Syringantha coulteri (Hooker f.) T. McDowell, a New Combination, and Remarks on the Relationships of the Monotypic Mexican Genus Syringantha Standley (Rubiaceae) Tim McDowell Department of Botany, Duke University, Durham, North Carolina 27708, U.S.A. (author's e-mail address:
[email protected])ABSTRACT. Exostema coulteri Hooker f. is trans-ferred to the monotypic genus Syringantha Stan-dley, and the new combination Syringantha coulteri(Hooker f.) T. McDowell is made. Analysis of leaf,flower, fruit, and pollen characters indicates the af-finities of Syringantha are with the tribe Hame-lieae. A description, illustrations, and micrographsof pollen and leaves of S. coulteri (Hooker f.) T.McDowell are provided.HISTORY OF SYRINGANTHA Exostema coulteri Hooker f. was first published by W. B. Hemsley (1879), who attributed the spe-cies to J. D. Hooker based on a manuscript in the Kew herbarium. The specific epithet honors Tho-mas Coulter (1793-1846), an Irish botanist and cu-rator of the herbarium of Trinity College, Dublin, whose collections from central Mexico include the type for E. coulteri. The assignment of this species to the genus Exostema (Persoon) Bonpland is not discussed in the published description, but evi-dently was based on characters of the corolla, sta-men, capsule, and seeds. In particular, a more or less tubular corolla, basifixed linear anthers with filaments inserted near the base of the corolla tube, and a septicidally dehiscent bilocular capsule with winged seeds are characters shared with the genus Exostema. However, the fully exserted stamens, for which Exostema was named, are lacking in E. coul-teri, which has the stamens only partially exserted. Standley (1921) maintained E. coulteri in histreatment of Exostema for the North American Flo-ra, but noted "description compiled," and appar-ently did not see any specimens of this species. Inthe dichotomous key to the 26 species of Exostemaincluded in Standley's treatment, E. coulteri is theonly one with "Stamens included; corolla lobesshort, rounded." In 1930 Standley described the monotypic genusSyringantha Standley with one species, Syringan-tha loranthoides Standley, based on Mexican col-lections by Karwinsky on loan from the LeningradBotanical Garden herbarium. Standley stated, "Syr-ingantha is evidently a close relative of Exostema... but the corollas are conspicuously different,"noting that the short, broad and erect corolla lobesof Syringantha contrast with the long, narrow andrecurved lobes in Exostema. He overlooked Syrin-gantha's resemblance to the anomalous Exostemaspecies E. coulteri. Lorence recognized that Syringantha loranthoi-des is identical to Exostema coulteri and used the earlier name E. coulteri Hooker f. in his annotations of specimens and in a checklist of Mexican genera of the Rubiaceae (Lorence, 1990). This reduction of S. loranthoides to E. coulteri has been followed in a recent review of Rubiaceae classification by Robbrecht (1994). In the course of a revision of the genus Exostema I have examined the morphology and pollen struc-ture of E. coulteri and other Exostema species and have determined that E. coulteri should be excluded from Exostema. Differences between Syringantha and Exostema are summarized in Table 1. The syn-apomorphies for Exostema-the corolla with a nar-rowly cylindrical tube, ligulate recurved corolla lobes, and the anthers long exserted-are lacking in Syringantha (Fig. 1). Syringantha also lacks char-acters common to the genera closest to Exostema, notably Coutarea Aublet and Portlandia P. Browne (Andersson & Persson, 1991: Bremer, 1992), which include anthers with latrorse dehiscence and tricol-porate pollen with spinulose, foveolate exine (Fig. 2). Exostema and closely related genera lack several characters of Syringantha, such as caducous stip-ules, a subconical nectar disc, and apiculate stamen connectives. Moreover, Syringantha has abundant raphide bundles in stems, leaves, flowers and fruits (Fig. 3), which have not been previously reported and are absent from all Exostema species (pers. obs.). Consequently, the monotypic genus Syringan-tha is here maintained, and the new combinationSyringantha coulteri (Hooker f.) T. McDowell ismade for this species.NOVON 6: 273-279. 1996.
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