Phaeostemma E. Fourn. (Apocynaceae, Asclepiadoideae, Gonolobinae) is a South American genus of twining vines, distributed from southeastern Venezuela to Argentina. Heretofore, five names have been published in the genus — Phaeostemma brandonianum Silveira, Phaeostemma glaziovii E. Fourn., Phaeostemma grandifolia Rusby, Phaeostemma riedelii E. Fourn. and Phaeostemma tigrina Woodson—however, based on morphology, neither Phaeostemma grandifolia [syn. Matelea dasytricha (Schltr.) Woodson] nor Phaeostemma tigrina appear to belong to Phaeostemma. The latter species appear to belong to the Andean lineage Lachnostoma Kunth (Morillo 2012). Members of Phaeostemma s.l. are recognized by stems, leaves and inflorescences densely pubescent, with brown to yellowish-red medium to long (0.9–3 mm) eglandular trichomes, mixed with some shorter (0.5–1 mm) eglandular, multiseptate trichomes, and in some species short (0.15–0.4 mm) glandular trichomes, leaves membranous to coriaceous, often broadly ovate, ovate-elliptic, elliptic or widely oblong, medium to large in size (8–19.5 × 3.8–12 cm), bases shortly cordate, flowers large and broadly campanulate (corolla 23–37 mm in diameter; Fig. 1), corollas green to greenish-yellow, lobes ovate to deltate, sometimes reticulate, spreading, not ocellate, gynostegium stipitate, nectar chambers present, anthers subtriangular almost horizontal, radially prominent, pollinia narrowly or triangular pearshaped, and staminal corona fleshy, of 5 digitate lobes, partly adnate to the stipe and to the corolla tube. Phaeostemma is somewhat similar to Lachnostoma, but in Lachnostoma, the pubescence is usually shorter, with short to long (0.12–1.60 mm) eglandular trichomes, and rarely short (0.10–0.25 mm) glandular trichomes, and corollas usually smaller (12–20 mm in diameter; 26–34 mm in Lachnostoma uribei (Morillo) Morillo, inedit.), with lobes in natural position ovate-oblong to narrowly ovate-elliptic, longer than wide, partly due to recurved margins, in few species as long as wide, and bases narrowly campanulate or subtubular (Fig. 2). Species of Lachnostoma are known only from wet mountain forests (usually above 1400 m) from the Andes of Peru, Ecuador and Colombia to the Costal Range of northern Venezuela, whereas species of Phaeostemma are known mainly from Tropical Rain Forests or in Austral Forests with araucarias (mostly below 1000 m), from southeastern Guayana to northern Argentina.

Lachnostoma and Phaeostemma have been treated as nomenclatural synomyms of Matelea Aubl. sensu lato (Spellman and Morillo 1976; Morillo 1984; Fontella-Pereira et al. 1985), however, recent morphological studies (Morillo 2012), indicate that these genera differ from Matelea by the combination of dense, ubiquitous, long brown or yellowish-red pubescence in stems, leaves and inflorescences, corollas medium to large (12–37 mm in diameter), narrowly to broadly tubular, campanulate to subcampanulate, staminal corona segments (Cs) well-developed, connate to the base of the anthers, apically bifid or digitate, nectar chambers conspicuous, anthers radially prominent, with a concave outer face, retinacula broadly sagittate, and follicles (mature follicles unknown for Phaeostemma) ovate-oblong, with 5 wings and several conical projections. Members of Matelea s.s., in contrast, are recognized by stems glabrous to variously pubescent, pubescence in one or two lines, rarely ubiquitous, eglandular trichomes white or translucent, glandular capitate trichomes white, translucent, or with blackish capitula, corollas small to medium-size (less than 15 mm in diameter), usually rotate to subcampanulate, staminal corona segments (Cs) not distinct, usually appearing as ridges emanating from the central stipe, nectar chambers absent, retinacula narrowly sagittate or ovate-sagittate, and follicles narrowly ovate or fusiform, unwinged, costate, or 5-winged, conical projects absent.

On-going work for various regional projects, including the Flora of the Guianas and the Biological Diversity of the Guiana Shield, has resulted in the discovery of a new species of Phaeostemma: Phaeostemma surinamensis Morillo & Krings [initially misidentified as Matelea glaziovii (E. Fourn.) Morillo = Phaeostemma glaziovii E. Fourn (Morillo 1997)]. A more careful study of the known species of the genus resulted in the present recognition of its distinctness. The new species is morphologically similar to Phaeostemma fucata (Woodson) Morillo & Krings, an endemic to Ptari-tepui, a sandstone mountain in the Venezuelan Guayana. The new species is described and distinguished below, and the requisite new combination made.