Perennial herbaceous fern ally 35 cm - 1.15 m tall Spores: green, spheric, 34 - 45 microns in diameter, and released from lengthwise slit in spore sacs (sporangia). Stems: one to several, unbranched or branched at middle nodes, green (dying back in winter), 2.5 - 9 mm diameter, round with hollow center and twelve to twenty-four obscure lengthwise ridges and alternating valleys on outside, which is smooth and not particularly rough. The stem joints have distinct nodes, and the stomates are scattered or in bands on each side of the stem valleys, but never sunken. Branches: often absent or at middle nodes, whorled, spreading, distinctly jointed at sheathed nodes, rounded with hollow center and four to six lengthwise ridges and alternating rounded valleys. The first internode of each branch is only 2 - 6 mm long and shorter than the subtending stem sheath from which it originates. Sheaths: green with narrow black teeth at top, square in face view, 0.4 - 1 cm long, 0.4 - 1 cm wide. The sheaths are actually small, fused, whorls of leaves. Sheath teeth: twelve to twenty-four, black, occasionally with narrow white border, narrow, 2 - 3 mm long.
Similar species: Equisetum fluviatile is most similar to E. palustre, but that species has elongate stem sheaths, less than eleven teeth per sheath, and obvious white edges on the dark-centered teeth. If unbranched, other members of this subgenus can be distinguished because the stems are not green. If branched, they can be distinguished because the first internode of the branches will be at least the same length as the subtending stem sheath that the branch arises from. Members of the subgenus Hippochaete will always have pointed tips on the fertile cones. However, E. laevigatum still may be confused since its cone is sometimes rounded, but that species has non-persistent teeth on the stem sheaths. The sterile hybrid between this species and E. arvense is E. x litorale, which always has branched stems, the first internode of the branches are nearly equal in length to the the subtending stem sheath, and the spores are always white and misshapen.
Habitat and ecology: Somewhat common in shaded shallow water, ditches, shores, and marshes.
Occurence in the Chicago region: native
Notes: This plant has the aboveground stems surviving for only one year or even less. As in other members of this genus, E. fluviatile is able to absorb and deposit silica in its stems. The genus Equisetum (the only living members of the Equisetaceae family) has been studied often as a means to understand the evolution of vascular plants. They are often also used to interpret fossil plants in a group called the sphenopsids, which first appeared in the Devonian Period (about 360 to 415 million years ago). At that time, the plants were treelike and reached sizes of well over 10 m tall and even 1 m in diameter. The group reached a point of high diversity in the Carboniferous (300 to 360 million years ago) when the majority of our coal deposits started forming from these and other plants. Today only these small plants (up to about 2 m tall and under 4 cm diameter in our largest species) survive from this once land-dominating group.
Stems annual, all alike, to 1 m or more, shallowly 9-25-ridged, the ridges smooth or nearly so, the stomates in a single broad band in each furrow; central cavity commonly more than ޠthe diameter of the stem, the vallecular cavities commonly lacking except near the base; sheaths green, 4-9 mm, with persistent, narrow, sharp, black or blackish teeth 1.5-3+ mm, these not or scarcely hyaline- margined; branches none to often numerous and whorled at the middle and upper nodes, 4-6-angled, simple, the
first internode slightly shorter than the associated stem-sheath; cone pedunculate, 1-2 cm, deciduous. In shallow water, along muddy shores, and in marshes and bogs; circumboreal, in Amer. s. to Pa., Ill., Io., Nebr., and Wash.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.