[*][*] Fruiting in summer; stems all of one kind, or the fertile contemporaneous with and like the sterile, equally herbaceous, producing mostly simple branches, or sometimes nearly naked.
4. E. palústre, L. Stems (10–18´ high) slender, very deeply 5–9-grooved, the ridges narrow and acute, roughish, the lance-awl shaped teeth whitish-margined; branches always hollow, 4–7-angled, rather few in a whorl.—Wet places, Niagara River (Clinton), Wisc. (Austin), and northward. June. (Eu.)
5. E. littoràle, Kühlewein. Stems (8–18´ high) slender, deeply 6–16-grooved, the ridges rounded, the teeth shorter than in the last, narrowly white-margined; branches often solid, 3–4-angled, 2–6 in a whorl.—Wet sandy shores, Vt. and N. Y., and northward.—Spores always abortive, whence the plant has been considered a hybrid, perhaps of E. arvense and E. limosum. July. (Eu.)
6. E. limòsum, L. ([Pl. 21], fig. 1–5.) Stems (2–5° high) slightly many-furrowed, smooth, sometimes continuing unbranched, but usually producing ascending branches after fructification; sheaths appressed, with 10–22 (commonly about 18) dark-brown and acute rigid short teeth.—In shallow water; rather common.—Air-cavities none under the grooves, but small ones under the ridges. A form in which the branches bear numerous small spikes is var. polystàchyum, Brückner. June, July. (Eu.)
§ 2. Stems all alike, evergreen, unbranched, or producing a few slender erect branches; fruiting in summer. Central air-cavity of the stem very large.
[*] Stems tall and stout (1½–4° or even 6° high), simple, or casually branched, evenly many-grooved; sheaths appressed.
7. E. hyemàle, L. (Scouring-Rush. Shave-Grass.) Stems 1½–4° high, 8–34 grooved, the ridges roughened by two more or less distinct lines of tubercles; sheaths elongated, with a black girdle above the base and a black limb; ridges of the sheaths obscurely 4-carinate, the teeth blackish, membranaceous, soon falling off.—Wet banks; common northward. Formerly in common use for polishing wood and metal. (Eu.)
8. E. robústum, Braun. Stems tall and stout (sometimes 8–10° high and nearly an inch thick), 20–48-grooved, the ridges roughened with one line of transversely oblong tubercles; sheaths rather short, with a black girdle at base and a black limb; ridges of the sheaths tricarinate, the blackish teeth soon falling off.—River-banks, Ohio and westward.
9. E. lævigàtum, Braun. Stems 1–4° high, rather slender, pale green, 14–30-grooved, the ridges almost smooth; sheath slightly enlarged upward, with a black girdle at the base of the mostly deciduous white-margined teeth, and rarely also at the base of the sheath; ridges of the sheath with one keel, or sometimes obscurely tricarinate.—By streams and in clayey places, Ohio to Minn., and westward.
[*][*] Stems slender, in tufts, 5–10-grooved; sheaths looser.