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| 1 Stems annual, deciduous, the sterile stems flexible; sterile and fertile stems dimorphic or monomorphic, usually branched (often copiously so) but sometimes unbranched or sparsely and irregularly branched; [subgenus Equisetum]. |
| ..2 Sterile and fertile stems monomorphic; sterile and fertile stems sparsely and irregularly branched; stem ridges 12-24, indistinct; diameter of the central cavity of the stem about 4/5's of the stem diameter |
| ..2 Sterile and fertile stems dimorphic; sterile stems copiously branched and green, fertile stems unbranched or branched, green, tan, brown, or purplish; stem ridges 4-18, distinct; diameter of the central cavity of the stem usually < 3/4's of the stem diameter. |
| ....3 Sheaths of the sterile stems 3-10 mm long, the teeth dark brown with white margins; sterile stems regularly whorled with simple branches. |
| ....3 Sheaths of the sterile stems 10-30 mm long, the teeth reddish-brown with brown margins; sterile stems regularly whorled with branches which regularly rebranch |
| 1 Stems perennial (or annual in E. laevigatum), evergreen, stiff; sterile and fertile stems monomorphic and either unbranched or with 2-3 short and unequal branches per node; [subgenus Hippochaete]. |
| ......4 Stems 0.3-5 dm tall; stem ridges 3-12; [PA, OH, IN, IL northwards]. |
| ........5 Stem ridges 6; teeth 3 per sheath; stems tortuous; stem lacking a central cavity |
| ........5 Stem ridges 3-12; teeth 3-12 per sheath; stems straight; stem with a central cavity |
| ......4 Stems 2-20 dm tall; stem ridges 10-50; [collectively widespread]. |
| ..........6 Main erect stems usually with 2-3 branches at the nodes; stems 1.5-7 mm in diameter; stomatal lines 1-2 on each slope of the stem ridges |
| ..........6 Main erect stems unbranched (rarely branched as a result of injury); stems 3-18 mm in diameter; stomatal lines 1 on each slope of the stem ridges. |
| ............ 7 Cone apex rounded; aerial stems annual |
| ............ 7 Cone apex pointed; aerial stems perennial. |
| ............ ..8 Spores not produced, or white and misshapen; most stem sheaths lacking a blackish band well below the teeth |
| ............ ..8 Spores green, spherical; most stem sheaths with a narrow to broad blackish band well below the teeth |