American Mertensia in which he accepted only 24 species, far fewer than Macbride and Rydberg. M. pilosa was not mentioned anywhere in the monograph by Williams, not even as a synonym. Popov (1953) recognized the same 24 North American species as Williams as well as 14 species from Asia, including M. pilosa. Phylogenetic analysis of DNA data has shown that many of the species are polyphyletic, but no comprehensive revision of the Asian or of the North American species has been attempted since 1953. In 1967, one of the sections delineated by Popov was raised to the rank of genus as Pseudomertensia. This reclassification has been confirmed by molecular phylogenies which place Pseudomertensia closer to Myosotis than any of the genera that have been sampled so far. Evolution In the earlier infrageneric classifications of Mertensia, some of the groups were based on shared "primitive" characters rather than the derived character states that show true phylogenetic relationships. In Mertensia, as elsewhere, such groups have often proved to be paraphyletic. More recently, molecular phylogenetics has greatly clarified the ancestral and derived character states in Mertensia. Some of the traits evolving later have appeared independently as many as seven times. Ancestral states in Mertensia include short plant height (< 40 cm), long stamens (> 1.5 mm), filaments inserted higher in the corolla, calyces divided at least 2⁄3 of the way to the base, and acute to acuminate calyx lobe apices. Their derived alternatives are greater plant height (> 40 cm), short stamens (< 1.5 mm), filaments attached lower in the corolla, and calyces divided less than halfway to the base, and obtuse calyx lobe apices. The nutlets and pollen of Mertensia are nearly uniform and consequently, are not of much taxonomic value. There are no known fossils of Mertensia. A molecular clock analysis has estimated that Mertensia diverged from Asperugo in the late Oligocene or early Miocene. Asperugo and Mertensia do not closely resemble each other morphologically. Being mostly plants of subarctic climates, Mertensia spread southward and to lower elevation during periods of Pleistocene glaciation, then retreated northward and to higher elevation during interglacials. Mertensia originated in Asia and dispersed over the Beringian land bridge to North America. Most of the groups that originated in Asia are more diverse there, but Mertensia is a conspicuous exception in having most of its species in North America. Gallery References External links Mabberley's Plant-book Mertensia (Search)