False-colour scanning electron micrograph of the cyanobacteria (blue-green algae), Anabaena azollae seen here in a leaf cavity of the water fern Azolla. This Anabaena forms a symbiotic relationship with the water fern, whereby the rate of photosynthesis is doubled and the nitrogen requirements of the plant are fulfilled internally by the cyanobacteria. Both these factors give the water fern a competitive advantage over other water plants, a factor spotted by the Chinese 2000 years ago. The Anabaena-Azolla association is used as a green manure for rice crops. Anabaena is a filamentous organism, which, apart from vegetative cells, forms heterocysts and akinetes.