What tree produces slightly poisonous seeds known in Britain and Ireland as conkers?

The tree that produces the slightly poisonous seeds called conkers in Britain and Ireland is the horse chestnut.

The species is Aesculus hippocastanum, also known as the common horse-chestnut or conker tree. Its glossy brown seeds develop inside green, spiky capsules and are collected for the traditional children’s game of conkers. The seeds are especially poisonous when young and fresh, containing compounds including aesculin, saponins, and glucosides.

Horse chestnut is not the same as sweet chestnut, Castanea sativa. Sweet chestnuts are edible when properly cooked, whereas horse-chestnut seeds should not be eaten. The two trees also belong to different plant families and have distinctly different leaves and fruits.

The horse chestnut reached Britain in the early seventeenth century after being introduced westward from southeastern Europe. During the First World War, conkers were collected as a possible starch source for acetone production, although they proved inefficient. The game remains the seed’s best-known cultural association.