What type of bacteria is $Rhizobium$?

Nitrogen fixation in root nodules of $Alnus$ is brought about by

Assertion : Leguminous plants are nitrogen fixers.
Reason : Leguminous plants have $Rhizobium$ in their root nodules.

Root nodules contain

Trace the events starting from the coming in contact of Rhizobium to a leguminous root till nodule formation. Add a note on the importance of leghemoglobin.

The given organism is symbiotic with the roots of $Alnus$ plants.