Common Name: Caltrop
Description: Annual or short-lived perennial herbs, usually hairy, mostly with elongated prostrate branches.
Leaves paripinnate with 3–12 pairs of ± oblong leaflets, opposite or alternate by abortion.
Flowers solitary, axillary, pedicellate. Sepals and petals 5, caducous. Petals yellow. Stamens 10 or rarely fewer by abortion. Nectariferous glands 5 between the calyx and corolla, 5 between the inner stamens and ovary. Ovary 5-locular, with 2–5 ovules per loculus (rarely 1 by abortion); stigma pyramidal to globose.
Fruit a schizocarp, separating into 5 (sometimes fewer by abortion) bony indehiscent usually spiny cocci [occasionally winged]; seeds without endosperm, separated by transverse or oblique septa that develop after fertilization.
Distribution and occurrence: World: c. 25 species, temperate to tropical regions of Europe, Africa, Asia, Australia. Australia: 11 species (9 species endemic), all mainland States.
Species can cause various disorders in sheep. The spiny fruits injure feet of animals and contaminate wool and stock-food.
Text by K. L. Wilson
Taxon concept:
| | Key to the species | |
| 1 | Stigma elongated, pyramidal; style no longer than stigma; spines on fruit well-developed; cocci 4–14 mm long | 2 |
| Stigma hemispherical or globose, occasionally elongated but then style relatively much longer; spines on fruit not well- developed; cocci 2–5 mm long | 3 |
| 2 | Fruit with numerous spines; cocci 8–14 mm long | Tribulus eichlerianus |
| Fruit with 2 upper and 2 lower spines; cocci 4–8 mm long
Back to 1 | Tribulus terrestris |
| 3 | Style longer than stigma, often 2.5 times as long; petals 5–13 mm long | Tribulus micrococcus |
| Style much shorter than stigma, often nearly absent; petals 3–4 mm long
Back to 1 | Tribulus minutus |
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