and the Kuril Islands. It is the only species of Mertensia that is native to both Asia and North America. It resembles M. rivularis and might be closer to that species than to the other species of North America. It also resembles M. platyphylla and some authors have placed it in synonymy under Mertensia platyphylla variety platyphylla. It has not yet been sampled in a molecular phylogenetic study. With the possible exception of M. pilosa, North American Mertensia is a monophyletic group consisting of three clades that are known informally as the Pacific Northwest clade, the Southern Rocky Mountain clade, and the Central Rocky Mountain clade. These groups are named for the region where most of their species occur, but each includes species from well outside of that region. History In 1753, in his landmark Species Plantarum, Swedish botanist Carl Linnaeus placed five species in the genus Pulmonaria. Albrecht Wilhelm Roth, in 1797, separated what are now M. virginica, M. maritima, and M. sibirica from Pulmonaria to form the genus Mertensia, based on their smaller and differently structured calyx, their different anther position, and the presence of nectar glands on the inner surface of the corolla. Roth described one species as Mertensia pulmonarioides, apparently unaware that Linnaeus had already described it as Pulmonaria virginica. He thus created a superfluous synonym that has been a source of confusion ever since. Mertensia was named after the German botanist, Franz Carl Mertens. In the time since Mertensia was erected in 1797, it has been the subject of six major revisions. These, in chronological order, were done by George Don (George Don, the younger (G. Don), not George Don, the elder (Don)), Asa Gray, James Francis Macbride, Per Axel Rydberg, Louis Otho Williams, and Mikhail Grigorevich Popov. Asa Gray divided Mertensia into two sections: Stenhammaria and Mertensia. The section Stenhammaria was named for the Swedish naturalist and clergyman Christian Stenhammar, who is best known for his work in lichenology. Gray defined the section Stenhammaria as consisting only of the littoral species M. maritima, but in 2014, it was expanded to comprise 12 species. In 1886, Asa Gray described seven species in Mertensia. After Gray completed his monograph of Mertensia, many species were discovered by botanical expeditions in the western United States. Many of the species recognized by Macbride and Rydberg were later placed in synonymy by Louis O. Williams in his treatment of North