The nitrogen-fixing symbiotic organism present in $Azolla$ is:

$A$: Bacteria enter only the polyploid cells of the root cortex.
$R$: These cells provide cytokinins to bacteria,which promote bacterial cell division.

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

Nitrogen-fixing bacteria are associated with which of the following plant families?

Which of the following is a symbiotic nitrogen fixer?

We find that $Rhizobium$ forms nodules on the roots of leguminous plants. Also,$Frankia$,another microbe,forms nitrogen-fixing nodules on the roots of the non-leguminous plant $Alnus$. Is it necessary for a microbe to be in close association with a plant to provide mineral nutrition? Explain with the help of one example.