Final answer:
Mendel developed purebred pea plants by selective breeding of self-fertilizing plants, ensuring they produced identical offspring. He carried out cross-pollination manually and removed anthers to prevent self-pollination.
Step-by-step explanation:
Gregor Mendel ensured he had purebred pea plants by using plants that were naturally self-fertilizing, which means they possessed both male and female parts and could self-pollinate. Mendel performed selective breeding to establish plants that always bred true, known as the P1 generation, by only continuing to breed plants that produced offspring identical to themselves. To carry out cross-pollination in his experiments, he manually transferred pollen from the anther of one plant to the stigma of another plant and prevented self-pollination by removing the anthers from flowers before they matured. This meticulous approach over several years allowed Mendel to produce true-breeding plants, which were fundamental for his inheritance experiments.