What are the prominent apertures on the exine where sporopollenin is absent called?

Step-by-step solution

Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known . Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent. Pollen grains are wellpreserved as fossils because of the pre