Plants perennial, rhizomatous. Aerial stems annual or perennial. Stems with hollow center and series of small carinal (under the ridges) and larger vallecular (under the valleys) canals. Leaves in whorls, fused part of length into sheaths. Stem ridges traversing length of internode and continuing into sheaths, terminating in sheath teeth. Branches when present borne at nodes, erupting through base of subtending sheath. Cones terminal on green stems or, in some species, terminating special, reproductive, brown stems, composed of whorls of peltate sporophylls; cone apices rounded or sharply pointed; sporangia 5–10 per sporophyll, pendent, attached to inner surface of sporophylls, elongate, dehiscing longitudinally. x =108.

Distribution

Nearly worldwide.

Discussion

Equisetum occurs in moist places such as riverbanks, lakeshores, roadsides, ditches, seepage areas, meadows, marshes, and wet woodlands. Aerial stems of Equisetum vary considerably in habit and appearance, even on individual plants, because of environmentally induced modifications affecting height and branching. Many taxonomically trivial varieties and forms have been named. For an extended discussion of this, see R.L. Hauke (1966). Four widespread, named hybrids are treated in the key and fully described below.

In species descriptions and in the key, length and width are given for the leaf sheath, excluding the free teeth. If the length and width of flattened sheaths are approximately equal and the sides are straight, the sheath is more or less square in face view, i.e., about as long as broad; if the length is greater than the width and the sides are straight, the sheath is more or less elongate in face view, i.e., longer than broad; if the length is greater than the width and the sides are slightly convex, the sheath is elliptic in face view. Stomates are usually visible at 20× magnification.

Reticulation in Equisetum is summarized in the reticulograms, which show the known and expected hybrids in North America. Most of those in Equisetum subg. Equisetum are still unknown in North America, but they should be sought, especially north of 45° N latitude. According to W.J. Cody and D.M. Britton (1989), E. × font-queri occurs rarely in British Columbia and materials possibly representing E. × arcticum Rothmaler have been taken in the Richardson Mountain region of Mackenzie. R.L. Hauke (1978) cited collections of E. × font-queri from British Columbia and California.

Species 15 (11 in the flora).

Selected References

Hauke, R.L. 1966. A systematic study of Equisetum arvense. Nova Hedwigia 13: 81--109.

Hauke, R.L. 1979. Equisetum ramosissimum in North America. Amer. Fern J. 69: 1--5.

Lower Taxa

Equisetum subg. Equisetum, Equisetum subg. Hippochaete

Key

1	Aerial stems persisting only 1 year or less, usually with regular whorls of branches; stomates on surface, scattered or in bands; cone apex rounded.	Subg. Equisetum
1	Aerial stems persisting more than a year (except some Equisetum laevigatum), usually unbranched; stomates sunken, in single lines; cone apex pointed (except some E. laevigatum).	Subg. Hippochaete
2	Aerial stems unbranched.	> 3
2	Aerial stems branched with regular whorls of branches.	> 8
3	Aerial stems green.	> 4
3	Aerial stems not green.	> 5
4	Sheaths square in face view; teeth more than 11 per sheath, often black throughout or with narrow white margins, 2–3 mm.	Equisetum fluviatile
4	Sheaths elongate in face view; teeth fewer than 11 per sheath, with prominent white margins and dark centers, 2–5 mm.	Equisetum palustre
5	Sheath teeth reddish, papery, coherent into 3–4 large groups.	Equisetum sylvaticum
5	Sheath teeth black or brown, firm, separate or coherent in more than 4 small groups.	> 6
6	Aerial stems with stomates, persistent, becoming green and branched.	Equisetum pratense
6	Aerial stems lacking stomates, dying back after spores are shed.	> 7
7	Sheath teeth more than 14.	Equisetum telmateia
7	Sheath teeth fewer than 14.	Equisetum arvense
8	First internode of each branch shorter than subtending stem sheath; branch valleys rounded.	> 9
8	First internode of each branch equal to or longer than subtending stem sheath; branch valleys channeled.	> 11
9	Branches solid, branch ridges furrowed.	Equisetum telmateia
9	Branches hollow, branch ridges rounded.	> 10
10	Sheaths square in face view; teeth more than 11 per sheath, dark, occasionally with narrow white margins, 2–3 mm.	Equisetum fluviatile
10	Sheaths elongate in face view; teeth fewer than 11 per sheath, with prominent white margins and dark centers, 2–5 mm.	Equisetum palustre
11	Aerial stem sheath teeth reddish, papery, coherent in 3–4 large groups; stem branches also branched.	Equisetum sylvaticum
11	Aerial stem sheath teeth dark, firm, separate or coherent in more than 4 small groups; stem branches unbranched.	> 12
12	Branch sheath teeth deltate; branches spreading.	Equisetum pratense
12	Branch sheath teeth attenuate; branches ascending.	> 13
13	Lowest whorls of branches with 1st internode longer than sheath; spores green, spheric.	Equisetum arvense
13	Lowest whorls of branches with 1st internode nearly equal to sheath; spores white, misshapen.	Equisetum ×litorale
14	Aerial stems regularly branched; stomatal lines occasionally doubled.	Equisetum ramosissimum
14	Aerial stems unbranched or with scattered branches; stomatal lines always single.	> 15
15	Cone apex rounded; aerial stems annual.	Equisetum laevigatum
15	Cone apex pointed; aerial stems perennial (at least persisting over winter in California populations of E. laevigatum).	> 16
16	Spores white, misshapen.	> 17
16	Spores green, spheric.	> 19
17	Sheaths green; teeth persistent.	Equisetum ×nelsonii
17	Sheaths dark-girdled; teeth persistent or shed.	> 18
18	Teeth 14 or fewer per sheath, persistent.	Equisetum ×mackaii
18	Teeth more than 14 per sheath, usually shed.	Equisetum ×ferrissii
19	Sheaths dark-girdled at most nodes of stem; teeth 14 or more per sheath, usually shed; articulation line visible.	Equisetum hyemale
19	Sheaths green or obscurely girdled at nodes near base of stem; teeth 32 or fewer per sheath, usually persistent, shed in some species; articulation line lacking.	> 20
20	Teeth 3–32 per sheath; stem ridges same number as teeth; aerial stems erect and straight.	> 21
20	Teeth 3 per sheath; stem ridges 6; aerial stems inclined and tortuous.	Equisetum scirpoides
21	Sheath teeth usually shed; cone apex rounded to apiculate with blunt tip; stem ridges flattened or convex.	Equisetum laevigatum
21	Sheath teeth usually persistent throughout; cone apex sharply apiculate; stem ridges minutely grooved.	Equisetum variegatum