Pollen grains land on the stigma of a flower to begin the process of fertilization.

Here's why:

* Stigma: The stigma is the sticky, often feathery top part of the pistil (female reproductive organ of a flower).

* Pollen: Pollen grains contain the male gametes (sperm cells) of a flower.

* Fertilization: When a pollen grain lands on the stigma, it germinates and grows a pollen tube down through the style (the stalk connecting the stigma to the ovary). This tube carries the sperm cells to the ovules (eggs) located within the ovary, where fertilization occurs.

Let me know if you'd like more details about the fertilization process!