plant order containing two families of tiny ferns that float on water: Salviniaceae and Azollaceae, each consisting of one genus. Salvinia (water spangle, or floating moss) has about 10 species, and Azolla ( mosquito fern) has six species. Roots are present in Azolla but missing from Salvinia. The two families differ further in their leaf structure, Azolla having two rows of bilobed leaves and Salvinia having a whorl of three leaves, two of which are round and emergent while the third is submerged and finely divided to function as a root. Azolla and the blue-green alga Anabaena azollae maintain a symbiotic relationship: the alga provides nitrogen to the fern, and the fern provides a habitat for the alga. This property of nitrogen fixation has made Azolla extremely important economically in the cultivation of rice, particularly in Asia. The species of the order are widely distributed, especially in warmer areas, and, because of their delicate beauty, they are used to decorate aquariums and garden pools. However, some species of Salvinia, especially S. molesta, have escaped from cultivation into lakes and other waterways, where they are very aggressive and noxious weeds.