I've also frequently mentioned frogbit (Hydrocharis morsus-ranae), which is very similar to Nymphoides (and also widespread) and which usually has three petals.
Actually, I usually mention them together. The main difference is that frogbit doesn't usually have an extended pistil and Nyphoides sometimes does. Other water lilies never have this kind of pistil, it's shaped quite differently in the Nuphars and Nenuphars.
Oh speaking of which, the names were shuffled around after Linnaeus came on the scene. Nenuphar and Nuphar were used almost interchangeably and rather indiscriminantly for all sorts of water lilies (including small ones like Nymphoides and frogbit) during the medieval period.
Actually, I usually mention them together. The main difference is that frogbit doesn't usually have an extended pistil and Nyphoides sometimes does. Other water lilies never have this kind of pistil, it's shaped quite differently in the Nuphars and Nenuphars.
Oh speaking of which, the names were shuffled around after Linnaeus came on the scene. Nenuphar and Nuphar were used almost interchangeably and rather indiscriminantly for all sorts of water lilies (including small ones like Nymphoides and frogbit) during the medieval period.