JULIANA VILLELA PAULINO, VIDAL DE FREITAS MANSANO
and SIMONE PÁDUA TEIXEIRA
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In this study, we evaluated the ﬂoral ontogeny of Swartzia dipetala, which has peculiar ﬂoral features compared with other legumes, such as an entire calyx in the ﬂoral bud, a corolla with one or two petals, a dimorphic and polyandrous androecium and a bicarpellate gynoecium. We provide new information on the function of pollen in both stamen morphs and whether both carpels of a ﬂower are able to form fruit. Floral buds, ﬂowers and fruits were processed for observation under light, scanning and transmission electron microscopy and for quantitative analyses. The entire calyx results from the initiation, elongation and fusion of three sepal primordia. A unique petal primordium (or rarely two) is produced on the adaxial side of a ring meristem, which is formed after the initiation of the calyx. The polyandrous and dimorphic androecium also originates from the activity of the ring meristem. It produces three larger stamen primordia on the abaxial side and numerous smaller stamen primordia on the adaxial side. These two types of stamens bear morphologically similar ripening pollen grains. However, prior to the dehiscence of thecae and presentation of pollen in the anther, only the pollen grains of the larger stamens contain amyloplasts. Two carpel primordia are initiated as distinct protuberances, alternating with the larger stamens, in a slightly inner position in the ﬂoral meristem, constituting the bicarpellate gynoecium. Both carpels are able to form fruit, although only one fruit is generally produced in a ﬂower. The increase in gynoecium merism probably results in an increase in the surface deposition of pollen grains and consequently in the chance of pollination. This is the ﬁrst study to thoroughly investigate organogenesis and the ability of the carpel to form fruit in a bicarpellate
ﬂower from a member of Fabaceae, in addition to the pollen ultrastructure in the heteromorphic stamens
associated with the ‘division of labour’ sensu Darwin.