2. Isoetes engelmannii A. Braun var. engelmannii (Engelmann's quillwort) Pl. 18h,i,j; Map 35

Leaves 14–38(–60) cm long, the outer surface of the base white to pale green, not shiny. Velum covering less than 1/4 of sporangium. Sporangium walls usually lacking brown dots or streaks. Sporangia 6–13 mm long, oblong. Megaspores white, mostly 0.40–0.55 mm in diameter, the surface with a raised network of narrow ridges. Microspores gray, smooth to papillose. 2n=22. May–August.

Scattered in the eastern Ozarks north historically to St. Louis County (eastern U.S. and adjacent Canada). Submerged to emergent aquatic in sinkhole ponds and along mossy margins of small creeks.

This species is the largest quillwort in Missouri and the only one with a truly aquatic habit. Most of the collections to document its distributions were made prior to 1950. Since that time the vegetation of many of the state's sinkhole ponds has changed, and the species has apparently been extirpated at some of its historical sites.