Virtually any sample of Azolla examined under a microscope will have filaments of Anabaena living within ovoid cavities inside the leaves. Like nitrogen-fixing bacteria living inside the root nodules of legumes, the relationship appears to be mutually beneficial. Since Azolla is easy to maintain in aquarium cultures, it is an excellent source of prokaryotic cells and heterocysts for general biology laboratory exercises on cell structure and function. It also has an interesting heterosporous life cycle and can readily be adapted to laboratory exercises on symbiosis. In addition, this little fern and its algal partner provide an important contribution toward the production of rice for a hungry world.