This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Annual much-branched herbs, more or less pubescent with flattened hairs, with opposite, usually unequal leaves, and axillary or lateral long-peduncled involucres subtending numerous pink or whitish flowers, the involucral bracts separate. Perianth-tube elongated, the limb 5-lobed. Stamens 5, inserted on the perianth-tube; filaments short. Fruit dry, leathery, completely encircled by 2 to 4 broad reticulated membranous wings. [Greek, three-winged calyx.]
About 5 species, natives of western North America, the following the generic type.
Tripteridium micranthum Torr. Frem. Rep. 96. 1845.
Tripterocalyx micranthus Hook. Kew Journ. Bot. 5: 261. 1853. [By typographical error macranthus.]
Annual, glabrous below, more or less glandular-pubescent above, stem ascending, branched, 1°-2° high. Leaves quite similar to those of Abronia fragrans in size and outline; involucral bracts ovate or ovate-lanceolate, acute or acuminate; flowers several or numerous, about 6" long, greenish-white; calyx-limb about 4" broad; fruit nearly 1' high, its 2-4 membranous wings 4"-7" broad, entire-margined, shining, very conspicuous, glabrous, united over the body of the fruit, beautifully reticulate-veined.
In dry soil, South Dakota to Montana, Nevada, Kansas and New Mexico. June-Aug.