The flowers are unisexual, without a perianth, consisting of a single stamen or a single 2-carpellate pistil. Callitriche terrestris and C. hermaphroditica are reasonably distinctive in the genus. Insofar as fruiting or other characters are adequate on specimens examined, almost all other Michigan populations are C. palustris. Submersed leaves have a distinctive bidentate apex, which readily identifies specimens that might otherwise be confused with depauperate Elodea (if only opposite-leaved) and any similar aquatics.
1. Plants entirely terrestrial, mat forming; leaves all alike; flowers on a short pedicel.
1. Plants fully or partially submerged or floating; when floating, leaves dimorphic, occasionally ± terrestrial on wet shores, but the aquatic and floating leaves usually still discernable; flowers sessile.
2. Leaves all submersed, linear; fruit becoming fully ripe very late in the season (apparently September–November, if at all), and then conspicuously winged the full length of each margin.
2. Leaves rarely (only when young) all submersed and linear, some or all usually crowded into a terminal floating rosette (or ± terrestrial) and/or oblanceolate to spatulate; fruit ripening early to late summer and wingless or with a narrow wing noticeable toward the apex.
3. Fruit about as long as wide, wingless, with pits of punctate surface not in rows.
3. Fruit mostly longer (by 0.2 mm) than wide, winged across the apex, the wings abruptly narrowed or absent basally, with pits of punctate surface tending to run in longitudinal rows.
All species found in Callitriche
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. November 13, 2019. https://www.michiganflora.net/genus.aspx?id=Callitriche.