In order to obtain the `F_1` generation, Mendel pollinated a true-breeding, say, tall plant with a true-breeding dwarf plant. But for getting the `F_2` generation, he simply self-pollinated the tall `F_1` plants. Why?

Explanation

Genotype of 50% of the offspring would resemble one parent and the rest the other parent. All the `F_1` offsprings of the cross are heterozygous so allowing self pollination is sufficient to raise `F_2` offspring. Also he intended to understand the inheritance of the selected trait over generations.