Isoëtes engelmannii A. Braun, Flora. 29: 178. 1846.
Isoëtes valida (Engelmann) Clute
Plants aquatic, emergent. Rootstock nearly globose, 2-lobed. Leaves evergreen, bright green, pale toward base, spirally arranged, to 60(--90) cm, pliant, gradually tapering to tip. Velum covering less than 1/4 of sporangium. Sporangium wall usually unpigmented, occasionally ± brown-streaked. Megaspores white, 400--560 μm diam., reticulate, with unbroken lamellate ridges; girdle obscure. Microspores gray in mass, 20--30 μm, smooth to papillose. 2n = 22.
Spores mature in summer. Emergent or in shallow water of lakes, ponds, streams, and ditches; Ont.; Ala., Ark., Conn., Del., Fla., Ga., Ill., Ind., Ky., Md., Mass., Mich., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Vt., Va., W. Va.
Isoëtes engelmannii is the most widely distributed quillwort of eastern North America. Plants with larger megaspores, ranging from 480--560 μm, have been called I. engelmannii var. georgiana Engelmann. This variety may represent a tetraploid cytotype. A tetraploid population of I. engelmannii (2n = 44) from northern Florida has larger megaspores characteristic of var. georgiana.
Isoëtes engelmannii hybridizes with I. echinospora [ = I. x eatonii Dodge (later synonym = I. x gravesii A. A. Eaton); I. flaccida; I. tuckermanii [ = I. x foveolata A. A. Eaton ex Dodge]; I. lacustris; and I. riparia [ = I. x brittonii Brunton & W. C. Taylor].