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..
Herbs, shrubs, or some tropical species trees, the branches often jointed to the nodes. Leaves mostly opposite, stipulate, pinnate, or 2-3-foliolate, the leaflets entire. Stipules persistent. Flowers perfect, axillary, peduncled. Sepals usually 5, distinct, or united by their bases. Petals the same number as the sepals, or none. Stamens as many as the petals, or 2-3 times as many, inserted on the base of the receptacle, the alternate ones sometimes longer; anthers versatile, longitudinally dehiscent; filaments usually with a small scale at the base or near the middle. Ovary 4-12-celled; style terminal; stigma usually simple; ovules I-numerous in each cavity, pendulous, or ascending. Fruit various, dry in our species. Endosperm of the seed copious or none; embryo straight or curved; cotyledons linear or oblong.
About 20 genera and 160 species, widely distributed in warm and tropical regions.
Fruit spiny, splitting into 5 3-5-seeded segments.
Fruit not spiny, often tubercled, splitting into 10-12 1-seeded segments.
1. TRÍBULUS [Tourn.] L. Sp. Pl. 386. 1753.
Herbs, mostly diffuse or prostrate, with evenly pinnate stipulate leaves and peduncled axillary yellow flowers. Sepals 5, deciduous. Petals 5, deciduous. Stamens 10, hypogynous, the alternate ones somewhat longer. Ovary sessile, 5-lobed, 5-celled, hairy; disk 10-lobed; style short; stigma 5-ridged; ovules 3-10 in each cavity, pendulous. Fruit 5-angled, spiny, splitting into five 3-5-seeded segments. [Greek, three-pronged, Caltrop, from the resemblance of the fruit to that implement.]
About 12 species, natives of warm and tropical regions. Besides the following typical one, another occurs in the Southern States.
Tribulus terrestris L. Sp. Pl. 387. 1753.
Annual, pubescent, branched from the base, the stem prostrate or ascending, sometimes 1° long or more. Leaves petioled; stipules small; leaflets 4-8 pairs, oblong, inequilateral, opposite, short-stalked, acutish or obtuse, 3"-8" long; flowers solitary, about 6" broad; peduncles shorter than the leaves; petals oblong, about as long as the sepals; segments of the fruit usually with 2 long spines, 2 shorter ones, and a row of very short ones forming a crest on the back, also commonly with some bristle-like hairs.
In ballast and waste places about the eastern seaports, and from Illinois to Nebraska, Arkansas, Arizona and Mexico. Adventive from Europe. June-Sept.