Sparganium hyperboreum

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.

Phenology: Flowering summer (Jul–Aug).
Habitat: Cold, quiet, shallow, oligo- to mesotrophic arctic-alpine waters
Elevation: 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

Discussion

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).