13. Equisetum ×mackaii (Newman) Brichan, Phytologist. 1: 369. 1843 (Nov. 1842.
Equisetum hyemale Linnaeus var. mackaii Newman, Phytologist 1: 305. 1843 (Sept. 1842); E. hyemale subsp. trachyodon A.Braun; E. ×trachyodon (A.Braun) Koch; E. variegatum Schleicher ex F. Weber & D. Mohr var. jesupii A.A. Eaton
Aerial stems persisting more than a year, unbranched, 20--86 cm; lines of stomates single; ridges 7--16. Sheaths black or black-girdled and white distally, appressed, elongate, 3.5--8 × 2--5.5 mm; teeth 7--16, centers dark, margins white, prominent, not articulate, tip usually brown, long, filiform. Cone apex pointed; spores white, misshapen.
Cones maturing in late summer, but misshapen spores not shed. Lakeshores, riverbanks, marshes; 0--500 m; Greenland; B.C., N.B., Nfld., Ont., Que., Sask.; Conn., Ill., Ind., Maine, Mich., Minn., Mont., N.H., N.J., N.Y., Ohio, Oreg., Vt., Wis.; n Europe.
The hybrid between Equisetum hyemale and E . variegatum , E . × mackaii , is often mistaken for small forms of E . hyemale . I have not seen specimens for the reports from Connecticut and New Hampshire.