Development of root nodules happens in following steps:
a. Rhizobium bacteria contact a susceptible root hair and divides near it.
b. Successful infection of the root hair results in curling of the root hair.
c. The infected thread carries the bacteria to the inner cortex. The bacteria get modified into rodshaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation.
d. A mature nodule is complete with vascular tissues. The vascular tissues of the nodule are continuous with those of the root.