The tribe Paullinieae is distinguished by a monoecious reproductive system; it exhibits two floral morphs, namely staminate flowers, with gynoecium reduced to a pistillode, and morphologically hermaphrodite but functionally pistillate flowers. The aim of this study was to analyze the development of sporogenesis and gametogenesis in flowers of Thinouia mucronataRadlk. (subtribe Thinouiinae) and Serjania meridionalisCambess. (subtribe Paulliniinae) and to elucidate the moment when pollen grains of pistillate flowers and the ovule of staminate ones stopped their development. Light and scanning electron microscopy was applied using standard techniques. In T. mucronataflowers are actinomorphic; the pistillate ones have anatropous, pseudocrassinucellate ovules, without hypostase and with placental obturator, and stamens with indehiscent anthers and well-developed pollen grains that remain inside the pollen sac at the end of anthesis. Serjania meridionalishas zygomorphic flowers, the pistillate ones have campylotropous, crassinucellate ovules, with presence of hypostase, and funicular obturator, and stamens with indehiscent anthers collapsed at the end of anthesis, showing remnants of aborted sporogenous tissue in each pollen sac. Both species share the anatomy of the anther wall; microsporocytes form tetrahedral or decussate tetrads; monads are bicellular; the pistillode has abortive megaspore; gynoecium with bithegmic ovules and Polygonum-type megagametophyte. One difference in antheral wall ontogeny between species was that T. mucronatadisplayed the dicotyledoneus type development, whereas S. meridionalisexhibited the basic type. These embryological characters clearly support the basal position of Thinouiain the tribe recently validated from molecular phylogeny studies and supported by the present morphological data.