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. Anthers are with only one lobe and two pollen sacs . In fruit, the tepals are never conspicuously modified.
Photosynthesis pathway
The Polycnemoideae are all C3-plants. This is considered a primary character.
Distribution and evolution
The Polycnemoideae are distributed in the temperate regions of Eurasia, North America, and Australia. 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.
Systematics
The intrafamilial position of the Polycnemoideae has long been a matter of dispute. The taxon was recognized in 1827 as Tribus Polycnemeae within the family Chenopodiaceae by Dumortier . Later, it was treated as belonging to the family Amaranthaceae in 1849 by Moquin-Tandon. Oskar Eberhard Ulbrich raised it to subfamilial level in 1934, again within Chenopodiaceae. Today, both families are included in Amaranthaceae s.l. Phylogenetic research has revealed the Polycnemoideae represent a basal lineage of evolution within the family Amaranthaceae. The Polycnemoideae comprise only one tribe, Tribus Polycnemeae, with four genera and 13 species:
Hemichroa R.Br., with alternate, linear, succulent leaves, and stigmas papillous all around: only one species in Australia:
* Hemichroa pentandra
Nitrophila S.Watson - niterwort, with opposite leaves, and stigmas papillous only on the inside, 4 species in North, Middle and South America:
