Colchicine

Colchicine (COAL - cha - seen) is a powerful mutagen used initially for gout and now...

FROM WIKIPEDIA: 

Since chromosome segregation is driven by microtubules, colchicine is also used for inducing polyploidy in plant cells during cellular division by inhibiting chromosome segregation during meiosis; half the resulting gametes, therefore, contain no chromosomes, while the other half contains double the usual number of chromosomes (i.e., diploid instead of haploid, as gametes usually are), and lead to embryos with double the usual number of chromosomes (i.e., tetraploid (4N) instead of diploid (2N). While this would be fatal in most higher animal cells, in plant cells it is not only usually well tolerated, but also frequently results in larger, hardier, faster-growing, and in general more desirable plants than the normally diploid parents.