Enlargement or swelling at position of dormant adventitious root primordium (on the stem), inhabited by nitrogen-fixing bacteria. [source: TAIR:Katica_Ilic]
Comment
Unlike the root nodules, these contain functioning chloroplasts in the nodule cortex and are therefore capable of carbon fixation. Note: stem nodule is commonly used term for this structure, although the nodules are formed at positions of dormant adventitious root primordia.