In many aromatic plants, the leaves produce most of the essences and aromas thanks to their cells specialized in the synthesis of these substances. Some of these plants (mint, sage, etc.) have secretory cells extended by secretory hairs on the underside of the leaves. They release their essences and become particularly fragrant when crumpled between the fingers. In other plants, the essences can concentrate in the bark (cinnamon), the root (valerian), the seeds (fennel), or the flowers (chamomile), and crumpling the leaves between fingers would be useless.