Generally, spices and herbs get their specific scent and flavor from their essential oils. These oils infuse their aroma into the air because they are very volatile; they evaporate easily.
Various aromatic oils come from different parts of a plant. Some plants have several different oil sources each with an unique property. Depending on the plant, oil can be obtained from: balsam, bark, buds, flowers, gum, leaves or needles, resin, roots, seeds, stalks and wood.
Science is just beginning to understand the effects of aroma on humans. The preparation, mixing and use of oils are an ancient and continuing art.