Lvs mostly 5–15 cm × 0.5–1.5 mm, erect or sometimes spreading, soft, gradually tapering from the base to a long, very slender tip, often stomatiferous toward the tip, without peripheral strands; sporangia 4–7 mm, usually brown-dotted at maturity, in ours usually at least half covered by the velum; megaspores 0.25–0.6 mm wide, densely short-spiny, the spines sharp to blunt or even truncate, sometimes some of them bifid; 2n=22. Submersed or amphibious in ponds, lakes, and slow streams, rooted in sand or mud; common, circumboreal, s. to N.J., Pa., O., Mich., Minn., Colo., and Calif. The Amer. plants may be weakly distinguished as var. muricata (Durieu) Engelm. (I. muricata; I. braunii, a preoccupied name; I. setacea, misapplied)
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