Hemlock

Hemlock is a herbal poison got from several related plants in the Apiaceae family. The toxic chemicals differ in each plant, but there is a general similarity in the way they work.

Conium, two species. An extract of Conium maculatum was used to execute Socrates. This is the plant most often referred to as hemlock. There are eight alkaloids found in Conium maculatum. Two of them, gamma-coniceine and coniine  account for most of the plant's toxicity. Coniine disrupts the workings of the central nervous system by blocking the neuromuscular junction in a manner similar to curare. These alkaloids are made by the plant, and are no doubt effective to stop herbivores eating the plant.
Cicuta water hemlock or cowbane: highly poisonous. Water hemlock is one of North America's most toxic plants, being highly poisonous to humans.Schep L.J. et al 2009. Poisoning due to water hemlock. Clinical Toxicology. 47 (4): 270–78.  Three members of the genus contain a toxin, cicutoxin, which causes central nervous system stimulation including seizures.
Oenanthe crocata  and several other species of the water dropwort are extremely poisonous. It has been referred to as the most poisonous of all British plants, and is considered particularly dangerous because of its similarity to several edible plants. The active poison is oenanthotoxin. The poison produces the "sardonic grin" a spasm of the facial muscles that produces what appears to be a grin. The poison was said to be used in ancient  Phoenician Sardinia to kill old people who could no longer support themselves.

 References 

Category:Poisons
Category:Apiaceae