poison hemlock, lethally poisonous herbaceous plant ( Conium maculatum ) of the family Umbelliferae (parsley family). It has rank, finely divided foliage, flat-topped clusters of small white flowers, and a hollow, purple-mottled stem. Although native to the Old World, it is now naturalized and common in parts of the United States. The poisonous principle (the alkaloid coniine) causes paralysis, convulsions, and eventual death. Poison hemlock was used in ancient Greece in executions; a famous example was the philosopher Socrates. The related water hemlock (any species of Cicuta ) is similar in appearance and as poisonous. C. maculata, called also musquash-root, spotted cowbane, and beaver poison, is the common species of E North America. The evergreen trees called hemlock are unrelated. Poison hemlock is classified in the division Magnoliophyta, class Magnoliopsida, order Apiales, family Umbelliferae.