Identify '$a$' in the given figure:

In angiosperms,the pollen grain is normally shed at which stage?

How many meiotic divisions are necessary to produce $100$ pollen grains?

$A$ - Only some cells of the sporogenous tissue are capable of giving rise to microspore tetrad.
$R$ - The microspores are arranged in a cluster of four cells - the microspore tetrad.

The pollen tube develops from the ..........

How many wall layers surround a typical microsporangium?