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 herbs, with erect forking stems. Leaves opposite, rarely alternate on the lower part of the stem, narrow, equilateral, not oblique at the base, entire; stipules gland-like, often obsolete. Involucres delicate, short-peduncled in the upper forks. Glands 5, broader than long, subtended by petal-like appendages. Capsules long-pedicelled, 3-lobed. Seeds terete, usually narrowed upward, more or less papillose, the caruncle sometimes wanting. [Greek, resembling Zygophyllum.]
1826. Zygophyllidium hexagonum Small, Fl. SE. U. S. 714.
Annual, yellowish green, glabrous or sparingly pubescent. Stem slender, striate-angled, erect, or assurgent, 4'-20' tall, branched, the branches ascending, often almost filiform; leaves opposite, linear,' oblong or lanceolate, very short-petioled, obtuse or acute, equilateral; stipules obsolete or very narrow; involucres solitary in the axils, often clustered, 1"-1 1/2" long, ciliate, pubescent, short-peduncled, with 5 glands subtended by triangular-ovate whitish or green appendages; capsule glabrous, 2" in diameter; seeds ovoid or oblong-ovoid, 1 1/2" long, terete, papillose.
Iowa to Montana, Texas and Colorado. Also at Wilmington, Del. June-Oct.