Cal 10-nerved, equally or unequally 5-toothed; cor elongate, bilabiate, the upper lip galeate-hooded and barely or scarcely notched, the lower lip about as long as the upper, spreading, 3-lobed; stamens 4, ascending under the upper lip, the lower pair the longer; pollen-sacs nearly parallel, opening separately; nutlets smooth; fibrous-rooted perennials with glabrous herbage and showy pink-purple (white) fls borne singly in the axils of small bracts, forming elongate terminal racemes. 12, N. Amer. (Dracocephalum, misapplied)
In addition to the following spp., P. ledinghamii P. D. Cantino and P. intermedia (Nutt.) Engelm. & A. Gray may be sought at the margins of our range. The former is an apparent amphiploid of nos. 1 [Physostegia virginiana (L.) Benth.] and 3 [Physostegia parviflora Nutt.], known from N.D. and Man. to s. Mack. It has fls mostly 14-23 mm and is in general intermediate between its presumed ancestors. P. intermedia, of the lower Mississippi Valley, extends n. to se. Missouri. It has a short cal (tube 2-4 mm) without glandular hairs and a relatively small (to 20 mm) cor lacking glandular hairs; the base of the stem is often strongly swollen, and the uppermost lvs are generally clasping and widest at the base of the blade.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.