How did Gregor Mendel control pollination in his pea-plant experiments?

Gregor Mendel controlled pollination in his pea-plant experiments by deliberately transferring the pollen from one variety of the garden pea to the pistils of another.1

Because pea plants typically reproduce through self-pollination, Mendel was able to obtain "pure" varieties that had descended for many generations from plants with similar traits. By manually performing these cross-pollinations, he could precisely control the mating between plants with specific, contrasting characteristics—such as seed color or plant height—to trace the inheritance of those traits across successive generations.123