Evaluation of the Taxonomy of Phacelia purshii (Hydrophyllaceae)

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Phacelia purshii, one of a few phacelias with fringed corolla lobes, has historically been described as a variety of Phacelia fimbriata or as a distinct species. The geographical range is centered on the Ohio River Valley extending north to Pennsylvania, east to the Blue Ridge, and south to Alabama. Two related species were described, Phacelia bicknellii and Phacelia boykinii, but these names were rarely used in floras or applied to herbarium specimens. A morphometric and geographic analysis of herbarium specimens showed that the description of P. bicknellii corresponds to male sterile plants and therefore does not warrant taxonomic recognition. In a sample of 280 specimens, latitudinal variation was evident in a suite of characters, some of which were associated with the historical distinction between P. purshii and P. boykinii. Two previously unrecognized diagnostic characters, pubescent ovaries and pubescent abaxial surfaces of corolla lobes, distinguished plants from the southern part of the geographic range. Both pubescence characters had a relatively abrupt transition from more or less present to absent and these corresponded to southern and northern localities, respectively. Pedicel length, the historical diagnostic character for P. boykinii, had a bimodal distribution. A discriminant function correctly assigned > 94% of specimens to northern and southern morphs when the morphs were defined by ovary pubescence. The morphological and geographical data provide for accurate diagnosis of specimens and therefore support taxonomic recognition of the southern form. That form is recognized not as a distinct species, but as a variety of P. purshii, Phacelia purshii var. boykinii.