Lesser Duckweed (Lemna minor) is a small plant that lives in still waters. In summer it can cover the whole surface of a pond. In winter you can find it also but not in large quantity.
It floats on the water thanks to its round thallus with small air bags. It has a simple construction: a small round disk with some roots attached to it. In the tropics sometimes flowers develop, but in tamperate regions like France flowering seldom happens. To reproduce itself the plant makes more thalluses, and that's enough apparently.