Papilionaceous flower

Papilionaceous flowers (from Latin: papilion, a butterfly) are flowers with the characteristic irregular and butterfly-like corolla found in many, though not all, plants of the species-rich Faboideae subfamily of legumes. Tournefort suggested that the term Flores papilionacei originated with Valerius Cordus, who applied it to the flowers of the bean.

The flowers have a bilateral symmetry with the corolla consisting of five petals. A single, large, upper petal is known as the banner (also vexillum or standard petal). The semi-cylindrical base of the banner embraces and compresses two equal and smaller lateral wings (or alae). The wings in turn enclose a pair of small keel petals, that are situated somewhat lower than the wings, but are interior to them. They have concave sides and correspond with the shape of the wings. The two keel petals are fused at their bases or stuck together to form a boat-shaped structure that encloses the essential flower organs, namely the androecium and gynoecium. Typically these flowers have a vexillary (i.e. descendingly imbricate or overlapping) aestivation.

The stamens often have a diadelphous morphology, meaning that they are united in two sets with a distinct filament shape in each set. The inferior set forms a membranous sheath at its base, that envelops the single, superior pistil in a tube. The tube divides into nine filaments, which trace the base of the keel petals that enclose them. The single free filament lies above them. In Smithia the two sets are equal, with 5 stamens in each.

Many papilionaceous flowers however have ten distinct stamens. Genera that conform to the latter morphology include Sophora, Anagyris, Cercis, Daviesia, Dillwynia, Gompholobium, Mirbelia and Pultenaea.

...
Wikipedia