Laburnum
From Tardis Wiki, the free Doctor Who reference
A laburnum was a type of tree. Once, when engaged in a philosophical discussion with the Rani and the Master about how death was the essentially the conversion of animal into vegetative matter, the Sixth Doctor remarked that the Master would probably turn into a laburnum when he died. Laburnums, he pointed out, were poisonous. (DW: The Mark of the Rani)
