Isoëtes acadiensis Kott, Canad. J. Bot. 59: 2592. 1981.
Plants aquatic, submerged. Rootstock nearly globose 2(--3)-lobed. Leaves evergreen, olive green to reddish brown, pale toward base, spirally arranged, to 20 cm, pliant, gradually tapering to tip. Velum covering less than 1/2 of sporangium. Sporangium wall brown-streaked. Megaspores white, 400-650 μm diam., rugulate to reticulate with ridges having rounded or smooth crests; girdle smooth. Microspores gray in mass, 25--35 μm, spinulose. 2n = 44.
Spores mature in late summer. Emergent or in shalow water of slightly acid lakes, ponds, and streams; N.B., Nfld., N.S.; Mass., Maine, N.H., N.Y.
Isoëtes acadiensis can be distinguished from I. tuckermanii by its megaspore texture. Where these two taxa grow together, plants bearing megaspores with intermediate patterns occur. In Atlantic Canada, I. acadiensis typically occupies shallower water than I. tuckermanii.
Isoëtes acadiensis hybridizes with I. lacustris.