Colchicine is an alkaloid widely used in plant breeding for doubling the chromosome number. Colchicine is extracted from the corms of autumn crocus (Colchicine autumnale). The alkaloid does not allow the formation of spindle because it prevents assembly of microtubules. It is, therefore, called ‘mitotic poison’. Thus mitotic poisons are substances that affect the cells in mitosis or prevent them from entering it.
Colchicine holds the cells in metaphase. The enzyme ribonuclease is prophase poison. Colchicine does not inhibit chromosome replication. As a result the Colchicine treated to meristematic cells show doubling of chromosomes. This property of colchicine is being used to have polyploidy.