What tree's seeds contain strychnine, historically used as a poison?

The tree whose seeds contain strychnine is Strychnos nux-vomica, commonly called the strychnine tree or nux vomica. It is a medium-sized deciduous tree native to India and Southeast Asia, especially tropical parts of the Indian subcontinent and nearby regions.

Its round orange fruit contains several hard, flattened seeds. Those seeds are rich in the poisonous alkaloids strychnine and brucine; the bark also contains toxic compounds. Strychnine affects the nervous system by preventing inhibitory signals from properly controlling muscle activity, leading to severe spasms and convulsions in poisoning.

The plant’s toxic properties were known long before chemists isolated the compound. Strychnine was first identified in 1818 by French chemists Joseph Caventou and Pierre-Joseph Pelletier. In the nineteenth and early twentieth centuries, it was used in poisons, medicines, and even purported stimulants, although its narrow margin of safety made those uses extraordinarily hazardous.

Strychnos ignatii, or Saint Ignatius’s bean, is a related plant that also contains strychnine. However, Strychnos nux-vomica is the classic “strychnine tree” and the best answer to this question.