3. Equisetum laevigatum A. Braun (smooth scouring rush) Pl. 16a,b; Map 33

E. kansanum J.H. Schaffn.

Aerial stems monomorphic, 0.2–1.5 m long, usually only lasting for 1 growing season, green, smooth, 10–32-ridged, unbranched or sometimes irregularly branched at some nodes. Leaf sheaths green except for a thin, dark band at the tip, the teeth usually shed before maturity, narrowly triangular, attenuate, white to dark brown. Strobili 1.0–2.5 cm long, the tips rounded, less commonly bluntly acute, not sharply pointed. 2n=216. March–July.

Scattered nearly throughout Missouri (U.S., southern Canada, northern Mexico). Banks of streams, spring branches, and rivers, fens, roadsides, and railroad ballast, less commonly in upland prairies, particularly loess hill prairies.

For discussion of the hybrid with E. hyemale, see the treatment of that species. Another, more unusual, sterile hybrid in Missouri is E. ¥nelsonii (A.A. Eaton) J.H. Schaffn., which represents the cross E. laevigatum ¥ E. variegatum Schleich. ex F. Weber & D. Mohr. The latter parent is a northern species whose closest localities to Missouri are along the southern shore of Lake Michigan in northeastern Illinois. Surprisingly, the hybrid has been found growing as small, sterile colonies in fens in the Ozarks, in Shannon County, more than 600 km from the nearest extant localities in northeastern Illinois. This unusual hybrid apparently has persisted as a relict since the Pleistocene Ice Age, when climatic conditions potentially would have allowed E. variegatum to exist in the Ozarks. It is distinguished from other Missouri horsetails by its unbranched, slender stems with only 6–14 ridges, and in its leaf sheath teeth, which are green with white margins and brown, attenuate tips.