Mendel didn't use *all* tall plants in his F1 experiment. He actually crossed pure-breeding tall plants with pure-breeding short plants. This is a crucial detail that underpins his findings.

Here's why it was important:

* Pure-breeding lines ensure consistent traits: A pure-breeding tall plant will always produce tall offspring when self-fertilized. Similarly, a pure-breeding short plant will always produce short offspring. This ensures that the traits being studied are consistent and not influenced by other factors.

* Control over variables: By using pure-breeding lines, Mendel controlled the starting point of his experiment. He knew exactly what genes were present in each parent plant. This made it easier to observe and analyze the results of the cross.

If Mendel had used a mix of tall and short plants in his F1 generation, he would have gotten a wide range of heights in his offspring. This would have made it much more difficult to track the inheritance of the traits and would have clouded his observations.

Therefore, the use of pure-breeding lines in the parental generation was essential for Mendel's success in uncovering the principles of inheritance.