Flower diversity in the tribe Amorpheae encompasses many unusual modifications of the papilionoid flower, including petal loss and a unique tubular region on which petals and stamens are inserted. Barneby's phylogeny of Amorpheae indicates that this synorganization is derived within the tribe and is found only in Dalea and Marina, although he did not establish whether it originated once. Other Amorpheae are described as having petals inserted on a hypanthium rim. We sought to test (1) whether the synorganization is structurally homologous in Dalea and Marina and (2) whether this synorganization is present in other members of Amorpheae. Our results show that all investigated Amorpheae have a hypanthium (a synorganized zone subjacent to the sepals, petals, and stamens). In Dalea and Marina, distal to the hypanthium, there is a calyx tube and a separate region of synorganized stamens and petals. This region is unique in that it appears as fused filaments, not petaloid as in other angiosperms that have corolla-androecium synorganization (i.e., a corolla tube). The banner, wing and keel petals attach to this synorganized region at different heights, and these heights vary among species. We infer that the corolla-androecium synorganization found in Marina and Dalea are structurally homologous. Psorothamnus is a genus previously described as having petals inserted on the hypanthium rim. We observed this to be the case for some species; however, other species were observed to have a short zone of synorganization. The latter condition is structurally homologous with that in Dalea and Marina. Petal loss further complicates the interpretation of the hypanthium region in the genera Amorpha and Parryella, genera previously grouped with Psorothamnus as not having synorganized petals and stamens.

Key words: Amorpheae, floral morphology, homology, Papilionoideae, synorganization