Mapping remains tentative, given potential cryptic segregates. H. nigricans sensu lato varies greatly over its largely southwestern range, and it deserves further collection and analysis; reported 2n = 18, 20, 36 and 40 (Church & Taylor 2005). East of the Mississippi River, the range is fragmented into disjunct sections (K) that may have genetic distinction. The species may have been overlooked or undercollected in western regions of the state, where there are sight records from rocky glades that are not mapped here. Several southwestern segregates have been recognized, but the remaining widespread variety was not divided by Terrell (2001). It is likely that plants of the cedar glades in c. Tenn. and the Knobs region of Ky. (as mapped here) should be segreated to form a variant distinct from more upright or robust plants of the Ozarks and Gulf Coastal Plain; see also notes under rupestris. Based on seed characters and chromosome numbers, Terrell (1996, 2001) proposed that nigricans sensu lato belongs in a new genus, as Stenaria nigricans (Raf.) Terrell. Morphological and molecular data do not provide strong support for distinction of this genus from Houstonia (Church & Taylor 2005; W), but it could be a reasonable section.