Step 1: Understanding the Problem:
The outer hard layer of a pollen grain, known as the exine, is highly resistant to environmental degradation, which allows pollen to survive for millions of years as fossils.
Step 2: Approach and Formula:
Identify the specific chemical substance forming the exine of pollen grains that provides this extreme resistance.
Step 3: Detailed Explanation:
- The exine of pollen grains is primarily composed of Sporopollenin.
- Sporopollenin is one of the most resistant organic materials known in the biological world.
- It can withstand extreme temperatures, strong acids, and strong alkalis. Furthermore, no known enzymes can degrade sporopollenin.
- Because of this incredible resistance to decomposition, pollen grains are perfectly preserved as microfossils in geological strata.
Step 4: Final Answer:
Pollen grains are well preserved due to Sporopollenin.