The function of endothecium is

The stalk of the stamen is known as:

$A$: The generative cell comes to lie freely in the cytoplasm of the tube cell.
$R$: The cellulosic wall around the generative cell is dissolved.

In over $60\%$ of angiosperms,pollen grains are shed at the $2$-celled stage. In less than $40\%$ of angiosperms,pollen grains are shed at the $3$-celled stage.

Which is an incorrect statement?
$I$. Each cell of sporogenous tissue in anther is capable of giving rise to a microspore tetrad.
$II$. The pollen grain represents the male gametophyte.
$III$. Pollen grains are usually triangular and $10-15 \mu m$ in diameter.
$IV$. Sporopollenin is one of the most resistant organic materials,which can be destroyed only by strong acids and alkalis.

Identify the incorrect statement regarding sporopollenin.