Describe the structure of pollen grain of angiosperms.

Structure of pollen grains: Pollen grains represents the male gametophyte.
1. The pollen grain are surrounded by prominent two layered wall.
2. The hard outer thick layer is called exine and is made up of sporopollenin, which can withstand against high temperature, enzymes and strong acids and alkali.
3. It has prominent aperture called germ pore where sporopollenin is absent.
4. The inner thin wall of the pollen grain is called intine. It is made up of cellulose and pectin.
5. The cytoplasm of pollen grain is surrounded by plasma membrane.
6. Pollen grain consists of two cells, the vegetative cell and generative cell on maturity.
7. The vegetative cell is larger in size with large amount of reserve food material and a large irregularly shaped nucleus. 
8. The generative cell is smaller in size and floats inside the cytoplasm of vegetative cell.
9. Internally, each pollen grain posses two type of nucleus called generative nucleus and tube nucleus.
10. The generative nucleus divides and forms two male gametes nucleus and develops pollen tube after pollination.
11. In majority of angiosperms, pollen grains are shed at two celled (vegetative cell and generative cell) and in few, three celled stage (vegetative cell and two male gametes).
12. In some plant species, pollen grains externally posse sharp teeth like projection help in adhering of pollen grains to the stigmatic surface.