A genus of 2 species, herbs, of e. North America and Asia. The disjunct populations in e. North America and e. Asia have been variously treated as species, varieties, or races; following the analysis of Nie et al. (2006), I opt to recognize the continentally disjunct populations as being morphologically and genetically different enough (and with a long enough time since separation) to warrant specific status.
Ref: Barker et al. (2012); Cantino In Kadereit (2004); Lee et al. (1996); Nie et al. (2006a); Thieret (1972); Wagstaff & Olmstead (1997); Walker (2019) In Flora of North America Editorial Committee (2019). Show full citations.