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Each plant from which essential oil is derived belongs to one of a number of botanical families. These origins are easy to forget once the oils have been put into little glass bottles. Just as plants show family characteristics in their physical appearance, essential oils show family characteristics in their therapeutic effects, which can be helpful when selecting oils for any particular condition. For would-be therapists, knowledge of the families helps to give a system of, and a pattern for, learning.

Not all plants produce essential oils and of those which do, not all contain a sufficient quantity to justify extraction, either by distillation or solvent extraction. There are well over 200 plants from which essential oils are taken and although most may present possibilities to the perfume industry (also for the study of smell – osmology), not all are popular, free enough from toxicity or necessary for the practice of aromatherapy.

The yield of essential oil from each plant varies not only from year to year in the same country (because of climatic changes), but also from country to country, depending on the growing conditions, so the yields given can only be approximate and are given after the plant name. Those quoted in this chapter have been taken from Guenther (The Essential Oils) and The H & R Book of Perfume; a 1 per cent yield means that 100 kilos of plant yields approximately 1 litre of essential oil.

There are 21 families which yield the oils used in aromatherapy. Sixteen families yield the most well-known oils; some, like Santalaceae, having hardly any plants in their repertoire and others, like Lamiaceae, having an enormous number.

Unfortunately, botanical names are changed from time to time (which adds to the difficulty of learning them!) Where this is happening, or has happened, all names will be given. Where the common name is almost the same as the Latin name, the former will not be shown.

All plants to be referred to in chapter 5 are entered there under their common name, and this is also the case in all charts, to make them more easily referred to until Latin names become the norm in the UK (see Table 3, for easy reference).

General properties and effects characteristic to the plant family will be indicated; any special property or effect of note for any particular oil is given in addition to these family traits. Further properties and effects of each oil can be found in the various charts and lists in chapters chapter 3, chapter 6 and chapter 14.

Cautions are given on the results of toxicity tests on ingestion by animals, plus the assumption that oils may be wrongly used; these cautions do not necessarily apply to the methods of use (and dilutions used) employed by trained aromatherapists.