Pollen grain are generally $25-50 \mu m$ in diameter:

Pollen grains have two main layers

(i) Outer Layer It is also called exine. It is made up of sporopollenin. It is hard and protective in nature. Due to sporopollenin pollen can with stand extreme temperatures.

(ii) Inner layer It is also called intine. It is made up of cellulose and pectin. It is very thin as compared to the outer layer