These are absorbed by the Azolla which releases carbohydrates that are absorbed by the CBs. It is almost impossible to separate the symbiotic partners once they become established. This relationship has been used for centuries in rice cultivation because it provides a cheap, renewable and pollution-free source of fertilizer. The CBs live in a circular zone that develops in upward growing root nodules. The nodules develop a special tissue layer that undergoes mitosis and autolysis in the presence of the CBs.
Cycads occupy poor habitats and are almost extinct.
Their symbiotic relationship with CBs is one reason why they still can be found in nature. Cycas Upward Growing Root Nodules Cross Section through a root nodule showing the dark zone that contains Cyanobacteria. These produce papillose outgrowths near the base of their enormous leaves. CBs are able to colonize these areas and even penetrate the cells in this structure. At least one Gunnera species grows in Hawaii.