Approximate diameter of pollen grain is

Pollen grains are generally 25 – 50 micrometer 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 withstand 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.