New Combinations in Silphium perfoliatum L. (Compositae)

In my Master’s Thesis, (1960) I hypothesized a common Pliocene ancestor for the glabrous, petiolate leaved Silphium perfoliatum L. and the pubescent, sessile leaved S. connatum L. This ancestral species had a distribution from North Carolina north into Ohio, along the preglacial Teays River, across the midwest throughout the tall grass prairies from the Dakotas south to Kansas and Oklahoma. The population exhibited several character clines from east to west, e.g. pubescent to glabrous stems, sessile to petiolate leaves, and small to large number of ray flowers. During the Pleistocene the population was divided in two by the action of glaciers and glacial lakes. The eastern and western segments are now, respectively, S. connatum and S. perfoliatum.