They contain many chemical constituents that are manufactured by the plants during their growth cycles.
Plants store their essential oils in different places. The rose has its oil stored in its flowers, the orange in its peel, and the geranium’s oil is stored in its leaves. Try crushing a geranium leaf and you will smell the aroma.
Have you ever walked through a forest of pine trees and smelt the strong smell of the trees. Well that is the smell of the essential oil molecules as they evaporate from the pine needles. A stroll through such a place is a natural tonic for our lungs.