Azolla waterfern. View of the floating leaves of the waterfern Azolla filiculoides. The leaves of waterfern support colonies of Anabaena, a blue- green alga which fixes atmospheric nitrogen and enriches the water in which it grows. For this nitrogen-fixing ability, species of waterfern are grown in rice paddies in India as a fertiliser plant. The roots which are unanchored have a very simple structure of scientific interest; the roots are generated by multiple divisions in various planes from one apical cell. Azolla filiculoides is an American waterfern, now naturalised in Britain after being introduced in 1899. Magnification: x0.5 at 6x6cm size.