To maintain a line of pure-breeding peas, selective breeders would use a technique called artificial cross-pollination. This is a technique were breeders transfer pollen from the anthers of one pea plant (male parent) to a stigma of a flower (female parent).
This technique was developed by Gregor Mendel, the founder of the science of genetics. He was a monk who dedicated his time in research about how genes of plants are carried over through specific species. In his controlled experiment he used the pea plant because they are cheap and readily available from merchants who sold them in different classifications. He also used pea plants because they have the capacity to self-pollinate or cross-pollinate.
