Pollen grain (male gametophyte):-

• Generally spherical. 25-50 μm in diameter. Cytoplasm is surrounded by a plasma membrane.
• A pollen grain has a two-layered wall, exine and intine.
• Exine: The hard outer layer. Made up of sporopollenin (highly resistant organic material). It can withstand high temperatures and strong acids and alkalis.
• Enzymes cannot degrade sporopollenin.
• Exine has apertures called germ pores where sporopollenin is absent.
• Pollen grains are well preserved as fossils due to the presence of sporopollenin.
• Exine exhibits a fascinating array of patterns and designs.
• Intine: The inner wall. It is a thin and continuous layer made up of cellulose and pectin.
• A matured pollen grain contains 2 cells
• Vegetative cell: is bigger, has abundant food reserve and a large, irregularly shaped nucleus.
• Generative cell: is small and floats in the cytoplasm of
  the vegetative cell.
• It is spindle-shaped with dense cytoplasm and a nucleus.