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oak-moss The common name for the lichen Evernia prunastri. The thallus consists of flattened, strap-like branches which are greenish-grey on the upper surface and white beneath. Whitish, powdery soredia are often present. Oak-moss is common in areas of relatively low air pollution, on a range of substrates: trees, fences, rocks, walls, etc. It is extensively used in the perfume industry as a fixative and as a scent component. Compare PSEUDEVERNIA.