Hydrogen cyanide
Plants in the "Prunus" species contain amygdalin and prunasin, substances which break down in water to produce hydrogen cyanide. Hydrogen cyanide is a colorless, extremely poisonous chemical that gives almonds their characteristic flavour. These substances are found mainly in the leaves and seed and can be detected by the bitter taste. It is usually present in too small a quantity to do any harm but any very bitter seed or fruit should not be eaten.