Monographs Details: Equisetum myriochaetum Schltdl. & Cham.
Authority: Mickel, John T. & Smith, Alan R. 2004. The pteridophytes of Mexico. Mem. New York Bot. Gard. 88: 1-1054.
Description:Species Description - Aerial stems to 8 m tall (mostly 2–4 m tall), with regular whorls of branches; stems 3–23 mm diam., with 16–18 ridges; stem cross sections with both carinal and vallecular collenchyma well developed, the former usually extending to the vascular bundles and the latter to the vallecular canals; endodermis commonly double; sheath length to width ratio about 1:1.5, with dark margins on distal edge, otherwise green, occasional dark sheath at base of large specimens; stomata in one line on each side of grooves; branches with 6–8 ridges, ridges bearing tubercles resembling resembling saw teeth and pointing towards apices of stems; strobili terminal on branches and main stems blunt, or with slight apiculum; stem strobili to 30 mm long, 12 mm diam.; branch strobili 10 mm long, 4 mm diam.


Type. Mexico. Veracruz: Misantla, “in sylvis humidis,” Schiede & Deppe 833 (B!).

Equisetum myriochaetum has flattened sheath segments (also found in E. xhaukeanum) and often has poor tubercle development. Generally, large Mexican horsetails with regular whorls of branches belong to E. myriochaetum. Equisetum myriochaetum crosses with E. giganteum to form E. xschaffneri: Equisetum xschaffneri Milde, Verh. Zool.-Bot. Ges.Wien 11: 345. 1861 (Fig. 144d). Type. Mexico. Veracruz: Orizaba, Schaffner 315 (B?). For synonyms, see Hauke (1963). Its stems have regular whorls of branches and are 2–4 m tall (to 7 m tall in Costa Rica), 3–22 mm diam., with 12–48 ridges; vascular bundles each with its own endodermis; carinal collenchyma extending to vascular bundles; vallecular collenchyma extending to the vallecular canal or nearly so; sheaths green to brown at bases; stomata in bands of 1–2(–3) in each groove; branches with 6–8 ridges with saw teeth or irregular tubercles; strobili terminal on branches and main stems, acute or each with a slight apiculum; spores malformed. Equisetum x schaffneri rarely occurs from Mexico to Peru, in Mexico at 500–950 m elevation (Tam, Ver, Pue, Oax, Chis). Equisetum giganteum is not known to occur in Mexico, and Hauke (1963) suggested that E. x schaffneri is dispersed by an occasional good spore. Equisetum x schaffneri can be distinguished by its malformed, non-greens spores, intermediacy in number of rows of stomata (1–2 rows on each side of grooves); E. giganteum with broad stomatal band, 3–4 rows (vs. E. myriochaetum in one row on sides of each groove), branch ridges with flattened tubercles (vs. stomata in single lines and branch ridges with curved saw teeth in E. myriochaetum), and endodermis around individual bundles, as in E. giganteum (vs. continuous in E. myriochaetum).

Terrestrial in swampy or marshy places, usually seepage areas, usually more or less exposed, in pine-oak forests, grassy flats, lower montane rain forests; (350–)700–2420 m. Mexico; Guat, Hond, Nic, CR, Pan; Col, Ven, Ec, Peru. Selected Specimens Examined. Chis (Breedlove 21386, NY). Col (González s.n., 25 May 1977, IBUG). Gro (Lorea 1138, FCME). Gto (Pérez Calix & Carranza 2898, IEB). Hgo (Gimate 842, NY). Jal (Cochrane et al. 5186, IBUG, UC, WIS). Méx (Hinton 2402, NY, US). Mich (Moore et al. 5739, UC). Nay (McVaugh & Koelz 762, NY). NL (Sánchez et al. 438, MEXU). Oax (Hallberg 1020, ENCB, MICH, US). Pue (Kelly 567, US). Qro (Servi´n 1375, IEB). Sin (Vega A. 863, IEB). SLP (Pringle 3294, BR, NY, UC, US). Tam (González Medrano 16961, MEXU). Ver (Copeland herb. 164, GH, MICH, UC, US).