Not yet mappable. Diploids in americana may become treated as a distinct species, perhaps with a generally more southern range, but more research is needed; see notes under americana. Rafinesque (1836, 3:38-39) described a series of three segregates, with increasing tree size and leaf roughness: "obovata", "alba" and "americana". Upper leaf surfaces are indeed often scabrid to short-hairy, especially on seedlings and younger trees, not just glabrous as sometimes stated (FNA 3, W). Plants on drier uplands in Ky. and Tenn., including trees up to 10 m tall, usually have relatively small, rough leaves and slightly corky branches; these may be referable to var. aspera Chapm. (described from Fla. in 1860). Such leaves can be distinguished from those of U. alata by their relatively broad shape, with more asymmetric base and more acuminate apex. On the southeastern Coastal Plain at least Ala. to N.Car., var. floridana (Chapm.) Little differs from var. americana in its relatively small leaves (7-10 cm long versus 10-15 cm), with less asymmetric base and less upwardly curved acuminate serrations (W); it is likely that this taxon extends up the Mississippi Valley, and similar plants (mostly diploids) occur as far west as Tex. and Okl. (A. Whittemore colls. at NA).