Some records mapped here (B, M) need to be rechecked. True interior is a tetraploid (2n = 44) distributed in the central Mississippi Valley (perhaps on more base-rich soil), while typical mariana is largely southeastern (on acid infertile soils). Some plants mapped here as mariana appear transitional to interior (including those that have been called var. leiosperma Fern. & Grisc.). R. mariana var. ventricosa (Fern. & Grisc.) Kral & Bostick is a somewhat intermediate taxon that occurs only east of the Appalachians, but it has been reported from Ky. in error (M). R. interior can be distinguished from virginica as follows (FNA 10; see also R.J. LeBlond in W): roots not tuberiferous (versus tuberiferous); stem angles sharply to narrowly winged at mid-stem (versus conspicuously winged); hypanthium neck as long or longer than the body (versus shorter). R. interior differs from the diploid mariana in its stem faces being subequal (see notes under mariana); also, leaves tend to be larger on average (ca. 2-7 x 1-2.5 cm versus 2-4 x 0.5-2 cm); flowers tend to be brighter lavendar-rose (versus wihte or pale to dull lavendar).