The stem of the aquatic plants is flexible because it helps the leaves to float. They bend with the flow of water so the plants do not get damaged by strong current. The stems of such plants may have hollow, air filled pockets which help the plants to remain at the sunlit surface.
I guess this was the answer to one question I read. It is really designed for strong currents and I wonder about the very small plants if they can protect themselves?
Most plants have mat fine hairs underneath the leaves that helps them with holding to currents that also protect them from preys