Mendel began his experiments by allowing pea plants to self-pollinate for several generations for a very important reason: to ensure he was working with purebred lines.

Here's why:

* Purebred lines are essential for controlled experiments. Mendel wanted to study how traits were passed down from parents to offspring. To do this, he needed to be certain that the plants he started with were homozygous for the traits he was interested in. This means that each plant had two identical copies of the gene for that trait.

* Self-pollination creates purebred lines. When a plant self-pollinates, it fertilizes itself with its own pollen. This results in offspring that are genetically identical to the parent plant, ensuring that the line remains homozygous for the trait.

* Avoiding unexpected variations. By starting with purebred lines, Mendel could be confident that any variations he observed in the offspring were due to the traits he was studying, and not to random mutations or variations from unknown sources.

In summary, Mendel's careful use of self-pollination to create purebred lines was crucial for the success of his experiments. It allowed him to control the genetic makeup of his plants and draw accurate conclusions about the inheritance of traits.