Yew tree berries. Red berries on a yew tree (Taxus baccata). The red part of these berries, called the aril, is the only part of the yew tree that is not poisonous. The rest of the plant contains a heart-paralysing alkaloid called taxine, the effect of which is so sudden that poisoned animals are sometimes found dead with half-chewed leaves still in their mouths. Birds can eat the berries with impunity as the poisonous seeds pass through them undigested. The yew gains from this arrange- ment as the birds unwittingly disperse the seeds over wide areas, fertilising them with their dung. Yews grow wild in hilly woodland throughout Europe and can sometimes live up to 2000 years old.