Plants slender, grasslike, to 0.8 m; leaves and inflorescences usually floating. Leaves limp, unkeeled, flat, 0.1--0.4(--0.8) m 1--5 mm. Inflorescences: rachis unbranched, flexuous; bracts ascending, lower bracts slightly inflated near base; pistillate heads 1--4, axillary, contiguous, sessile, or most proximal peduncled and supra-axillary, 0.5--1.4 cm diam. in fruit; staminate heads 1(--2), terminal, contiguous or not with distalmost pistillate head. Flowers: tepals without subapical dark spot, erose; stigmas 1, ovate. Fruits brown or yellowish, dull, subsessile, body ellipsoid to obovoid, not faceted, ±more or less constricted at equator, 2--5 1.5--2.5 mm; beak less than 0.5 mm, or absent; tepals attached at base, not reaching equator. Seeds 1. 2n = 30. Flowering summer (Jul--Aug). Cold, quiet, shallow, oligo- to mesotrophic arctic-alpine waters; 0--3000 m; Greenland; St. Pierre and Miquelon; Alta., B.C., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Yukon; Alaska; circumboreal. Sparganium hyperboreum is distinguished from other floating-leaved species by its beakless fruits with sessile stigmas. Putative hybrids between Sparganium hyperboreum and S. natans have been found in Manitoba, Newfoundland, Northwest Territories, and Alaska by V. L. Harms (1973), who discussed variation in both species. The hybrids have wider (2--5 mm) leaves, golden-brown fruits with short (1 mm) beaks, and supra-axillary pistillate heads (V. L. Harms 1973; C. D. K. Cook and M. S. Nicholls 1986).
OTHER REFERENCES Beal, E. O. 1977. A Manual of Marsh and Aquatic Vascular Plants of North Carolina, with Habitat Data. Raleigh. [North Carolina Agric. Exp. Sta. Techn. Bull. 247.] Brayshaw, T. C. 1985. Pondweeds and Bur-reeds, and Their Relatives: Aquatic Families of Vascular Plants in British Columbia. Victoria. [Brit. Columbia Prov. Mus., Occas. Pap. 26.] Cook, C. D. K. 1985. Sparganium: Some old names and their types. Bot. Jahrb. Syst. 107: 269--276. Larson, G. E. 1993. Aquatic and Wetland Vascular Plants of the Northern Great Plains. Fort Collins, Colo. [U.S.D.A. Forest Serv., Gen. Techn. Rep. RM-238.] Löve, Á. and D. Löve. 1981. In: IOPB chromosome number reports LXXII. Taxon 30: 694--708. Mason, H. L. 1957. A Flora of the Marshes of California. Berkeley. Müller-Doblies, D. 1970. Taylor, R. L. and G. A. Mulligan. 1968. Flora of the Queen Charlotte Islands. Part 2. Cytological Aspects of the Vascular Plants. Ottawa. Voss, E. G. 1966. Nomenclatural notes on monocots. Rhodora 68: 435--463.
