3 (2) Vegetative stems commonly elongating unless plant emersed from early age; petiolate leaf blade round to oblong, base cordate to truncate; distal central perianth limb lobe with lateral flanges 5 Heteranthera rotundifolia + Vegetative stems elongating only on plants in over 5 cm of water; petiolate leaf blade oblong to ovate, base truncate to cuneate; distal central perianth limb lobe without lateral flanges 6 Heteranthera limosa

Perianto glabro, blanco o morado; tubo 1,5-4,4 cm; lóbulos 0,5-1,6 cm, oblongos, obtusos y todos dispuestos simétricamente (los 3 lóbulos superiores hacia la base y el central con o sin una banda morada sobre otra amarilla).

perianto glabro, azul o blanco; tubo 1,1-2,9 cm, lóbulos 0,5-1,8 cm, oblongos, obtusos y dispuestos asimétricamente (3 lóbulos hacia arriba, 2 en posición transversal y uno, más ancho, hacia abajo); el superior con 2 apéndices en la base y una mancha amarilla rodeada por 2 bandas más obscuras

Aunque H. rotundifolia fue considerada recientemente como subespecie de H. limosa..., la diferencia entre sus caracteres florales evidencia su rango específico. Los apéndices del tépalo superior y los filamentos de los estambres superiores recurvados en el ápice contribuyen a la mayor zigomorifia de la flor de H. rotundifolia frente a H. limosa.

perianth blue or white, salverform, tube 15–44 mm, limbs essentially actinomorphic, lobes equal, narrowly elliptic, 5.2–26.3 mm, distal lobes yellow toward base; stamens unequal, lateral stamens 2.3–7.8 mm, central stamen 3.3–7.2 mm; filaments linear, glandular-pubescent

perianth blue or white, salverform, tube 11–29 mm, limbs essentially actinomorphic, lobes narrowly elliptic, 5.2–18.2 mm, proximal lobe longer, distal central lobe with lateral flanges toward base; stamens unequal, lateral stamens 2.8–8 mm, central stamen 3.9–8.5 mm; filaments linear, curved downward apically, glandular-pubescent;

Heteranthera rotundifolia is closely related to H. limosa and has been considered conspecific with it by many, including D. S. Correll and H. B. Correll (1972), who noted differences in stem development but did not recognize them taxonomically. Close study of reproduction and seedling development (C. N. Horn 1988) has revealed other differences, besides yet others in flower morphology (cf. the descriptions). Seedlings of H. rotundifolia have flexible petioles and commonly develop elongate stems, while those of H. limosa have rigid petioles, even in deep water, and rarely develop elongate stems.