Common name. Caltrops. Family Zygophyllaceae.
Habit and leaf form. Herbs; resinous, or not resinous. Plants succulent, or non-succulent. Annual, or perennial. Xerophytic (and often halophytic, in salt-deserts). Leaves alternate, or opposite (with one of each pair reduced); when alternate, spiral; ‘herbaceous’, or leathery, or fleshy, or modified into spines; subsessile; non-sheathing; compound; pinnate (leaflets in 3–12 pairs); paripinnate. Leaflets elliptic, or oblong; cordate, or oblique at the base, or rounded at the base. Leaf blades pinnately veined; cross-venulate. Leaves with stipules. Stipules free of one another; often spiny; persistent. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present, or absent. Stem anatomy. Secondary thickening developing from a conventional cambial ring, or anomalous.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Entomophilous.
Inflorescence and flower features. Flowers solitary (usually), or aggregated in ‘inflorescences’; axillary (or leaf-opposed, when leaves alternate). The terminal inflorescence unit cymose. Inflorescences axillary (to smaller leaf when leaves opposite), or leaf-opposed (when leaves alternate). Flowers pedicellate; ebracteate; ebracteolate; regular; 4 merous, or 5 merous; cyclic; tetracyclic, or pentacyclic, or polycyclic. Floral receptacle developing a gynophore, or with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk present; extrastaminal, or intrastaminal (nectary of 5 glands between sepals and petals, or 10 glands in an extra whorl between stamens and ovary); of separate members, or annular. Perianth with distinct calyx and corolla; 8, or 10; 2 -whorled; isomerous. Calyx present; 4, or 5; 1 -whorled; polysepalous; imbricate, or valvate; regular. Sepals ovate. Corolla present; 4, or 5; 1 -whorled; polypetalous; imbricate; regular; yellow. Petals ovate, or obovate. Androecial members definite in number. Androecium 8, or 10. Androecial members free of the perianth; free of one another; 2 -whorled. Androecium exclusively of fertile stamens, or including staminodes (the episepalous whorl sometimes sterile). Staminodes 4, or 5. Stamens 4–5, or 8–10; isomerous with the perianth, or diplostemonous; alternisepalous, or oppositisepalous. Filaments appendiculate, or not appendiculate. Anthers dorsifixed; versatile; dehiscing via longitudinal slits; introrse, or latrorse; tetrasporangiate. Gynoecium (2–)5 carpelled. The pistil 5–12 celled. Carpels isomerous with the perianth, or reduced in number relative to the perianth. Gynoecium syncarpous; eu-syncarpous; superior. Ovary plurilocular; 5–12 locular. Gynoecium stylate. Styles 1; attenuate from the ovary; apical; deciduous. Stigmas 1; 5 - lobed. Placentation axile. Ovules 1–5 per locule; pendulous; with ventral raphe; non-arillate; anatropous, or hemianatropous, or orthotropous, or campylotropous.
Fruit and seed features. Fruit non-fleshy (woody); a schizocarp. Mericarps 4, or 5. Seeds 2 or more per mericarp (2–5). Seeds endospermic, or non-endospermic. Endosperm oily. Cotyledons 2. Embryo straight, or curved.
Physiology, biochemistry. Mustard-oils present.
Etymology. From the Latin tribulus; an instrument of war which rested on three of its iron prongs while a fourth projected upwards, thrown on the ground to impede cavalry; refers to the resemblance of the carpel of the original species to such an instrument.