The H.verticillata is kept in very shallow water, at the deepest end it´s barely 7cm deep and it can very easily shoot its leaves out, which is what it prefers.
The H.vulgaris is very different and can be kept submerged. If it has sufficient light and nutritients it shoots huge leaves trying to reach the surface. I have a piece of it in my pyrrhogaster tank which is 27 cm deep at the moment, and one leave has recently reached the surface. I hope you can see in the pictures the very long stem and the pad almost touching the surface. Excuse the really bad quality.
Chris, i´ll send you some verticillata back if you want. I´m chuffed with the leucocephala, it´s doing GREAT for me now since i acquired Planorbis/Planorbarius that keep it clean and free of algae. The terrestrial portion i keep in the dunni tank is now exploding with flowers and growing really, really fast. I love the damn plant.
I´m also adding a picture of the H.leucocephala umbel.