Introduction: The essential oils
It has been known for several thousand years that the flowers, fruits, leaves, barks, and roots of many plants contain volatile, odorous substances, collectively called tbe essential oils. Highly prized by ancient man, the essential oils, in the form of herbs and spices, became one of the earliest recorded items of commerce. It is unlikely that any other commodity has provided such a stimulus to world exploration and to the development of international trade. From that ancient trade in raw spices, the isolation and preparation of odorous principles from plants and the later synthetic duplication of the essential oils and their constituents has developed, in modern times, into a large and flourishing industry.
While these oils are very useful to humanity, the classical view has held that essential oils represent inactive waste products of plant metabolism and thus possess no significant biological function. Only recently have we come to realize that the essential oils and their constituents may be of benefit to the plants which produce them; providing a means of chemical communication by which tbe plant maintains itself against competitors, predators, and pathogen.
The ability of essential oils to function as chemical signals appears to stem from their volatility in air (and thus the ability to influence another organism at some distance from the source plant) and from the complexity of composition and structure of the constituents (and thus the ability to transmit a complex and selective biological message). Additionally, recent evidence has shown that essential oil constituents are both rapidly synthesized and catabolized by plants, and thus may play a dynamic though as yet unknown role in metabolism. In this context of a functional role, it is important to note that the synthesis and accumulation of essential oils is generally associated with the presence of specialized glandular structures, most often situated on or near the surface of the plant (See figure 1.) Such a location is ideal for release of essential oil constituents to the atmosphere, and is consistent with a chemical defense function, The oil glands also provide a means of sequestering essential oil formation from other metabolic processes; yet, the sites are not so isolated as to prevent catabolism and the apparent metabolic recycling of the essential oil constituents.