Mapping here is provisional, since there has been much confusion with americana, and the two species may not be clearly distinct in some colls. (Rohrer et al. 2008; FNA 9). P. mexicana, sensu stricto, is centered in the Ozark region, and it may be largely absent north or east of Mo. (J. Shaw, pers. comm.), However, it has been reported from much of northeast Mexico, eastern Texas, the lower and central Mississippi Valley: also north to s. Wis., Ill., s. Ind. and s. Ohio. In Mo., there appears to be little difference from americana in range or habitat, and some colls. attributed to mexicana appear to be transitions from americana to hortulana (Y). Most plants mapped here may be closer to the hairy form of americana rather than typical mexicana, but there may be colls. of typical mexicana from lowlands of the Mississippi Embayment: e.g. BALL (GH), E.J. Palmer #16511, 20 Sep 1919, low hills, Wickliffe; and TRIG (APSC), W.H. Ellis #96, 15 Jun 1965, banks of Shanklin Cr. 1.5 mi N of US 68. Typical mexicana also appears to occur in Tenn. (at least Franklin, Marion & Marshall Cos.), but still generally difficult to separate from americana (Ch+ and SERNEC). P. americana var. lanata Sudsworth may be the appropriate name for the intermediate plants that prevail in Ky., especially on drier rocky sites, but see also notes under nigra. Typical mexicana reportedly differs from americana as follows (Cr, St, Y, W, FNA 9; Johnson 2013; J. Shaw, pers. comm.): flowering often earlier (always before leafing), with shorter petals (ca. 8-10 mm versus 10-15 mm); sepals hairy on lower surface (versus glabrous) and often more glandular at margins; calyx tube and pedicel more or less hairy (versus usually glabrous); leaves often larger and relatively broad (L/W = 1.9-2.6 versus 1.6-2.1), with obtuse to subcordate bases (versus cuneate to obtuse), usually with persistent soft hairs below (versus hairy to glabrate), the petioles or blade bases usually with a pair of glands (versus often glandless); truncks less thorny and less thicket-forming, often lacking suckers and with a more robust tree-like habit. Reported differences in twig pubescence (FNA 9) may not be reliable; there appears to be much variation within both mexicana and americana.