Notes on the Ragweeds of South America with the Description of Two New Species: Ambrosia pannosa and A. parvifolia (Compositae)

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Abstract
The approximately 39 American spp. of Ambrosia are divided between 2 regional floras; (a) 31 spp. native to North America, and (b) 8 spp. native to South America. The South American assemblage is comprised of 2 subgroups of evidently related plants. The larger of these, with 5 spp. (2 of which are the spp. newly described), is morphologically primitive and has little close relationship with North American taxa. The smaller, with 3 spp., appears to be closely related to the most advanced ragweeds of North America. It is suggested that the former may have been derived from forms which migrated to South America during an early phase of ragweed evolution and phylogeny, while the latter may be a southern extension of a recently evolved, predominantly North American complex.

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