The majority of woody plants have tannin cells in bark. Tannins are bitter polyterpenes that are rejected by many animals and, if ingested, often interfere with protein digestion. Tannins, as the name suggests, are used for tanning leather. Bark can also have bitter alkaloids.
Secondary phloem is a tissue that can contain either resin ducts or laticifers .
Bark may have pigments that give color to the layers.