Nearly all the parts of a plant can contain essential oils. However, the majority can usually be found in particular regions, depending on the plant. The most well-known are the citrus fruits, which store their essential oil in the cavities of the peel which you can usually see with your eye. When you break into an orange peel, the essential oil gets released into the air creating that distinct fragrance. Sage, marjoram and other Labiates (a plant family category) store them in glandular hair or glandular scales in their upper skin layer. Laurel plants such as cinnamon or cassia store theirs in oil and resin cells. Fennel, coriander and other umbellifers (another plant family) keep theirs in tubular ducts that only release oil after being cut or broken.