Within longifolia sensu lato, Terrell (1996) noted that the "tenuifolia group... is the most distinct... It is a morphological extreme of longifolia that has long internodes (20-80 mm), linear or filiform leaves, diffuse internodes with sometimes divaricate or slightly reflexed branches and filiform pedicels to 20 mm long, short calyx lobes usually 0.5-2.0 mm long, and small capsules (1.0-) 1.5-2.5 (-3.0 mm) long." W has provided support for recognizing these plants as a species, with the following differences from remaining variants of longifolia: leaves 1.3-4.7 cm long (versus 1.6-4 cm), 0.5-4.0 mm wide (versus 1.5-6 mm), with L/W = 7-20 (versus 4-11); inflorescence very diffuse and open, to 20 cm long (rather open to rather compact, < 12 cm long), the branches ascending to deflexed, slender to filiform, with 1-4 remote nodes bearing reduced leaves (versus ascending to spreading, slender, without such distinctive reduced leaves), the pedicels up to 14 mm long (versus 8 mm); internodes mostly 4-9 (versus 7-11); mature capsules mostly 1.5-2.5 mm long and wide (versus 1.8-3 mm); stem densely cinereous-puberulent, especially at the nodes (versus densely puberulent to glabrous). In Appalachian regions these plants are relatively distinct, but in Ozarkian regions similar plants (nuttaliana) intergrade extensively with Terrell's (1996) "Ozark-Ouachita group" of longifolia. Terrell preferred to combine all these groups within longifolia, but he did detail segregates. Further analysis is needed in Ky., including re-examination of types.