In the Part One lesson on lime peel and lime leaf, we saw two parts of one plant genus give two quite different smells.
This lesson extends that principle and takes it to its final conclusion.
Five parts, five chemistries
A plant does not make one essential oil and distribute it evenly throughout. Each tissue has its own function and accumulates its own group of compounds to serve that function.
Fruit peel holds oil in sacs just beneath the surface, mainly light monoterpenes. The function is to repel insects and resist fungi. The result: a bright, fast-evaporating smell.
Leaves accumulate defence compounds released when the tissue is damaged. The result: a green smell that appears when crushed.
Flowers accumulate attractants, often heavier molecules that last in the air and stay on surfaces. The result: a sweet smell in the middle or base notes.
Roots accumulate very heavy sesquiterpenes. The result: an earthy smell that hardly evaporates, like vetiver.
Seeds accumulate compounds that protect the embryo and its reserves. The result: often quite unlike the leaves of the same plant — toasted coriander seed is sweet and orangey while coriander leaf is green and pungent.
Three pairs worth smelling in a row
Cassia bark and cassia leaf. The bark is rich in cinnamaldehyde, so sweet and hot; the leaf is rich in eugenol, so it leans strongly towards clove.
Pomelo peel, pomelo leaf and pomelo blossom. Three parts of one tree, three different scent families in our classification.
Toasted coriander seed and coriander leaf. Two parts, two families, and they do not even share a leading group of compounds.
The practical consequence
There is no such thing as the essential oil of a plant. There is only the essential oil of a plant part.
A label that says cassia oil without saying bark or leaf is incomplete, and the two differ considerably in price. On the Vietnamese market, not a few bottles labelled cassia oil are in fact leaf oil — much cheaper because the leaves are a by-product of harvesting the bark.
This can be checked at once with an analysis: if eugenol dominates it is leaf, if cinnamaldehyde dominates it is bark. No argument needed, only reading.