If a diploid cell is treated with colchicine then it becomes : -

Colchicine, an alkaloid from Colchicum autumnale, inhibits the formation of the spindle fibres during cell division.
The chromosomes duplicate and separate, but the cell does not divide, so the chromosome number of the cell is doubled.
A diploid (2n) cell thus becomes tetraploid (4n).
Colchicine is used by plant breeders to produce polyploid plants.