A water fem which is used as green manure in rice fields is:

[HDquiz quiz = “1962”]

Azolla is a genus of aquatic fern. Azolla is also known as water fern, duckweed fern, or mosquito fern. Azolla belongs to the Salviniaceae family. Azolla is a weed and they grow very fast in a favorable environment. In rice fields, Azolla sps. are used as green manure. Azolla species contain the blue-green algae Anabaena. Anabaena is able to fix atmospheric nitrogen and provide it to the host Azolla. Upon decomposition, Azolla provides nitrogenous manure to the rice plant.