According to the American Museum of Natural History, the purpose of the stigma is to germinate pollen. The stigma is the top part of the pistil, which is where reproduction takes place.
Pollen is produced in the plant's anther, and then falls on the stigma. Often insects move one plant's pollen to another plant's stigma. The stigma is sticky to keep the pollen in place while it germinates. After germination, the sperm moves from the stigma down to the style of the pistil to the ovary to fertilize the eggs. After this, a fruit grows, in which are the seeds for a brand new plant.
Pollen that germinates on the stigma comes via self-pollination or cross-pollination. Self-pollination occurs when the pollen comes from the same plant as the ovules that are being fertilized. Cross-pollination occurs when pollen comes from another plant. Pollination occurs using various methods. In some cases, insects or even animals pollinate the plants. Pollinators are attracted by brightly colored flowers, strong scents and nectar glands. Some plants have evolved and developed flowers that look like insects or produce insect pheromones in order to attract them. Other plants, like grasses, pollinate through wind pollination; however, these plants tend to lack petals and or sepals.