A total of 134 chromosome counts representing 21 taxa of the genus Arnica are presented. Counts are published here for the first time for A. lonchophylla (n = 38) and A. nevadensis (n = 38). Ten previously unreported counts representing 9 taxa are also presented. The basic chromosome number of the genus is x = 19. With respect to chromosome number, the genus exhibits maximum diversity in the Rocky Mountains of Colorado and Wyoming. Arnica angustifolia, A. cordifolia and A. mollis are recognized as mature polyploid complexes, containing several wide ranging polyploid races and only a few highly restricted or scattered diploid races. Within the genus in general, diploids tend to be restricted to unglaciated areas while polyploids are much wider ranging, particularly in unglaciated areas.

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The Minuartia rossii complex, which consists of M. austromontana, M. elegans, and M. rossii s. str., is confined primarily to Arctic–Alpine North America. Previously, the three taxa have been recognized at various taxonomic levels. However morphological, cytological, and phytogeographical data support the recognition of the taxa at the specific level. The basic chromosome number of the complex is x = 15, with M. rossii having 2n = 58 and 60, M. austromontana 2n = 30, and M. elegans both 2n = 30 and 2n = 60. With respect to reproductive biology, the complex as a whole exhibits pronounced protandry and is gynodioecious; however, M. rossii s. str. rarely flowers and relies almost exclusively on vegetative reproduction via bulbils. The three taxa have distinct geographical distributions: M. rossii. Wrangel Island, high Arctic North America, Greenland, and Spitzbergen; M. elegans. Eastern Siberia, Alaska, the Yukon, and south to Alberta; and M. austromontana, confined primarily to the front ranges of the northern and central Rocky Mountains. This distribution pattern is probably the result of Pleistocene survival of the taxa in three different areas: the Canadian Arctic Archipelago, Beringia, and south of the ice. The epithet columbiana frequently applied to the Rocky Mountain taxon is invalid. The name Minuartia austromontana S. J. Wolf et Packer is proposed for this taxon and it is herein described, having never previously been given formal taxonomic recognition.

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