The two very species-rich families Apocynaceae and Orchidaceae contain the most complex flowers in the angiosperms. Although they are not related they show a number of amazing convergent similarities in flower structure.
Endress reviews and compares development and evolution of this complexity in both families. It has arisen by progressive synorganization of the basic floral organs, especially androecium and gynoecium, and partly also by the emergence of novel floral parts. As a result the flowers look puzzling and the basic organs can no longer be easily recognized at first sight.
Peter K. Endress, 2015, 'Development and evolution of extreme synorganization in angiosperm flowers and diversity: a comparison of Apocynaceae and Orchidaceae', Annals of Botany, vol. 117, no. 5, pp. 749-767 http://dx.doi.org/10.1093/aob/mcv119