What plant's seeds contain the toxin ricin?

The plant whose seeds contain the toxin ricin is the castor bean, Ricinus communis. Despite its common name, the castor bean is not a true bean: it belongs to the spurge family, Euphorbiaceae.

Ricin is a highly potent protein toxin produced in the seed’s endosperm. The seeds also contain 40–60% oil, which is extracted to make castor oil. Ricin is not oil-soluble, so properly processed commercial castor oil contains little or no active ricin and is not equivalent to raw castor seeds.

A common mix-up is assuming that every castor product is equally poisonous. The major danger comes from chewing or crushing raw seeds, which breaks the seed coat and releases the toxin. The plant itself is widespread in tropical and subtropical regions and is also grown ornamentally. Its spiny seed capsules and large, palmate leaves make it visually distinctive, but the attractive plant should never be treated as edible.