Isoëtes lacustris Linnaeus, Sp. Pl. 2: 1100. 1753.
Lake quillwort, isoète lacustre
Isoëtes hieroglyphica A. A. Eaton; I. macrospora Durieu
Plants aquatic, submerged. Rootstock nearly globose, 2-lobed. Leaves evergreen, dark green to reddish green, pala brown toward base, spirally arranged, to 25 cm, rigid, abruptly tapering to tip. Velum covering less than 1/2 of sporangium. Sporangium wall ± brown-streaked. Megaspores white, 55--750 μm diam., cristate to reticulate with branching to anastomosing ridges; girdle densely papillate or rarely smooth. Microspores gray in mass, 33--45 μm, paillose. 2n = 110.
Spores mature late summer. Cool, oligotrophic, slightly acidic lakes and streams; Greenland; Man., N.B., Nfld., N.S., Ont., P.E.I. Que., Sask.; Conn.; Maine, Mass., Mich., Minn., N.H., N.Y., R.I., Tenn., Vt., Va., Wis.; n,c Europe.
Population of Isoëtes lacustris in Tennessee and Virginia are disjunct.
North American plants of Isoëtes lacustris have been segregated as I. macrospora; both taxa are decaploids (2n = 110) and have similar leaf and spore morphology. Then cannot reliably be distinguished from each other except on the basis of geography.
Isoëtes lacustris is a totally submerged aquatic. Plants have been found at depths of more than 3 m. Plants with rugulate megaspores bearing smooth, rounded ridges and a smooth girdle have been called I. hieroglyphica A. A. Eaton [I. macrospora f. hieroglyphica (A. A. Eaton) N. E. Pfeiffer].
Isoëtes lacustris hibridizes with I. echinospora [ = I. x hickeyi W. C. Taylor & Luebke]; I. engelmannii; I. riparia [ = I. x jeffreyi D. M. Britton & Brunton]; and with I. tuckermanii [ = I. x harveyi A. A. Eaton].