This is widespread across temperate regions of North America. Old or obscure records of amplifolius from Ky. are provisionally included here, pending more detailed examination; see notes under that name. Submersed and floating leaves are ca. 2-5 cm wide in both species, but illinoensis differs in several characters (FNA 22); 2n = 104 (versus 52). Hybrids of these two species have been reported. Leaves of illinoensis tend to be larger than other species of the genus in Ky., but are similar in size to nodosus (FNA 22). Its submersed leaves are 5-20 x 2-4.5 cm (versus 9-20 x 1-3.5 cm in nodosus); floating leaves are 4-19 x 2-6.5 cm (versus 3-11 x 1.5-4.5 cm). However, petioles of illinoensis tend to be shorter: submersed are 0-4 cm (versus 2-13 cm) and floating 2-9 cm (versus 3.5-26 cm). Distinction of illinoensis from nodosus is based on its larger submersed leaves being acuminate at apex with sharp awl-like tip (versus just acute); also fruits grayish green to olive-green (versus red to reddish brown)..