Final answer:
Sporopollenin is the primary constituent of the exine of pollen grains, offering substantial protection to the pollen and aiding in the preservation of pollen fossils.
Step-by-step explanation:
Sporopollenin is the primary constituent of the exine, the thicker and outer covering of the pollen grain. It is a complex waterproofing substance that is supplied by the tapetal cells and is known for its significant resistance to various forms of degradation. This resilience is why sporopollenin is credited with the ability to protect pollen under different circumstances, including situations involving wind, water, or biological agents. Its toughness also explains the existence of well-preserved pollen fossils.