Ann. New York Acad. Sci. 7: 170, 1892
We recognise two subspecies, I. amnicola subsp. amnicola and I. amnicola subsp. chiliantha (Hallier f.) J.R.I.Wood & Scotland, with the later presenting larger corollas. Intermediates between both subspecies also occur.
Subsp. amnicola has an amphitropical distribution; it is found in the southern United States and South America. In South America it is most common as a species of dry Chaco scrub near the Andes in western Argentina, western Paraguay and southern Bolivia but penetrates the Andean cordillera along dry river valleys. It also occurs in dry areas of northwestern Peru and neighbouring parts of Ecuador and in the upper Magdalena valley in Colombia. In the United States it is perhaps introduced and is most common in the Río Grande region of Texas. No records from Mexico have been traced.
Subsp. chiliantha presents a more restricted distribution to seasonally flooded swampy ground in Bolivia, Paraguay and the Brazilian Pantanal.
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