AERIAL STEMS: dimorphic, erect, the surface smooth; fertile stems 7–35 cm long, ephemeral, tan to pink, unbranched; vegetative stems 10–60 cm long, 5–14-ridged, persisting through 1 growing season, green, with regular whorls of branches, these 3- or 4-ridged. SHEATHS: as long as or longer than wide, lacking bands or markings; teeth 5–14 per sheath (3–4 on branches), 1–3 mm long (-8 mm on fertile stems), mostly black, persistent. STROBILI: 5–35 mm long, the tips rounded. SPORES: 35–70 μm in diameter. NOTES: Wet soil of montane meadows and stream banks, less commonly on moist rock ledges: Apache, Coconino, Gila, Graham, Mohave, Navajo, Pima, Yavapai cos. (Fig. 1A); 1400–2700 m (4500–8800 ft); nearly worldwide. The fertile stems of E. arvense are produced in April and May before the vegetative ones and wither soon after the spores are shed. They tend to be stouter than the vegetative stems, with fewer ridges and larger sheaths. The vegetative stems have regular whorls of branches, these with only 3–4 ridges. Care must be taken not to confuse these branches with those of wound-forms of other Arizona species, which develop following damage by flood-waters, and which occur irregularly and generally have more ridges. REFERENCES: G.Yatskievych and M.D. Windham , 2008, Vascular Plants of Arizona: Equisetaceae. CANOTIA 4 (2): 41–45.