3 genera, see text
The Polycnemoideae are a small subfamily of plants in the Amaranthaceae family, representing a basal evolutionary lineage. The few relictual species are distributed in Eurasia and North Africa, North America, and Australia.
The subfamily Polycnemoideae comprises small herbs; some species are weakly lignified and grow shrublike. The subfamily is distinguishable from all other members of Amaranthaceae by normal secondary growth. The alternate or opposite leaves are often linear or subulate. The stomata of the leaves are arranged in parallel to the midveins.
The bisexual flowers are sitting solitary in the axil of a bract and two bracteoles. The inconspicuous perianth is formed of chartaceous, scarious, white or pinkish tepals. One to five stamens are present with their filaments united in a short but distinct filament tube (like in Amaranthoideae subfamily). Anthers are with only one lobe and two pollen sacs (bilocular, like in Gomphrenoideae subfamily). In fruit, the tepals are never conspicuously modified.
Distribution and evolution
Polycnemoideae began to diverge from Amaranthaceae s.str. in Eocene. At the edge from Eocene to Oligocene, the subfamily split into a lineage on the Northern Hemisphere, which was the ancestor of Polycnemum, and a lineage predominantly occurring on the Southern Hemisphere with the ancestors of Nitrophila, Hemichroa, and Surreya. An antarctic connection of these southern ancestors is assumed. The genus Nitrophila developed in South America and dispersed later to North America. The genera of the subfamily diversified during Miocene and Pliocene, with only a few rare species that seem to be relictual.
The intrafamilial position of the Polycnemoideae has long been a matter of dispute. The taxon was recognized in 1827 as Tribus Polycnemeae within the Chenopodiaceae family by Dumortier (In Florula Belgica). Later, it was treated as belonging to the Amaranthaceae family in 1849 by Moquin-Tandon (in Prodromus systematis naturalis.... Vol 13). Oskar Eberhard Ulbrich raised it to subfamilial level in 1934, again within Chenopodiaceae (in Engler & Prantl: Die natürlichen Pflanzenfamilien. Vol 16c). Today, both families are included in Amaranthaceae s.l.
- Hemichroa R.Br., with alternate, linear, succulent leaves, and stigmas papillous all around: only one species in Australia:
- Nitrophila S.Watson - niterwort, with opposite leaves, and stigmas papillous only on the inside, 4 species in North, Middle and South America:
- Polycnemum L., with alternate, subulate, non-succulent leaves, and stigmas papillous all around. 6 species in Eurasia (central and southern Europe, northwestern Africa, Central Asia):
- Surreya R. Masson & G. Kadereit, two species in Australia:
- Gudrun Kadereit, Thomas Borsch, K. Weising, and Helmut Freitag (2003): Phylogeny of Amaranthaceae and Chenopodiaceae and the evolution of C4 photosynthesis. - In: Int. J. Plant Sci. 164(6): p. 959–986.
- Rüdiger Masson & Gudrun Kadereit (2013): Phylogeny of Polycnemoideae (Amaranthaceae): Implications for biogeography, character evolution and taxonomy. Taxon 62 (1): 100-111. 
- Polycnemoideae, Germplasm Resources Information Network - (GRIN) Online Database
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