Isocarpha comprises five species ranging from southern Texas and the Bahamas to northern Peru and northeastern Brazil. The genus belongs in Eupatorieae rather than Heliantheae, but structure of the head of Isocarpha with its elongate, chaffy receptacle is anomalous within Eupatorieae. An hypothesis is presented suggesting the evolutionary derivation of this type of head through formation of floral buds in the axils of the involucral bracts. Generic ties of Isocarpha within Eupatorieae are unclear, but the closest affinities appear to be with Ayapana and relatives.



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