Modern-day filamentous cyanobacteria (Anabaena azollae) from cavities within the leaves of the ubiquitous water fern (Azolla filiculoides). The larger, oval cells are heterocysts (red arrow), the site of nitrogen-fixation where atmospheric nitrogen (N2) is converted into ammonia (NH3). Polar nodules are visible in some of the heterocysts. The water fern benefits from its bacterial partner by an "in house" supply of usable nitrogen. The cellular structure of these bacteria has changed very little in the past one billion years.