Pure tall plants are crossed with pure dwarf plants. In the F_1...

The phenotype produced by heterozygotes is identical with that produced by homozygous for the concerned dominant allele. The dominant allele in such a situation is known as completely dominant, eg, when pure tall plant is crossed with pure dwarf plant. In $F_1$ generation, all plants were tall. These tall plants of $\mathrm{F}_1$ generation were selfed than the ratio of tall to dwarf plants obtained was $3: 1$. So, allele of tallness is dominant to allele of dwarfness.