Describe the external structure of a pollen grain.

(N/A) Pollen grains are generally spherical,measuring about $25-50 \ \mu m$ in diameter.
They possess a prominent two-layered wall. The hard outer layer is called the exine.
It is made up of sporopollenin,which is one of the most resistant organic materials known.
It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known.