Oceanic crust See crust

oceanography The study of the oceans, their distribution and mapping, the physics, chemistry, biology and ecology of their waters, and the exploitation of their resources.

odor threshold The lowest concentration of a substance in air or water that has a detectable odor.

OECD See Organization for Economic Cooperation and Development.

OES See occupational exposure standards.

oil 1. A triacylglycerol that is liquid at or near room temperature. The commonest fatty acids in oils are oleic and linolenic acids, which are unsaturated. Storage oils, synthesized in the endoplasmic reticulum, make up as much as 60% of the dry weight of certain seeds, such as the castor bean (Ricinus communis). After oil is extracted from seeds, the residual 'cake' is used to feed livestock or, if poisonous (as in castor bean), as fertilizer.

Essential oils are volatile oils secreted by aromatic plants; they are the source of characteristic odors and flavors. Some essential oils repel insects or grazing animals, while others (allelochemicals) deter encroaching neighboring plants. Resinous oils help prevent loss of sap and protect against the entry of pathogens and parasites. Essential oils are common in xero-phytes, where in hot conditions the evaporating oils increase the density of the

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