and DNA data, Potter & Doyle (1994) pro- FIGURE 1.-Sphenostylis marginata (A, B, D) and S. angustifolia (C, E-H): A, flowers showing the twisted standard petals; B, flower showing the cuneate style tip; C, keel tip and cuneate style tip; D, leaf in adaxial view; E, flowering branch (note the stipules and stipels); F, pod; G, flower in front view (note the twisted petals); H, seed.Scale bars: A, 8 mm; B, 12 mm; C, E-G: 5 mm; D, H: 2 mm.Photographs: A, B, D by David Styles; C, E-H by Ben-Erik van Wyk.