1052 EQUISETACEAE 



Sheath close; central cavity one-half of the diameter of the stem or more. 

 sheaths 7-8 mm. long; stem stout, sparingly branched. 5. E. fluviatile. 

 sheaths less than 5 mm. long; stem slender, much branched. (Autumnal 



forms of) 1. E. arvense. 



Teeth of the sheaths deciduous ; plant unbranched or nearly so. 



6. E. kansanum. 

 stem perennial, evergreen (except sometimes in E. laevigatum), mostly simple; spike 



apiculate, with a rigid tip; stomata in regular rows. 



Central cavity present; stem more than 6-angled, except sometimes in E. variegatum. 

 Teeth of the sheaths deciduous; stem tall and stout, many-grooved. 

 Stem tubercled, rough. 



Sheath close, usually with a black or dark ring near the base, then grayish 



and with a narrow dark border at the base of the teeth. 

 Stem 1-2 m. high, stout; sheaths as broad as long. 7. E. robustum. 

 Stem 0.3-1 m. high, slender; sheath longer than broad. 



8. E. affine. 



Sheaths more ampliate above, green, with or without a narrow border, 



but no ring below. 9. E. intermedium. 



Stem not tubercled, smooth; sheaths enlarged upwards, with or without a dark 



margin, but rarely with a ring below. 10. E. laevigaium. 



Teeth of the sheaths persistent, white-margined: stem low, slender, 5-10-grooved. 



11. E. variegatum. 

 Central cavity wanting; stem 6-angled. 12. E. scirpoides. 



1. E. arvense L. Spore-bearing stems 1-2.5 dm. high, 3-5 mm. in diameter, 

 with loose 8-12-toothed sheaths, rarely with a few branches; cones peduncled, 

 2-4 cm. long, 5-10 mm. thick; sterile stems decumbent to erect, 1-3.5 dm. high, 

 2-3 mm. thick, 10-14-furrowed, branched; branches 3-4-angled, solid; teeth of 

 the sheaths lanceolate, acuminate. In the fall the sterile stems sometimes bear 

 small cones 4-10 mm. long, mostly with sterile spores (var. serotinum). Wet 

 banks: Greenl. S.C. Calif. Alaska; Eurasia. Plain Alp. 



2. E. sylvaticum L. Stems 1-4.5 dm. high, 8-14-riciged, 3-4 mm. thick; 

 both the fertile and the sterile ones branched; central cavity constituting half 

 the diameter; sheaths brown, loose, the teeth more or less coherent; primary 

 branches 4-5-angled, the secondary ones 3-angled; cones peduncled, 1-3 cm. 

 long, 5-8 mm. thick. Wet shady places: Newf. Va. la. B.C. Alaska; 

 Eurasia. Submont. Mont. My-Je. 



3. E. pratense Ehrh. Stems 2-4 dm. high, 3-5 mm. thick, 8-20-ridged, 

 the fertile ones at first nearly unbranched, some developing short spreading 

 branches, the sterile ones with long branches; central cavity one-third the diam- 

 eter; sheaths green, loose; teeth lanceolate with dark middle; branches 3-ridged; 

 teeth of the sheaths deltoid; cones peduncled, 1-2 cm. long, about 5 mm. thick. 

 Alluvial soil: N.S. N.J. Colo. Alaska; Eurasia. Submont. Subalp. Ap- 

 My. 



4. E. palustre L. Stems 2-9 dm. high, 3-5 mm. thick, 5-10-angled ; branches 

 long, ascending, hollow, 4-7-angled; sheaths widened upwards; teeth lance-subu- 

 late; cones 1.5-2.5 cm. long, 5-6 mm. thick. Wet places: Newf. Conn. Wyo. 



-Wash. Alaska; Eurasia. Submont. Mont. Je-Au. 



5. E. fluviatile L. Stems 3-15 dm. high, 4-8 mm. thick, 10-30-angled; 

 central cavity four-fifths of the diameter; branches spreading or upcurved, long, 

 4-6-angled, hollow; sheaths appressed; teeth dark brown, distinct, narrow; cones 

 short-peduncled, 1.5-2.5 cm. long, 5-7 mm. thick. E. limosum L. Shallow 

 water: Newf. N.Y. Wyo. Wash. B.C. Plain Mont. Je-Jl. 



6. E. kansanum J. H. Schaffner. Stems 3-5 dm. high, 15-30-grooved, 

 light green; sheaths long, dilated above, green, with a narrow black band at the 

 top, rarely with a faint one below; cones short-peduncled, ovate or ovate-oblong, 

 1.5-2.5 cm. long, 7-8 mm. thick. Clay banks: Mo. Utah Mont. Plain- 

 Submont. 



7. E. robustum A. Br. Stems 1-3 m. high, 8-12 mm. thick, 16-48-angled, 

 dark green; ridges rounded, rough, with 3 rows of tubercles; sheath tight, about 

 as broad as long; teeth mostly persistent, lance-linear, sharply 3-angled, black, 

 with white margins; spike sessile, 1.5-2.5 cm. long, 6-7 mm. thick, strongly 

 apiculate. N.Y.- -Md. La. Tex. Calif. B.C. Plain Submont. Je-Au. 



8. E. affine Engelm. Stems 3-10 dm. nigh, 4-8 mm. thick, 16-40-angled, 

 dark green; ridges rounded, rough with 2 rows of tubercles; sheaths longer than 



