Richards et. al. (2010) explains the unique Banana Plant morphology, physiology and reproduction well. These are the photos from that paper.

See:
http://www.amjbot.org/content/97/11/1761.full.pdf+html
(Open access)

"During sexual reproduction of species in the aquatic genus Nymphoides (Menyanthaceae), the flowers appear to arise directly from the petiole of a floating leaf"