Gregor Mendel didn't specifically try to prevent the plants with round seeds from self-pollinating. Instead, he employed a clever strategy to ensure cross-pollination between plants with different traits, such as round and wrinkled seeds. Here's how he did it:

1. Controlled Cross-Pollination: He carefully removed the anthers (male reproductive parts) from the flowers of the plants he wanted to be the female parent. This prevented self-pollination.

2. Dusting with Pollen: He then took pollen from the anthers of the plants he wanted to be the male parent and dusted it onto the stigma (female reproductive part) of the flowers from which he had removed the anthers.

By meticulously performing these steps, Mendel could control the parentage of the plants and ensure that they were cross-pollinated. This allowed him to observe the inheritance patterns of traits like seed shape and color, leading to his groundbreaking discoveries about genetics.