What is the process called when pollen unites with the stigma?

The process being referred to occurs when pollen lands on a stigma, which is part of the female reproductive structure of a flower. This process is known as pollination. During pollination, the pollen grain, which carries male gametes, adheres to the stigma and can lead to the formation of a pollen tube that facilitates the transfer of sperm cells to the ovule for fertilization.

Understanding this process is fundamental in botany and agriculture, as it plays a crucial role in the reproduction of flowering plants and, consequently, in the production of fruits and seeds. Pollination can occur through various agents, including wind, water, and animals, most notably insects.

The other options, while related to plant processes, do not specifically describe the union of pollen and stigma. Germination refers to the process of a seed developing into a new plant, fertilization involves the actual union of male and female gametes that occurs after successful pollination, and transpiration is the process of water movement through a plant and its evaporation from aerial parts, particularly leaves.