Nomenclatural changes in Onagraceae Author Hoch, Peter C. Missouri Botanical Garden, 4344 Shaw Blvd., St. Louis, Missouri 63110 - 2291, USA https://orcid.org/0000-0003-0754-0731 phoch@mobot.org Author Gandhi, Kanchi Harvard University Herbaria, 22 Divinity Ave., Cambridge, MA 02138, USA text PhytoKeys 2020 145 57 62 http://dx.doi.org/10.3897/phytokeys.145.51139 journal article http://dx.doi.org/10.3897/phytokeys.145.51139 1314-2003-145-57 03D4E388AEA153C7B33D25464476D52A Epilobium Linnaeus sect. Pachydium (Fischer & C.A. Meyer) Hoch & K. Gandhi comb. nov. Basionym. Oenothera sect. Pachydium Fischer & C.A. Meyer, Ind. sem. hort. petrop. 2: 45. 1836 [ "1835" ]. Boisduvalia [unranked] Pachydium (Fischer & C.A. Meyer) Endlicher, Gen. pl. 1191. 1840. B. subg. Pachydium (Fischer & C.A. Meyer) Reichenbach, Deut. Bot. Herb.-Buch. 170. 1841; Boisduvalia sect. Pachydium (Fischer & C.A. Meyer) Munz, N. Amer. Flora 5, II: 228. Type. Oenothera densiflora Lindley [= Epilobium densiflorum (Lindley) Hoch & P.H. Raven]. The distinctive, circumboreal/ circumpolar group commonly known as fireweeds has been treated either as a section of Epilobium ( Haussknecht 1884 ; Raven 1976 ; Chen et al. 1992 ) or as the separate genus Chamaenerion Seguier . Although the two groups share the distinctive comose seeds, several floral features, and a base chromosome number of x = 18, Chamaenerion differs from Epilobium in having leaves nearly always spirally arranged, rarely subopposite or verticillate near stem base (vs. opposite at least on proximal stem); lack of a floral tube (vs. more or less distinct floral tube); flowers slightly zygomorphic with subequal stamens that are erect, then deflexed, and styles that are deflexed, then erect (vs. actinomorphic with erect stamens in two series and erect styles); petals entire (vs. emarginate); and pollen shed in monads (vs. tetrads) ( Wagner et al. 2007 ). Recent molecular analyses ( Baum et al. 1994 ; Levin et al. 2004 ) also demonstrated that the fireweeds form a strongly supported clade sister to the rest of Epilobium ( Wagner et al. 2007 ). Holub (1972) argued that the correct name for the fireweeds at the generic level should be Chamerion (Raf.) Raf. ex Holub, not Chamaenerion , which he argued was illegitimate. However, as noted in personal correspondence between KG and Ulf Eliasson in 2009, and summarized by Sennikov (2011) , clarifications in the botanical code and in the lectotypification of Chamaenerion and Epilobium negate Holub's analysis, and Sennikov concluded that the correct and valid name for the fireweeds at the generic level is Chamaenerion Seguier . All but one of the eight species recognized in this genus have been treated at some point as species in Chamaenerion , the only exception being a species described in Epilobium that is endemic to the Himalayan region from Kashmir to Nepal and Xizang (Tibet), China ( Chen et al. 1992 ), for which the following new combination is provided: