Polycnemoideae is a poorly studied subfamily of the Amaranthaceae/Chenopodiaceae alliance which contains ca. 13 rare species in three disjunctly distributed genera. We present the results of a dated molecular phylogeny and a survey of morphology, micromorphology and stem anatomy revealing
the phylogenetic relationships and historical biogeography of the subfamily as well as its morphological evolution. The results are converted into a new classification of the group. The divergence of Polycnemoideae from Amaranthaceae s. str. dates back to the Eocene and the oldest split in
the subfamily separated a Northern Hemisphere lineage (Polycnemum) and a predominantly Southern Hemisphere lineage (Nitrophila, Hemichroa, Surreya gen. nov.) around the Eocene/Oligocene boundary. There is some evidence for a southern connection via Antarctica in
the ancestor of the Nitrophila/Hemichroa/Surreya clade. The amphitropical genus Nitrophila likely originated in South America and reached North America via long-distance dispersal. Genera of Polycnemoideae diversified during the Miocene and Pliocene. They are species-poor
and appear relictual. The morphological and micromorphological survey revealed a number of useful diagnostic characters which permit a clear definition of all four genera and 13 species recognized here. Hemichroa in its traditional circumscription lacks diagnostic characters. Hemichroa
pentandra is much more similar to Nitrophila and shares a number of symplesiomorphies with it, while H. diandra and H. mesembryanthema, which form a highly supported clade, show several clear synapomorphic characters. Therefore the latter two species are here described
within a new genus, Surreya R. Masson & G. Kadereit.
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