Structure of pollen grain
The pollen grain represents the male gametophyte. They are generally spherical measuring about 25-50 micrometer in diameter. The pollen grain is covered by a 2 layered wall called exine and intine. Exine is the hard outer layer and is made up of sporopollenin which is one of the most resistant organic material known. It can withstand temperatures and strong acids and alkalies. No enzyme which can degrade sporopollenin is known so far.
The pollen grain exine has a prominent aperture called germpore where sporopollenin is absent. Pollen grains are well preserved as fossils because of the presence of sporopollenin. The exine exhibits fascinating variety of pattern and designs which are having high taxonomic significance. The inner wall of the pollen grain is called 'intine' which is a thin continous layer made up of cellulose and pectin. The cytoplasm of the pollen grain is surrounded by the plasma membrane. On maturity pollen grain is made up of 2 cells, the vegetative celo and the generative cell. The vegetative cell is bigger, has abundant cytoplasm and a large irregular nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is lens or spindle shaped with dense cytoplasm and nucleus. More than 60% angiosperms shed their pollen grains in the 2 celled stage. In remaining species, the generative cell decides mitotically to give rise to 2 sperms before pollen grain is shed(3-celled).
