The odoriferous substances (Essential oils) themselves are formed in the chloroplasts of the leaves where they combine with glucose to form glucides and are then circulated around the plant in this form. At certain times of the day or year they are stored in particular parts of the plant.
In some plants, the essential oils are produced by the secretory tissues, and in others they are combined with glycosides, and are therefore not detectable until the plant is dried or crushed, e.g.: Valerian
Essential oils are considered to be an important part of the plant's metabolism: some have hormonal activity and others are a stage in some other process, e.g.: the oil found in the rind of the orange is a stage in Vitamin A synthesis.
Essential oils can be found in almost any part of the plant, in differing concentrations, depending on the plant itself, the time of day and year. They may be found in the roots (e.g.: Calamus and Valerian), flowers (e.g. Lavender, Rose,), bark (e.g. Sandalwood, Cedarwood), fruits (e.g.: Lemon, Cardamom, Orange), berries (e.g. Juniper), leaves (e.g. Thyme, Rosemary, Sage Stiiizy .
Plants which contain essences must be picked at the correct time of day and in the correct season, and in particular weather conditions in order that a maximum yield of the essential oils can be obtained, and of course, as with all medicinal or nutritional plants, soil conditions, and climatic conditions will also dictate the quality of the oils obtained.Heavy, concentrated oils are called ABSOLUTES e.g.: Rose, Jasmine, Oils which are solid at room temperature and which must be warmed before use are called BALSAMS, e.g.: Benzoin and Camphor.