The flowers are very much reduced, consisting of little or no perianth (the calyx at best a mere rim on the ovary), one stamen, and one carpel containing one ovule; they are presumably wind-pollinated. They form in the axils of ± firm aerial leaves, quite unlike the limp submersed leaves, which the plant produces under water.
All species found in Hippuris
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. January 21, 2021. https://michiganflora.net/genus.aspx?id=Hippuris.