Pollen grain

The pollen grain has a two-layered wall: an outer hard exine made of sporopollenin and an inner thin intine made of cellulose and pectin.
The exine has prominent apertures called germ pores, where sporopollenin is absent.
When the pollen grain germinates on the stigma, the intine grows out through one of the germ pores to form the pollen tube.
The radicle emerges and water is absorbed through the micropyle of the seed, not through the germ pore; male gametes are released from the tip of the pollen tube inside the synergid.
Hence the function of the germ pore is the initiation of the pollen tube.

The exine of the pollen grain is made of sporopollenin, one of the most resistant organic materials known.
It withstands high temperature, strong acids and alkali, and no enzyme that degrades it is known so far.
That is why pollen grains are well preserved as fossils.