Salviniopsida

Order of small, floating ferns with just two genera. Salvinia has leaves in whorls of three on the floating rhyzome. Two are small, undivided leaves covered with hairs for buoyancy, but the third is much larger and bears the sporangia. It resembles a mass of white threads that hang down into the water. Azolla is a tiny fern that floats in the paddy fields of Southeast Asia and is of considerable economic importance. There are pouches in its floating leaves that house the cyanobacterium Anabaena azollae. These bacteria fix nitrogen from the air and provide the rice fields with a valuable source of fertility.