5. Tribulus Linnaeus, Sp. Pl. 1: 386. 1753; Gen. Pl. ed. 5, 183. 1754.
Caltrop [Greek treis, three, and bolos, a point, for a kind of caltrop, alluding to fruits resembling that ancient metal instrument with three or four spines arranged so that one always projects upward, thrown on the ground to stop cavalry a Caltrop [Greek treis, three, and bolos, a point, for a kind of caltrop, alluding to fruits resembling that ancient metal instrument with three or four spines arranged so that one always projects upward, thrown on the ground to stop cavalry and foot soldiers]
Herbs, usually annual, sometimes perennial [rarely subshrubs]. Stems prostrate to suberect, diffusely branched, terete, becoming ridged on drying, to 1[–3] m, somewhat succulent, densely hairy to glabrate. Leaves opposite, even-pinnately compound, one of each pair alternately smaller or sometimes abortive; stipules persistent, herbaceous, subulate to falcate, apex acute; petiolules very short to absent, less than 1 mm; leaflets 6–16(–20), opposite, distinct, oblong to ovate or elliptic, middle pairs largest, base oblique, apex acute or obtuse, surfaces hairy, terminal pair pointed forward. Pedicels emerging from axils of alternately smaller leaves, erect. Flowers solitary, regular; sepals caducous, 5, distinct, green, equal, margins scarious, apex acute, hairy; petals deciduous, 5, imbricate, spreading hemispherically, not twisted, usually bright yellow, rarely white, base often darker, oblong to obovate-cuneate, base not clawed, apex rounded to slightly lobed; nectary 10 glands in 2 whorls, outer whorl 5 ± 2-lobed glands, inner whorls 5 distinct or connate glands; stamens [5–]10, 5 opposite petals somewhat longer; filaments adnate at base to petals, subulate [filiform], unappendaged; anthers cordate to oblong-cordate or sagittate; ovary sessile, 5-lobed, 5-locular, hirsute with stiff, upward-spreading bulbous-based hairs; ovules 3–5 per locule; style deciduous; stigma terminal, slightly asymmetric, 5-lobed, pyramidal or globose, papillose. Fruits schizocarps, horizontally depressed, 5-angled, at maturity dividing septicidally into 5 or rarely fewer mericarps and leaving no central axis; mericarps broadly triangular, apex dorsally crested, tuberculate, spiny [or winged or rarely only tuberculate abaxially], each divided internally by oblique transverse septa into 2–5 1-seeded compartments. Seeds usually 3–5 per mericarp, white, oblong-ovoid. x = 6.
Species ca. 25 (2 in the flora): introduced; Mexico, West Indies, Central America, South America, s Europe, w, s Asia, Africa, Indian Ocean Islands, Pacific Islands, Australia; generally weedy occupants of dry disturbed habitats, several species widely naturalized in temperate and tropical areas of the world.
SELECTED REFERENCES Porter, D. M. 1968. The basic chromosome number in Tribulus (Zygophyllaceae). Wasmann J. Biol. 26: 5–6. Porter, D. M. 1971. Notes on the floral glands in Tribulus (Zygophyllaceae). Ann. Missouri Bot. Gard. 58: 1–5.