Equisetum
See list of 8 species in this genusEquisetum hybrids are rare in New England, with the exception of two nothospecies— E. ×litorale and E. ×mackaii. Hybrids can be recognized by their white and misshapen spores (rather than green and spherical). Though spore shape is difficult to assess with a hand lens, color is not (i.e., separation of hybrid individuals from fertile parental species is possible without high magnification). Sequence data from cp DNA suggest that the division of horsetails into two genera ( Equisetum and Hippochaete) creates a paraphyletic Equisetum because of the basal position of E. bogotense Kunth (Guillon 2004). Therefore, only one genus is recognized here. References: Hauke (1963, 1978, 1993).
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1a. Aerial stems monomorphic, unbranched, persisting more than one year (i.e., evergreen); strobilus with an apiculus at its apex [Fig. 25]; stomates sunken below the surface, arranged in single lines on each side of stem grooves (view at 30 ×)
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2a. Stems 14- to 50-ridged; leaves articulated to and deciduous from the sheaths; central cavity greater than 50% of the stem diameter; strobili 10–20 mm tall
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2b. Stems 3- to 12-ridged; leaves not articulated to the sheaths and persistent on them; central cavity up to 35% of the stem diameter; strobili 2–10 mm tall
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3a. Stems tortuous, 6-ridged, 0.5–1 mm thick; sheaths with 3 leaves; central cavity absent; vallecular cavities 3
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3b. Stems erect and straight, 3- to 12-ridged, 1–4.5 mm thick; leaves numbering the same as the ridges; central cavity evident; vallecular cavities 5–12
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1b. Aerial stems monomorphic or dimorphic (i.e., the vegetative and spore-bearing stems differing in appearance), commonly with regular whorls of branches (except in unbranched forms of E. fluviatile and E. palustre), deciduous at the end of the growing season; strobilus rounded at its apex; stomates on the surface, scattered or in bands on each side of stem grooves
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4a. Strobili appearing in summer, borne on firmer, green, photosynthetic stems that persist throughout the season (i.e., stems monomorphic); length of the first internode of each branch shorter than its subtending stem sheath (or sometimes the branches absent) [Figs. 23,24]; grooves of branches rounded (i.e., U-shaped in cross-section)
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5a. Leaves numbering 12–24 per sheath, 1.5–3 mm long, dark throughout or with a narrow, white band [Fig. 23]; stems with 9–25 low ridges; central cavity 75% or more of the stem diameter
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5b. Leaves numbering 5–10 per sheath, 2–5 mm long, with prominent white margins [Fig. 24]; stems with 5–10 prominent ridges; central cavity less than 35% of the stem diameter
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4b. Strobili appearing in spring, borne on soft, pink to light brown stems, the green, photosynthetic stems forming or appearing later (i.e., stems dimorphic); length of the first internode of each branch equal to or exceeding its subtending stem sheath [Fig. 22]; grooves of branches channeled (i.e., C-shaped in cross-section)
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6a. Leaves red-brown, cohering in 3 or 4 large groups, papery; branches of the stem compound (i.e., again branched)
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6b. Leaves largely brown to black, separate or in 4 or more small groups, firm; branches of the stem simple (except sometimes in damaged plants)
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7a. Reproductive stems without stomates, promptly senescing; leaves of the main axis dark at the margin or with a very thin, obscure white margin, those of the branches subulate; central cavity ca. 25% of the stem diameter; stem internodes smooth to moderately scabrous
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7b. Reproductive stems with stomates, becoming green and branched; leaves of the main axis with a prominent white margin, those of the branches triangular; central cavity 35–50% of the stem diameter; stem internodes evidently scabrous
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Each photo represents one species in this genus.