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.The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The pollen tube is an extension of intine