Question

what are pollen grain and what is function of exine

Answer 1

pollen grains are the important one which is present in male flower and used for pollination(reproduction in plants) by cross or self pollination.function of exine is to protect the pollen grains from any other damages.

Answer 2

Pollengrain is male gametophyte.
Role of exine: It's highly resistant because of sporopollenin. Thus it helps pollen grain withstand harsh conditions, action of alkalis/acids, and also protects it from other environmental stresses.