The classification of some porophyllums can be somewhat problematic. Even if we completely discount the confusion over the common names of plants in this family there are still stumbling blocks present even in the Latin binomial nomenclature (1) of the species.

1. Binomial nomenclature (“two-term naming system”), a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages. ie: Porophyllum macrocephalum : poro-phyllum = pore – leaf and macro-cephalum = big/large – head (refers to the large size of the leaves and the presence of visible oil glands – pores – within the leaf)

Phylogenetic analyses (1) suggest that Pr. amplexicaule and P.scoparium (2) fall outside of the genus and do not belong to the Porophyllum family as such (Johnson, 1969) (Hansen etal 2016) but instead they form a strongly-supported clade that is the sister-group to a combined Pectis (3) + Porophyllum clade (4)

1. the study of evolutionary development of a species or a group of organisms or a particular characteristic of an organism.
2. See Post Porophyllum scoparium
3. See Post Chinchweed : Pectis papposa
4. Clade – a group of organisms believed to comprise all the evolutionary descendants of a common ancestor.

Porophyllum amplexicaule is restricted to southwestern Coahuila, and P.scoparium can be found in desert scrub from southern New Mexico, southeast into southwest Texas, and into Mexico from Coahuila to Nuevo Leon. (Johnson 1969) P.amplexicaule and P.scoparium hybridize where they co-occur.

P.amplexicaule is commonly found growing on limestone (as are other species of porophyllum, eg P.macrocephalum, P.calcicola, P.scoparium, P.gracile)

P.fruticulosum is considered a hybrid of P.scoparium and P.amplexicaule (See Posts Porophyllum scoparium and Porophyllum fruticulosum)

• Debra R. Hansen, Robert K. Jansen, Rowan F. Sage, José Luis Villaseñor, Beryl B. Simpson “Molecular Phylogeny of Pectis (Tageteae, Asteraceae), a C4 Genus of the Neotropics, and its Sister Genus Porophyllum,” Lundellia, 19(1), 6-38, (1 December 2016)
• Johnson, Roy.R. : Monograph of the Plant Genus Porophyllum (Compositae: Helenieae) : The University of Kansas Science Bulletin : Vol XLVII : No 7. Jan 31, 1969