(ethereal oil) Any of the volatile oils secreted by aromatic plants that give them their characteristic taste or odour. Most essential oils are *terpenoids
, while some are derivatives of benzene. Many xerophytic plants produce essential oils as a means of reducing their transpiration rates. In hot conditions the evaporating oils contribute to the density of the *boundary layer
at the leaf surface. The higher the density of the boundary layer, the greater resistance it offers to diffusion of water vapour. Some essential oils repel insects while others, e.g. those of Myoporum deserti, are toxic to grazing animals. Many allelopathic substances are essential oils.