Azolla

The Mosquito ferns, genus Azolla, also called water ferns and duckweed ferns because they are often found growing alongside with duckweed species, are a peculiar genus (the only genus in the family Azollaceae) of six species of aquatic ferns. They are extremely reduced in form and specialized, looking nothing like conventional ferns but greatly resembling moss.
Mosquito ferns float on the surface of water with their roots floating in the water. They form a symbiotic relationship with the cyanobacterium Anabaena azollae, which allows the plants to fix nitrogen from the air.

Because of their nitrogen-fixing capability, mosquito ferns have enabled an agricultural revolution in parts of southeast Asia. When rice paddies are flooded in the spring, they can be inoculated with Azolla, which then quickly multiplies to cover the water. As the plants die, they contribute nitrogen to the rice plants, and as the rice paddy dries out, the Azolla all eventually die, making an exceptional green fertilizer.