Plants of Callitriche hermaphroditica have sometimes been mistaken for vegetative plants of Elodea, but their leaves are strictly opposite and have a very characteristic bidentate apex.
The pistillate flowers of both our species are solitary and are carried to the surface (if water conditions are right) by elongation of the floral tube, which appears as a thread-like stalk. In both species the anthers dehisce after the perianth opens, spraying pollen onto the surface of the water, where it drifts and may reach a pistillate flower in a manner similar to the drifting entire flower of Vallisneria. Most reproduction is vegetative by fragmentation of the stem. Staminate plants of E. canadensis are relatively uncommon; however, in Michigan pistillate plants of E. nuttallii appear to be less common than staminate ones. Egeria densa Planch. is the common cultivated waterweed of aquaria and fish ponds. It resembles a large Elodea canadensis, but has longer leaves (in whorls usually of 4–6) and larger flowers. A native of South America, it is naturalized in several places in the northeastern United States and could occur in Michigan, as also the similar Hydrilla verticillata (L. f.) Royle, with visibly toothed leaves.
1. Well-developed leaves generally 1.5–4 (–5) mm wide (mostly about 2 mm) and 2–5 times as long as wide (or sometimes a little longer, especially in deep-water plants); staminate flower in an elongated spathe 4–8.5 (–14) mm long, sessile when immature, at length remaining attached by a long extremely delicate filiform stalk (floral tube); sepals of pistillate flowers ca. 2–4.5 mm long.
1. Well-developed leaves generally 0.8–1.5 mm wide (mostly about 1 mm, very rarely to 2 mm, but averaging less) and (4–) 5–10 (–13) times as long as wide; staminate flower in a ± spherical spathe with body ca. 2–3 mm long, sessile, breaking free from spathe at maturity; sepals of pistillate flowers ca. 1–1.5 mm long.
All species found in Elodea
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 12, 2017. http://www.michiganflora.net/genus.aspx?id=Elodea.