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.
Species 15 (11 in the flora): nearly worldwide.
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.
Hauke, R.L. 1966. A systematic study of
. Nova Hedwigia 13: 81--109. Hauke, R.L. 1979.
in North America. Amer. Fern J. 69: 1--5.
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