This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
About 25 species, of very wide geographic distribution. Type species: Equisetum ftuviatile L.
Stems annual; stomata scattered.
Stems of two kinds, the fertile appearing in early spring before the sterile.
Fertile stems simple, soon withering; sheaths of branches of sterile stems 4-toothed.
Fertile stems branched when old, only the apex withering.
Branches of the stem simple, their sheaths 3-toothed.
Stems all alike; spores mature in summer; branches simple or none.
Sheaths rather loose; branches usually long; stems bushy below, attenuate upwards.
Central cavity very small; spike long.
Central cavity about one-half the diameter of stem; spike short.
Sheaths appressed; branches usually short.
Stems mostly perennial, evergreen; spikes tipped with a rigid point; stomata in regular rows.
Stems tall, usually many-grooved.
Stems rough and tuberculate, prominently ridged.
Ridges with 1 line of tubercles; ridges of sheath tricarinate; stem stout.
Ridges of the stem with 2 indistinct lines of tubercles; ridges of sheath obscurely 4-
carinate; stem slender.
Stems not tuberculate; sheaths enlarged upward.
Stems low, slender, tufted, usually 5-10-grooved.
Central cavity small; sheaths 5-10-toothed.
Central cavity none; sheaths 3-toothed.