Assertion is true. Pollen grains are indeed well-preserved as fossils, and the reason behind this is that the exine of pollen grains is made up of a complex polymer called sporopollenin. 

Reason is also true. Sporopollenin is highly resistant to biodegradation and can withstand high temperatures, strong acids, alkalis, and enzymes. This makes it highly durable and resistant to decay, making pollen grains one of the most abundant and well-preserved fossil materials in the geological record.

Therefore, both the assertion and the reason are correct, and the reason is the correct explanation for the assertion.