Describe how mendel showed that the green-seed trait did not disappear but was only masked

In Gregor Mendel's experiments with purple and white pea-plant flowers, he first bred a pure-white and a pure-purple flowered plant together. This resulted in all purple flowers.

However, when he bred those flowers together, approximately 1 in every 4 of them were white. This showed that the white trait had not disappeared, but was just hidden, because it appeared i the second generation.