One could think of the essences given by the plants and trees as the life force, the Prana, or the Mana, of the plant or tree.  I like to imagine they are the blood and immune intelligence of the plant or tree that has evolved over aeons and has much to teach us.  Different parts of the plant are used for many different oils, such as leaves or the bark or twigs or the roots. Each of these aspects of the plants or trees has quite unique properties - some evident and some subtle from other parts of the plants.  The way the plant part functions in and for the plant, helps us discern how it will behave in us, and how it will support out health.  Likewise, the way the whole plant behaves in Nature helps enlighten its gifts as balancer and supporter of our health.  

Vetiver for instance, is a very grounding oil and roots us in times of intense “winds of change” in life.  It is no mistake the roots of the Vetiver grass can be up to 10 or 12 feet deep.  It is used traditionally for hedges in India and other parts of the world, so works with our having good boundaries in life with others, and is a champion of remediation and used in many ecological disaster sites and outdoor privies and likewise does a wonderful job of remediating all kinds of heavy metal toxicities in our livers.  In any case, the essential oils are all considered the Amrit, or the true liquid beauty of the plant, and we are so blessed to have them as our constant companions in our health crusades.