Tribulus terrestris L., puncture vine. Annual, taprooted, several—many–stemmed at base, matlike with radiating shoots often forked but not rooting at nodes, prostrate and trailing, < 5 cm tall; shoots strongly 2–dimensional (plagiotropic) with suntracking leaves, also closing upward in response to water stress, pubescent.
Stems cylindric, 2—3 mm in diameter, sometimes zigzagged, commonly swollen at base and nodes, greenish soon becoming brownish tan, tomentose and hispid.
Leaves opposite and alternate, even–1–pinnately compound, pairs of lateral leaflets (2—)4—6(—7), petiolate, with stipules; stipules 2, fused to lower stem and petiole at swollen node, awl–shaped to narrowly triangular on lower stem and lanceolate and crescent–shaped on upper side, (1—)1.5—5 mm long; petiole 4—7 mm long; rachis 10—30 mm long, 2–edged on upper side, tomentose and hispid; petiolules < 1 mm long; blades of leaflets oblong or elliptic to lanceolate, 5—10 × 2—5 mm, dull, ± asymmetric and oblique at base, entire, acute at tip, pinnately veined with midrib conspicuous and raised on lower surface, upper surface with hairs only along midrib, lower surface and margins sericeous, with irregular netlike of minor veins.
Inflorescence flowers solitary, axillary (opposite leaf at node when leaves alternate), lacking bracts; pedicel straight and slender, 4—9(—15) mm long.
Flower bisexual, radial, 6.5—10 mm across, dish–shaped; receptacle a low, broad disc; sepals 5, spreading, unequal, lanceolate to ovate, 3—4.2(—5) × 1.1—1.6 mm, green with membranous margins, acuminate or with short point at tip, upper surface glabrous, lower surface hairy, deciduous; petals 5, ± wedge–shaped, 2.5—4.7(—6) × 1.8—3.5 mm, bright yellow; nectaries 5, opposite sepals and at bases of outer stamens, rectangular or sometimes weakly 2–lobed, ca. 0.2 × 0.3 mm long, fleshy, green; stamens 10 in 2 whorls, free, attached to disc, unequal, sometimes the outer whorl having shorter filaments and larger anthers; filaments 2—3(—4) mm long, yellow; anthers versatile, dithecal, ± ellipsoid, 0.5—1.5 mm long, yellow, longitudinally dehiscent; pollen yellow; pistil 1, subtended by low, broad disc; ovary superior, (4—)5–lobed, green, lobes covered with erect, colorless bristles, lobes = chambers, each chamber with several ovules attached to center; style ± club–shaped, ca. 1.5 mm long, green; stigmas = (4—)5 grooved ridges, radiating from and fused at tip of style, ca. 1.3 mm long, densely and finely puberulent.
Fruit schizocarp, breaking into 4—5 dry, spiny mericarps (nutlets), each mericarp 2—3(—5)–seeded, boney and indehiscent, straw–colored; intact fruit body polygonal, ± 6—7 mm + 2(—4) spreading spines (prickles), with stout prickles on outer corners, the prickles to 7 mm long and diverging 70—130°, exposed facets of original fruit puberulent and with hard projections (from thick–based prickles) resembling short knobs and horns; inner edge acute with the 2 converging facets glabrous and without spination or other projections.
A. C. Gibson & B. A. Prigge