The anatomy of the stigma and style of three species of Oxalis (O. articulata, O. hispidula, and O. paludosa), belonging to different sections (Articulatae, Ionoxalis, and Corniculatae) was studied using light, scanning, and transmission electron microscopy. The stigma morphology of each of the different flower morphs of the three species (longistylous, medistylous, and brevistylous flowers) was compared in this work. The stigma is dry and has multicellular and multiseriate papillae. The morphology of the papillae does not differ between flower morphs. According to the Ca²⁺ concentration in pre-anthesis, anthesis, and post-anthesis, we hypothesize that the stigmas of the different morphs are equally receptive. Oxalis style is solid type. The cytoplasm of the transmitting tissue cells is dense with few vacuoles and abundant organelles. The transmitting tissue cells have large amounts of intercellular substance, mainly at the corners. This substance has moderate electron density in the species O. articulata and O. hispidula and shows some laxer areas in O. paludosa. The transmitting cell wall of the two first species has wall ingrowths like fingers with low electron density that protrude into the cytoplasm. The ultrastructural characteristics of the transmitting cells allow to characterize three of the sections of the genus.