Tannins. These water-soluble, bitter,
dark-colored substances are found in a variety of barks, woods, leaves, roots, and fruits.
Their role in the cell is uncertain. They likely serve as a deterrent to herbivores, and
may have antiseptic properties useful to the plant. Tannins
are used in the leather industry to cure hides. They bind with the proteins in animal
skins, making the proteins unavailable to decay organisms. Tanning a hide creates a stable
leather that is resistant to bacterial decay.
Essential oils. These fragrant substances probably
play a role in luring pollinators and repelling herbivores. Some essential oils also have
antiseptic properties. They are usually fragrant because they volatilize (evaporate) on
contact with air. In the plant, essential oils are usually contained in special oil glands
or in the region between the cell wall and cuticle of epidermal hairs. You may have
noticed that simply touching the fuzzy leaves of a mint plant brings out their fragrance.
Touching breaks the epidermal hairs and exposes the oils.
Essential oils are used in perfumes and flavorings.
Examples include peppermint, spearmint, citrus, eucalyptus, rose, juniper, lavender, and
rosemary oils. Many fragrant spices owe their smell to the essential oils they contain,
including cinnamon, clove, nutmeg, vanilla, pepper, and ginger.
