Nodulation : It is the process of formation of nodules. It can be described in in following points.

1. Rhizobiua multiply and colonise the surroundings of roots and get attached to the epidermal and root hair cells. 
2. The root hairs curl and bacteria invede it.
3. An infection thread is produced that carries the bacteria to the cortex of the root. 
4. Bacteria is then released into the cells and this leads to the differentiation of specialised nitrogen fixing cells. 
5. The nodule establishes a direct vascular connection with the root for exchange of nutrients.