BQUISETACE^. 127 



middle. Name from Lat. equus, horse, and seta, a bristle. 

 Contains about 25 species, widely distributed. 



§ I. EuEQUlSETUM. Stems annual, stomata scattered. 

 * Stems of two kinds, the pale or brownish fertile stems appearing 



earlier than the herbaceous sterile ones ; fruiting in spring. 

 t Fertile stems simple, sooti withering. 



1. E. arvense L. (HoRSETAlL.) Sterile stems green, 

 rather slender, 1° — 2° high, 6 — 19 furrowed ; branches numerous, 

 long, mostly simple, 4-angled, minutely roughened, lowest joint 

 commonly longer than the sheath of the stem ; fertile stems 

 4' — 10' high, light brown, the loose scarious sheath mostly dis- 

 tant, whitish, ending in about 12 brown acuminate teeth ; spike 

 rarely over i' long. {E. boreale Bong.) Virginia to California 

 and northward to Greenland. 



2. E. telmateia Ehrh. Sterile stems ivory white or greenish, 

 stout, 2° — 6° high, 20 — 40 furrowed ; branches very numerous, 

 erect-spreading, simple, 4 — 5 angled, the ridges rough and sul- 

 cate, the lowest joint shorter than the sheath of the stem ; fer- 

 tile stems 10' — 15' high, white, many-furrowed, the loose brown- 

 ish sheaths elongate, deeply 20 — 30 toothed. {E. fluviatile 

 Sm., E. eburneum Schreb., E. maximum Auct, not of Lam.) 

 California, Oregon, and northward. 



tt Fertile stems when older producing herbaceous branches, only 

 the naked apex withering. 



3. E. pratense Ehrh. Sterile and finally the fertile stems 

 producing straight, simple branches ; sheaths of the stem with 

 about 1 1 short, ovate-lanceolate teeth, those of the branches 3- 

 toothed. (E. umbrosum Willd., E. triquetrum Bory., E. Drum 

 mondii Hook.) Michigan, Wisconsin, and northward. 



4. E. silvaticum- L. Sterile and fertile stems usually 12- 

 furrowed, producing compound branches, the branchlets curved 

 downward ; sheaths loose, those of the stem with 8 — I4bluntish 

 teeth, those of the branches with 4 — 5, and of the branchlets 

 with 3 divergent teeth. Virginia to Michigan, and northward 

 to Labrador. 



** Stems of one kind, herbaceous ; branches simple or none ; 



fruiting in summer. 



+ Sheaths somewhat loose. 



