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Castor Oil Plant
Not long ago, a Bulgarian political refugee in London died three days after a mysterious stranger jabbed him in the leg with a sharpened umbrella tip. Autopsy revealed a minute hollow pellet deep in his wound that, it was believed, contained a fatal poison. 

From the nature of his slow death, it was determined that this substance was ricin, an intensely toxic substance (more poisonous than snake venom) present in the castor oil bean. It is not present in castor oil but remains in the bean "cake" after the oil has been extracted. The cake can be rendered safe for human parts of the world, including Mexico.