Stems 1-4 dm, usually erect, branched from the base; rhizome elongate, ending in a slender white tuber 1-2 cm; lvs all submersed, narrowly linear, 5-12 cm נ0.2-1.5 mm, obtuse to merely acute, 1-nerved, with remote cross-veins; lacunar cells none; stipular sheaths 0.5-2 cm, tightly clasping the stem, adnate to the blade for 2-10 mm, the margins connate below; peduncles slender, 2-15 cm; spikes submersed and hydrophilous, 1-5 cm, with mostly 2-5 unequally spaced whorls of fls, at least the lower whorls generally well separated; frs obovoid, 2-3 mm, the beak very low and truncate, the dorsal keel low and rounded, the lateral keels obscure or none; 2n=78. Shallow calcareous waters; circumboreal, in Amer. s. to Me., N.H., Pa., Mich., Minn., and Ariz. Our plants are mostly var. borealis (Raf.) H. St. John, with the uppermost whorls of fls in the spike adjacent, and the lower ones at most 7 mm apart. (P. interior) The chiefly northern var. filiformis, seldom found in our range, has laxer spikes, even the upper whorls separated.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.