Isoëtes riparia Engelmann ex A. Braun, Flora. 29: 178. 1846.
Shore quillwort, isoète riparien
Isoëtes canadensis (Engelmann) A. A. Eaton; I. saccharata Engelmann
Plants usually aquatic, occasionally emergent. Rootstock nearly globose, 2-lobed. Leaves evergreen, bright green, pale toward base, spirally arranged, often twisted, to 35(--50) cm, pliant, gradually tapering to tip. Velum covering less than 1/2 of sporangium. Sporangium wall brown-streaked to completely brown. Megaspores white, 450-650 μm diam., cristate with isolated and branching, lamellate ridges; girdle obscure. Microspores gray to light brown in mass, 30--40 μm, tuberculate. 2n = 44.
Spores mature late summer. Margins of lakes, ponds, and streams; tidal shores and estuaries; calcareous to slightly acidic substrates in fresh, usually oligotrophic, water; Ont., Que.; Conn., Del., Fla., Maine, Md., Mass., N.H., N.J., N.Y., N.C., Pa., R.I., S.C., Vt., Va., W. Va.
Megaspore morphology, electrophoretic profiles of leaf enzymes, and chromosome number provide evidence that Isoëtes riparia is an allotetraploid, formed by chromosome doubling in I. x eatonii, the primary diploid hybrid between I. echinospora and I. engelmanii. The considerable variation in leaf and spore morphology possibly results from multiple allotetraploid origins.
Isoëtes riparia hybridizes with I. echinospora [ = I. x dodgei A. A. Eaton]; I. engelmannii [ = I. x brittonii Brunton & W. C. Taylor]; I. lacustris [ = I. x jeffreyi D. M. Britton & Brunton); and I. tuckermanii.