1. Why tricellular pollen is formed when generative cell divides before pollen is shed ?2. Why bicellular pollen is formed when generative cell divides within pollen tube ?

The engulfed generative cell divides once more by mitosis (pollen mitosis II) to form the two sperm cells completely enclosed within the vegetative cell cytoplasm either before pollen is shed (tricellular pollen) or within the pollen tube (bicellular pollen)