Macroptilium arenarium (Bacigalupo) S.I.Drewes & R.A.Palacios produces two floral morphs, aerial chasmogamous flowers and cleistogamous flowers in geophyte racemes. A comparative study of the sporogenesis, gametogenesis and the development of the related sporophytic structures in both floral morphs is reported. The anther is tetrasporangiate, its wall consists of epidermis, endothecium, one or two middle layers and an uninucleate secretory tapetum. The mature endothecium presents fibrilar thickenings that are more developed in cleistogamous flowers. Pollen grains are tricolporate, angulaperturate, and are shed at bicellular stage. The ovule is crassinucelate, bitegmic and anacampylotropous. Megaspore tetrads with linear arrangement have been observed in chasmogamous flowers, whereas only megaspore dyads have been found in cleistogamous flowers. In both floral morphs the chalazal megaspore develops into an embryo sac of Polygonum type. Apomixis is considered as a possible replacement for sexual reproduction in cleistogamous flowers.