Some nitrogen-fixing microorganisms, such as nitrogen-fixing snails and paspalum nitrogen-fixing bacteria, can live among cortical cells in the roots of maize, paspalum, rice and sugarcane. These nitrogen-fixing microorganisms have certain specificity with plants, but they will not form special structures like nodules. These microorganisms can also fix nitrogen by themselves, and their nitrogen fixation characteristics are between autotrophic nitrogen fixation and biological nitrogen fixation. This form of nitrogen fixation is called combined nitrogen fixation.