South American Spongeplant or Frog’s Bit

 A floating to rooted stoloniferous perennial. Floating rosettes send runners out into the water, the ends of

which form juvenile plants. Juvenile form has thick, spongy, floating ovate to spatulate leaves, usually with

rounded tips and on an inflated stalk. Juvenile leaves and sometimes mature leaves have a patch of spongy tissue (aerenchyma) on lower surfaces. (from Weed Control Handbook)