This is a distinct polyploid (2n = 64), closely related to typical officinale, but sometimes treated as a variety (e.g. F). It has been reported from Ky. by Rollins (1993), but colls. have not been located. It is scattered in cooler regions of eastern states (F, Cr, W; Al-Shehbaz 1988b). Compared to officinale (2n = 32), microphylla has narrower siliques (1-1.5 mm versus 1.8-3 mm), with seeds in one row (versus two); seeds have more polygonal depressions on each side (ca. 75-175 versus 25-60); flowers tend to be larger (petals mostly 4.5- 6 mm versus 3-4.5 mm). Both species can become robust in optimal habitats, up to 1.5 m tall or more.