P. glabra (Willd.) M. Gómez, in its broad sense, is a rhizomatous subaquatic species with a pantropical range; 2n = 40, 60 (FNA 5, W). Plants native to the Americas have been segregated as densiflora by W but were combined with glabra in FNA 5. There are only two confirmed Ky. records: from DAVI (APSC) in natural wetlands, and from MADI (BEREA) in an artificial reservoir. Other colls. may exist but have not been confirmed (BA, BT, M): there is a reported coll. from CLAR (MO). P. densiflora is known from adjacent w. Tenn., se. Mo., s. Ill. (K) and s. Ind. (S. Namestnik, pers. comm.; Weakley et al. 2023). It can easily be overlooked in beds of hydropiperoides or setacea, but densiflora is a generally more robust plant (with blades mostly 15-30 x 2-5.4 cm versus 5-25 x 0.4-3.7 cm), and its ocreae are eciliate. It has two styles and biconvex achenes, as in amphibia (versus trigonous in the hydropiperoides complex). Mitchell (1976) found that it had intermediate flooding tolerance, between amphibia (generally in deeper water) and hydropiperoides (shallower), but considerable variation in tolerance appears to exist within all three species.