This is largely restricted to warmer American regions; rare to absent in Appalachian regions, and virtually unknown in glaciated regions north of Ky. (F, PL). It tends to occur in drier rockier sites than perfoliata, on average; see notes under perfoliata. Distinction from typical perfoliata may be most reliably based on capsules (F, Y, W), which are more slender with L/W ca. 4 (versus 3); pores are located at or within 0.5 mm of the apex (versus usually 1-1.5 mm below). Also, seeds are smooth and lustrous (versus muriculate, dull or slightly lustrous); open, chasmogamous corollas are usually restricted to the single terminal flower or rarely 2-3 (versus usually one per node along upper half of stem); stem leaves have L/W ca. 1.5-3 (versus mostly ca. 1), not or slightly clasping at base (versus clasping);