Vines (erect herbs), herbaceous; latex clear. Stems twining, sometimes only at tips [not twining], unarmed, minutely pilosulous with eglandular trichomes in decurrent lines from nodes to glabrate [densely pubescent or glabrous]. Leaves persistent, opposite, petiolate (sessile); stipular colleters apparently absent; laminar colleters present or absent. Inflorescences axillary, cymose, pedunculate. Flowers: calycine colleters present; corolla pinkish tan, pale reddish brown, or dark purple [cream, yellowish], campanulate to rotate, aestivation contort-dextrorse (nearly valvate); coralline corona absent; androecium and gynoecium united into a gynostegium adnate to corolla tube; gynostegial corona annular or of 1 whorl of 5 thick, laminar to prismatic segments; anthers adnate to style, locules 2; pollen in each theca massed into a rigid, vertically oriented pollinium, pollinia lacrimiform, joined from adjacent anthers by translators to common corpusculum and together forming a pollinarium. Fruits follicles, solitary or paired, pendulous, narrowly lance-ovoid to fusiform, smooth, glabrous. Seeds winged, not beaked, ovate or lanceolate, lenticular, comose, not arillate. x = 11.

Distribution

Introduced; Europe, Asia.

Discussion

Species ca. 28–35 (2 in the flora).

Vincetoxicum has been treated in many regional floras in North America as a synonym of a large and polyphyletic Cynanchum; however, these genera have been shown to be rather dis­tantly related (S. Liede and A. Täuber 2002; Liede et al. 2012). The circumscription adopted here corresponds in part to the Vincetoxicum clade of Liede et al. (2016). The later homonym Vincetoxicum Walter refers to distantly related species of Gonolobus and Matelea (subtribe Gonolobinae) and is a source of some taxonomic confusion because numerous American species of those genera were described under Vincetoxicum.

Vincetoxicum is well supported as belonging to the Tylophorinae K. Schumann (Asclepiadeae Duby). S. Liede et al. (2012, 2016) recommended uniting Vincetoxicum with Tylophora R. Brown and all other genera of Tylophorinae, except Pentatropis R. Brown ex Wight & Arnott, in order to resolve the non-monophyly of Tylophora and Vincetoxicum. This extreme proposal does not affect species in the flora area, but it seems prudent to exclude Tylophora from the synonymy of Vincetoxicum until a more considered approach to a revision of Tylophorinae is undertaken, weighing the advantages and disadvantages of broadly versus narrowly circumscribed genera.

Reports of Vincetoxicum hirundinaria Medikus [synonyms: Cynanchum medium (Decaisne) K. Schumann, not R. Brown, C. vincetoxicum, V. medium, V. officinale] are based upon plants not persisting outside cultivation or mis­application to plants of V. rossicum. Vincetoxicum hirundinaria was occasionally cultivated in the late nineteenth and early twentieth centuries at scattered sites in the northern United States and southern Canada, but none of the occurrences outside of cultivation appears to have persisted and no extant populations are known. Inconsistent recognition of the specific distinction between V. hirundinaria and V. rossicum, coupled with misapplication of the name C. medium to plants of V. rossicum in the Americas, has led to many erroneous reports of naturalized populations of V. hirundinaria in the flora area. For example, the concept of V. hirundinaria of H. A. Gleason and A. Cronquist (1991) combines attributes of both species. Vincetoxicum hirundinaria can be distinguished most readily from the two naturalized species in the flora area by the cream to yellowish cream corollas.

The two naturalized species in the region, Vincetoxicum nigrum and V. rossicum, are considered invasive and to be threats to native forest understory vegetation. Means to control these species are an active area of research (A. DiTomasso et al. 2005) and the continuing use of V. nigrum as a horticultural species should be curtailed. Monarch butterfly larvae (Danaus plexippus) do not complete development on V. nigrum or V. rossicum; however, there is equivocal evidence for monarch oviposition on Vincetoxicum (DiTomasso et al.). An unusual reproductive characteristic of many species of Vincetoxicum, shared by V. nigrum and V. rossicum, is polyembryonic seeds (DiTomasso et al.).

Selected References

None.

Lower Taxa

Vincetoxicum nigrum, Vincetoxicum rossicum

Key

1	Corollas dark purple, adaxially pilosulous to hispidulous, lobes deltate, ± as long as wide, apically planar, coronas annular, shallowly lobed; peduncles 0.5–1.5 cm; seeds 6–8 mm.	Vincetoxicum nigrum
1	Corollas pinkish tan to reddish brown, adaxially glabrous, lobes lanceolate, 1.5–2 times longer than wide, apically twisted, coronas of 5 prismatic segments united basally; peduncles 1.5–2.5 cm; seeds 4–6.5 mm.	Vincetoxicum rossicum