Comparative studies of floral development and morphology have largely contributed to the understanding of taxonomic classification, phylogenetic relationships and evolutionary trends across many angiosperm clades, particularly in the florally diverse family Leguminosae (alternatively Fabaceae). This study aimed to characterize the middle to late stages of floral development and morphological variation of the caesalpinioid genus Tachigali, an evolutionary radiation of predominantly neotropical rainforest trees. Floral buds and flowers of five representative species from Tachigali were analyzed under stereo microscopy, light microscopy and scanning electron microscopy to evaluate informative morphological and developmental characters. Although the genus displays relatively small flowers measuring up to 14 mm long, they are variable in terms of symmetry, structure and size, which have influenced the main taxonomic subdivisions among the species. Here, we show that the floral architecture of Tachigali involves a double whorl of stamens, anthers with dome-shaped connective extension and monosymmetrical hypanthium, owing to the unequal development of its wall at different stages of the floral ontogeny. Such developmental patterns are likely new diagnostic floral characters of Tachigali in the context of the early diverging caesalpinioid clades and reaffirm the circumscription of the genus in order to include the species previously classified within Sclerolobium.
