Explain the stages of root nodule formation.

(N/A) The process of root nodule formation occurs in the following stages:
$1$. $Rhizobia$ multiply and colonize the surroundings of roots,attaching themselves to epidermal and root hair cells. The root hairs curl,and the bacteria invade the root hair.
$2$. An infection thread is produced,carrying the bacteria into the cortex of the root,where they initiate nodule formation.
$3$. The bacteria are then released from the infection thread into the cortical cells.
$4$. This leads to the differentiation of specialized nitrogen-fixing cells. The nodule thus formed establishes a direct vascular connection with the host for the exchange of nutrients.