A small family, largely of the southern hemisphere and largely aquatic (including all of our species). The submersed leaves in all our species (except one essentially leafless Myriophyllum) are pectinate, i.e., with a straight central axis and simple filiform lateral segments on both sides, like a 2-sided comb. Other aquatics with dissected leaves may have them forked dichotomously (Ceratophyllum) or more irregularly branched (Ranunculus, Utricularia, Bidens beckii), but the only other species with a straight central axis is Rorippa aquatica; it has alternate leaves with the lateral segments usually further divided.
1. Leaves all or mostly whorled (or nearly so), deeply pectinate, or (in M. tenellum) essentially absent (but alternate); flowers mostly unisexual, 4-merous, in axils of submersed deeply pectinate leaves or (most species) of emersed alternate or whorled bracts.
1. Leaves all clearly alternate, serrate to deeply pectinate; flowers bisexual, 3-merous, in axils of emersed alternate leaves.
All species found in Haloragaceae
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. July 12, 2020. https://michiganflora.net/family.aspx?id=Haloragaceae.