Why colchicine is called as the mitotic poison?

(a)Colchicine is an alkaloid obtained from the corms of Autumn crocus. The alkaloid inhibits the formation of spindle by preventing assembly of microtubules.
(b)The chromosomes duplicate but they remain within the same cell, increasing the number of chromosomes. As a result, the colchicine treated meristematic cells show doubling of chromosome.