Hemlock
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.