Kallstroemia Scop. is a genus of roughly 17–20 species of flowering plants in the caltrop family, Zygophyllaceae, within the order Zygophyllales. The genus is closely allied to Tribulus and shares much of its floral and fruit architecture: flowers are typically yellow to orange with five petals, and the ovary is 10-locular with a style that elongates and hardens into a prominent columnar beak as the fruit matures. The fruit itself is a schizocarp that splits at maturity into approximately 10 one-seeded, tuberculate mericarps (half-carpels) that disperse individually. Sepals persist beneath the fruit, and seedling germination is epigeal.
All species are diffusely branched, prostrate to ascending annual or perennial herbs native to tropical and warm-temperate regions of the New World, ranging from the southwestern United States south through Mexico, Central America, and into South America, with greatest diversity in arid and semi-arid habitats. A few species (K. maxima, K. pubescens) extend into the Caribbean and West Africa as introduced or naturalised weeds.
Notable members include Kallstroemia grandiflora (Arizona poppy or orange caltrop), a showy annual of the Sonoran and Chihuahuan deserts valued in horticulture for its large orange flowers, and Kallstroemia hirsutissima, a common annual weed of disturbed ground across the southwestern United States. The genus was named in honour of A. Kallstroem, an 18th-century naturalist, by the Italian botanist Giovanni Antonio Scopoli.
Etymology
The genus name Kallstroemia honours A. Kallstroem, an 18th-century figure, as commemorated by the Italian botanist Giovanni Antonio Scopoli (Scop.) who formally described the genus. The common name "caltrop" (shared with the related Tribulus) refers to the spiny, multi-pointed mericarps that resemble the medieval weapon of the same name.
Distribution
Kallstroemia is native to tropical and warm-temperate regions of the Americas, with its centre of diversity in arid and semi-arid zones from the southwestern United States (Arizona, New Mexico, Texas, California) south through Mexico and Central America into South America. Several species (K. maxima, K. pubescens) occur as weeds in the Caribbean, West Africa, and other warm regions beyond the New World.
Ecology
Members of Kallstroemia are characteristic plants of disturbed ground, desert grasslands, roadsides, and waste places in arid to semi-arid climates. They are diffusely branched prostrate to ascending annuals or short-lived perennials that colonise open, sandy or gravelly soils. The tuberculate, spiny mericarps aid dispersal by adhering to animal fur and footwear, contributing to the weedy spread of several species.
