Rhizobia live in the roots of legumenous plants like alfalfa, clovers, peas, vetch, cowpeas, beans, lupines, and soybeans.

The bacteria spread all over the roots and get into the plant cells.

The picture below shows how a thread of rhizobia grows into a plant through the root hairs. They threads break and release bacteria in the plant cells.

Once inside the plant cells, they multiply and the cell fills up.

When the cells fill up the expand and form nodules on the roots. These nodules look like lumps on the root. The size and abundance of nodules differ depending on what kind of plant or rhizobia is involved.