1 INTRODUCTION
Cynanchum L. is one of the largest genera of Asclepiadeae (Apocynaceae), and mainly distributed in Africa and Asia (Endress et al., 2018). It has been considered as a ”dustbin genus” due to its complex morphological characteristics, generally containing a gynostegial, at least basally fused corona (Liede & Kunze, 1993). The extent and classification of Cynanchum are controversial. Markgraf (1972) and Ali et Khatoon (1982) supported thatCynanchum should be divided into Cynanchum ,Vincetoxicum Wolf, etc. But this taxonomic revision was not supported by other scholars (Tsiang & Li 1977; Li et al, 1995). Woodson (1941) and Liede (1997) suggested that more than twenty allied genera were included in Cynanchum according to the characters of fused corona etc. More than ten genera,which included RaphistemmaWall., are merged into Cynanchum based on abundant molecular and morphological evidence recently (Khanum et al., 2016). ButVincetoxicum was separated from Cynanchum with the evidence of molecular, chemical and morphological characteristics (Qiu et al., 1989; Liede, 1996; Liede, 2001; Rapini et al., 2007). Therefore, the morphological diagnostic characters of Cynanchum were expanded as: corolla 3–10 (–40) mm long; gynostegial corona highly variable, usually with a ring of fused staminal and interstaminal parts, staminal parts occasionally with adaxial appendage; occasionally with additional free staminal lobe connate to ring (Endress et al., 2018).
In China, 57 species of Cynanchum were recorded in Flora of China (Li et al., 1995). According to the recent revised circumscription of Cynanchum and Vincetoxicum , 27 species of Cynanchum have been transferred to Vincetoxicum , and nine species of the allied genera have been included in Cynanchumwhich contain Adelostemma gracillimum (Wallich ex Wight) J. D. Hooker, Graphistemma pictum (Champ. ex Bentham) Benth. et Hook. f. ex Maxim., Holostemma ada-kodien Schultes, Metaplexis hemsleyana Oliv., M. japonica (Thunb.) Makino, Raphistemma hooperianum (Blume) Dec., R. pulchellum (Roxb.) Wall.,Sichuania alterniloba M. G. Gilbert & P. T. Li. (Khanum et al., 2016), Sarcostemma acidum (Roxb.) Voigt (Liede & Kunze 2002; Liede & Täuber 2002). Recently, two new species ofCynanchum were reported from China (Shen et al., 2019; Xu et al., 2021). There are 41 species of Cynanchum recorded in China by now.
We collected an unknown species in Ruili, Yunnan during field survey in 2020. This species has many typical characters of formerRaphistemma (included in Cynanchum ) with large and reniform leaves, large and campanulate corolla. We are unable to match it with any recorded species after the careful examination of the specimens and literature from China and neighboring countries, and the molecular phylogenetic results strongly support that it is an independent species within Cynanchum also. Therefore, we describe it here as new.