White Oak Bark, Quercus alba. There are a host of vegetable astringents used for their tannin. White oak bark deprived of its corky layer is one of the best for external use, since the only active agent therein is tannin, no resins or bitter principles interfering with the action. A decoction is used to prevent the formation of bed-sores, and in other indications for an astringent. A fl. is made, however, and is given in 15-minim doses. In the U. S. P. IX galls are recognized, but oak bark has been deleted. Various species of oak are recognized in a few of the pharmacopeias. Principally used as a commercial source of tannic acid, q. v.