Surely this is not a fern?? Yes it is!! It's an unusual hardy fern that can be used as an aquatic plant or planted as a bog garden plant in full sun or light shade. Maximum depth of water is 25cm. If planted at the surface the four leaf clover-like fronds stand upright, up to 15cm high, if planted underwater most will float on the surface with a few standing upright.
It creeps along with its rhizomes to make an unusual ground cover, an ideal bog plant. Noone will believe you when you tell then they are ferns.

Marsilea quadrifolia 9cm €5.95
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Frank Vassen from Brussels, Belgium, CC BY 2.0, via Wikimedia Commons