Azolla is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, having a significantly different appearance to other ferns and more resembling some mosses or even duckweeds. Azolla filiculoides is one of two fern species for which a reference genome has been published. It is believed that this genus grew so prolifically during the Eocene that it triggered a global cooling event that has lasted to the present.