An Illustrated Flora of the Northern United States, Canada and the British Possessions/Equisetaceae

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. Branches compound. 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.