The massive, starchy, spongy rhizomes (several cm in diameter) may be distinguished from those of Nymphaea by the more numerous closely spaced spiral rows of leaf scars, which are ± triangular, winged, or semicircular. In Nymphaea, the leaf scars are circular and more irregularly spaced (and there may also be tuberous branches or scars thereof).
1. Sepals 5, at most about 15 mm long; stigmatic rays 10 or fewer, on a red disc; pistil ca. 1 cm long or less at anthesis, ripening into a smooth fruit at most 2 cm long; anthers ca. 1–2 (–2.5) mm long, shorter than filaments, the stamens deciduous from the base of the ripe fruit; blades of floating leaves up to 10 cm long, the sinus usually at least two-thirds as long as the midrib.
1. Sepals usually 6 and longer; stigmatic rays (9–) 10–21, on a yellow or green (rarely reddish) disc; pistil often larger, ripening into a ridged or striate fruit up to 4 cm long; anthers ca. (3–) 4–6 (–7) mm long, longer than filaments, the withered stamens persisting at the base of the fruit; blades of floating and emersed leaves usually larger, the sinus rarely more than half as long as the midrib.
2. Petioles rounded in cross-section; sepals with a green patch basally.
2. Petioles flattened above, or even slightly winged; sepals with a prominent maroon (very rarely green) patch basally.
All species found in Nuphar
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=Nuphar.