Aerial stems persisting more than a year, unbranched, 18--220 cm; lines of stomates single; ridges 14--50. Sheaths when mature dark-girdled, brown to gray above girdle, squarish in face view, 4.5--17 × 3.5--18 mm; teeth 14--50, articulate and promptly shed or persistent. Cone apex pointed; spores green, spheric. 2 n =216. Cones maturing in summer, old stems sometimes developing branches with cones in spring. Moist roadsides, riverbanks, lakeshores, woodlands; 0--3000 m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska including the Aleutian Islands, all other states; Mexico; Central America in Guatemala. In southern and central to western regions plants tend to be taller and have more persistent teeth ( Equisetum robustum , E . prealtum ); in the Far West they often have bituberculate ridges ( E . hyemale var. californicum ). Equisetum hyemale subsp. hyemale is found in Europe and Asia to northwestern China in Xinjiang.

STEMS: with minute tubercles in 1 row along the ridges. TEETH: irregularly persistent. NOTES: See also parent taxon. Banks of streams and rivers, marshy meadows, roadside ditches: Apache, Cochise, Coconino, Gila, Graham, Maricopa, Mohave, Navajo, Pima, Santa Cruz, and Yavapai cos. (Fig. 1B); 900–2600 m (2850–8500 ft); June–Sep; U.S., Can. Mex., Guatemala REFERENCES: G.Yatskievych and M.D. Windham , 2008, Vascular Plants of Arizona: Equisetaceae. CANOTIA 4 (2): 41–45.