This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The gum-resins consist, as their name indicates, chiefly at least of resin and gum. With these constituents, however, there are always associated small quantities of other substances such as volatile oil, bitter principle, enzyme, etc. They are secreted in schizogenous or schizolysigenous ducts, being formed in the resinogenous layer of the walls of the tapetal cells, and discharged into the ducts in the form of milky liquids which are exuded when the ducts are punctured.
The resins contained in the gum-resins exhibit in general the characters detailed on pp. 453-4.
The gum of most of the gum-resins resembles, but is not identical with acacia gum, very possibly it consists of two of more glucosidal acids in varying proportions. It is always accompanied by an enzyme from which it has never yet been freed; it therefore always contains nitrogen.