The Azolla (Azolla imbricata) belongs to Azollaceae (the Azolla family). It is a small free-floating fern that is distributed from Tokai region of Honshu west to Shikoku, Kyushu in Japan, as well as the Korean Peninsula, China, and India. It is the world's smallest fern plant floating in paddy fields and ponds. The feathers are triangular and alternate, green in summer but reddish in winter. It is similar to "Japanse mosquitofern" (Azolla japonica), but can be distinguished by the fact that the root hairs remain until late.