Chemically, the essential oils of plants consist largely of mixtures of lipids called terpenes a Terpenes are small organic molecules that have an immense diversity of structure. Thousands of different terpenes are known; some are hydrocarbons and others contain oxygen; some are open chain molecules, and others contain rings◼ Chemically, the essential oils of plants consist largely of mixtures of lipids called terpenes. ◼ Terpenes are small organic molecules that have an immense diversity of structure. Thousands of different terpenes are known; some are hydrocarbons, and others contain oxygen; some are open￾chain molecules, and others contain rings