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. Resins are colorless or colored aromatic, combustible, terpene hydrocarbons, such as found in pines and other conifers, and the liquid occurs under pressure in the ducts, which are surrounded by living cells (epithelium) that make the terpenes to be stored in the canals. Laticifers mostly are narrow tubes that produce and contain the isoprene (terpenes), which, when exposed to air, solidify as a long-chain polymer (rubber, latex).
Bark may have pigments that give color to the layers. In some cases, bark is light-colored when initially exposed but darkens when the pigment is oxidized (eucalyptus and sycamore).