The dried bark varies considerably in appearance, according to the age of the branch or stem from which it has been taken. It is of a greyish or blackish-brown colour outside, with numerous small, whitish corky warts. When gently scraped, the inner layers are seen to be crimson in colour. The inner surface of the bark is smooth, of a pale, yellowish brown and very finely striated. The fracture is short.
Older bark is rougher externally, thicker and usually in single quills or channelled pieces.
When masticated, it colours the saliva yellow. The chemical constituents of Frangula Bark, especially those to which the laxative properties are due, are but imperfectly known. A yellow, crystalline glucoside, Frangulin has been isolated from it. Possibly other glucosides are also present and contribute to the laxative action, but the evidence in favour of this assumption is not conclusive. Two resins, resinous bitter matter and a little tannic acid are likewise present in the bark.