Rhizobia Root Nodule (image) Princeton University Caption Legume trees nurture bigger, healthier trees by pumping nitrogen fertilizer into tropical-forest soil. The process, known as "nitrogen fixing," is carried out with the infectious bacteria rhizobia, which dwell in little pods inside the tree's roots known as root nodules (above). Legumes use secretions to invite rhizobia living in the soil to infect their roots, and the bacteria signal back to initiate nodule growth. The rhizobia move into the root cells of the host plant and -- in exchange for carbohydrates the tree produces by photosynthesis -- convert nitrogen in the air into the fertilizer form that plants need. Credit Photo by Sarah Batterman Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.