Hemlock. 18th century engraving of a hemlock plant. Hemlock (Conium maculatum) is a highly poisonous perennial herbaceous flowering plant. Among the many pyridine alkoloids present in hemlock, the most important is coniine. It is a neurotoxin that can cause respiratory collapse and death. About 6-8 leaves of the plant can be a fatal dose. In Ancient Greece, condemned prisoners were given an infusion of hemlock. The most famous person to suffer this end was Socrates, the philosopher, condemned to death for impiety. This illustration was first published in the Gentleman's Magazine in 1762.