What are pollen grains?

Pollen grains

Each pollen grain is a tiny spherical structure, surrounded by 2 layers:

Exine

• Hard, outer layer
• Composed of sporopollenin
• Highly resistant

Intine

• Thin inner layer
• Composed of cellulose & pectin

 Formation of a pollen grain from microspore involves the following changes:

• Expansion of the microspore
• Formation of a large vacuole
• Microspore nucleus displaced to an eccentric position against microspore wall
• Nucleus undergoes first pollen mitosis
• A large vegetative cell & a small generative cell are formed. This is the 2-cell stage. In many plants, pollen grains are released at this 2-celled stage. However, in some others, they are released at a 3-celled stage.
• Generative cell detaches from pollen grain wall
• Generative cell is engulfed by the vegetative cell
• ‘Cell within a cell' structure is formed
• Generative cell again undergo mitosis
• Two sperm cells are formed enclosed within the vegetative cell cytoplasm