It is also critical that only the correct part of the plant be used for distilling essential oils. Although essential oils are produced by many parts of the plant, different plant parts produce essential oils in varying amounts and compositions. Sometimes, the essential oil derived from different parts of the same plant can even have an entirely different chemical make-up. For example, Coriandrum sativum can produce two different essential oils: cilantro from the leaves and coriander from the seeds. Delaquis et al. (2002) showed the cleansing properties and chemical differences between cilantro and coriander essential oils.
Expert testing has determined which plant parts produce the optimal quality of essential oil. Some plants are distilled whole, while others are distilled from only certain parts of the plant. For instance, peppermint essential oil is gently distilled from the whole plant while helichrysum essential oil comes only from the flowering tops. Below is a table of some common essential oils and the plant part from which they are distilled.