Azolla filiculoides (water fern) is a species of Azolla, native to warm temperate and tropical regions of the Americas as well as most of the old world including Asia and Australia. It is a floating aquatic fern, with very fast growth, capable of spreading over lake surfaces to give complete coverage of the water in only a few months. Each individual plant is 1–2 cm across, green tinged pink, orange or red at the edges, branching freely, and breaking into smaller sections as it grows. It is not tolerant of cold temperatures and, in temperate regions it largely dies back in winter, surviving by means of submerged buds. It harbors the diazotrophic organism, Nostoc azollae, in specialized leaf pockets. This ancient symbiosis allows N. azollae to fix nitrogen from the air and contribute to the