Two new and peculiar species of Heteranthera are herein described. Heteranthera catharinensis is unique in the genus due to its glomerulate, many-flowered inflorescences, in which the flowers are restricted to the base and apex of the cincinni. It also possesses the biggest flowers in the H. reniformis Ruiz & Pavón species complex, with glabrous perianth lobes, medial filament, and style. On the other hand, Heteranthera pumila is described as the smallest known species of Pontederiaceae, with its dwarf stature, petiolate leaves with especially diminute blades, inflorescences 1-2-(3)-flowered, peduncle densely covered with glandular hairs, basal bract with glandular hairs at base, and smooth seeds, rarely possessing 7-9 inconspicuous longitudinal wings. We present detailed descriptions, illustrations, comments, a distribution map, conservation assessments for the new species, and an identification key to the Brazilian species of Heterantheras.l. Finally, we discuss inflorescence morphology and terminology in Pontederiaceae, characterizing it as thyrsoid.
Keywords: Atlantic Forest; Commelinales; Neotropical flora; aquatic flora; mudplantains; pickerelweed.