1. Pollen Grain Germination:
* When a pollen grain lands on the stigma of a flower, it germinates.
* The pollen grain forms a pollen tube, a long, tubular structure that grows down through the style towards the ovary.
2. Travel of the Sperm Nuclei:
* Inside the pollen grain, there are two sperm nuclei.
* These sperm nuclei travel down the pollen tube towards the ovary.
3. Reaching the Ovule:
* The pollen tube eventually reaches an ovule, which contains the female egg cell.
* The pollen tube enters the ovule through a small opening called the micropyle.
4. Double Fertilization:
* Inside the ovule, the two sperm nuclei are released.
* One sperm nucleus fuses with the egg cell, forming a diploid zygote. This zygote will develop into the embryo of the new plant.
* The other sperm nucleus fuses with the polar nuclei, forming a triploid endosperm. The endosperm acts as a food source for the developing embryo.
Therefore, it's not the entire nucleus of the pollen grain that fertilizes the egg cell, but one of the sperm nuclei within the pollen grain. This process of double fertilization is unique to flowering plants and ensures the formation of both the embryo and the endosperm.
