Flowers of Dipterygeae (Fabaceae, Papilionoideae) exhibit an unusual petaloid calyx. The two adaxial sepals are large and petaloid, and the three abaxial sepals form a three-toothed lobe. The goal of this study was to elucidate the ontogenetic pathways of this peculiar calyx in light of the floral development of the three genera that comprise the tribe. Floral buds of Dipteryx alata, Pterodon pubescens and Taralea oppositifolia were analysed using scanning electron microscopy and light microscopy. The order of bracteole and sepal initiation varies among the species. The androecium is asymmetric. The carpel cleft is positioned to the right or to the left, and is opposite the adaxial antepetalous stamen. The peculiarity of the calyx becomes noticeable in the intermediate stages of floral development. It results from the differential growth of the sepal primordia, in which the abaxial and lateral primordia remain diminutive during floral development, compared with the adaxial ones that enlarge and elongate. Bracteoles, abaxial sepals, petals and anthers are appendiculate, except in T. oppositifolia, in which the appendices were not found in bracteoles or anthers. These appendices comprise secretory canals or cavities. Considering that the ontogenetic pathway for the formation of the petaloid calyx is similar and exclusive for Dipterygeae, it might be a potential synapomorphy for the group, with the presence of secretory canals in the appendices of abaxial and lateral sepals and petals.

The presence of glandular appendages in the anthers is a rare condition in angiosperms. In Leguminosae it occurs in species of the Mimosoid clade and in earlybranching clades of papilionoids such as Dipterygeae. In Dipterygeae such appendages surprisingly exhibit a secretory cavity instead of secretory emergences as is the case for the Mimosoid clade. Thus, the objective of this study was to elucidate the function of anther glands in Dipteryx alata and Pterodon pubescens, species in the Dipterygeae clade that exhibit a pollen release mechanism that is intermediate between the explosive and valvular types.• Flower buds and flowers were processed for surface, anatomical, histochemical and ultrastructural analyses.• Anther glands consist of a cavity secreting sticky substances (oleoresins and polysaccharides) that play a key role during the flower's lifespan by aggregating pollen grains and attaching them to the floral visitor's body. Other floral features that are important for understanding the pollen release mechanism that is intermediate between the valvular and the explosive types are: (i) keel petals intertwined with tector trichomes; (ii) glandular appendages in the abaxial and lateral sepals and in petals composed of secretory ducts; and (iii) a continuous secretion process of the anther glands followed by an asynchronous dehiscence of anthers.• The well-adapted papilionoid flag blossom with anther glands and keel petals intertwined with trichomes provided the foundation for a successful canalisation toward a pollen release mechanism intermediate between the explosive and valvular types inside early-branching papilionoids.

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