The essential oils, or volatile oils as they are often called, are found in many different species of plants. These oils are distinguished from fatty oils by the fact that they evaporate or volatilize in contact with the air and possess a pleasant taste and strong aromatic odor. They can be readily removed from plant tissues without any change in composition. Essential oils are very complex in their chemical nature. The two principal groups are the terpenes, which are hydrocarbons, and the oxygenated and sulphuretted oils.
The physiological significance of these oils as far as the plant is concerned is not obvious. They probably represent by-products of metabolism rather than foods. The characteristic flavor and aroma that they impart are probably of advantage in attracting insects and other animals which play a role in pollination or the dispersal of the fruits and seeds. When present in high concentration, these same odors may serve to repel enemies. The oils may also have some antiseptic and bactericidal value. There is some evidence that they may play an even more vital role as hydrogen donors in oxidoreduction reactions, as potential sources of energy, or in affecting transpiration and other physiological processes.
All distinctly aromatic plants contain essential oils. They occur in some 60 families and are particularly characteristic of the Lauraceae, Myrtaceae, Umbelliferae, Labiatae, and Compositae. The amount, of oil varies from an infinitesimal quantity to as much as 1 to 2 per cent. The oils are secreted in internal glands or in hairlike structures. In some instances, as in wintergreen and mustard, the oil is not present in the plant, but develops only as the result of chemical action when the ground-up plant tissue is extracted with water. Almost any organ of a plant may be the source of the oil: flowers (rose), fruits (orange), leaves (mint), bark (cinnamon), root (ginger), wood (cedar), or seeds (cardamorn), and many resinous exudations as well.
Essential oils are extracted from the plant tissues in various ways depending on the quantity and stability of the compound. There are three principal methods: distillation, expression, and extraction by solvents.
Distillation . The oldest and simplest type of distillation is boiling in water, but this is now of no practical importance. The more usual