There are also resins, a volatile oil and several other constituents.
In this group may be included the oleo-resins, such as copaiba, cubebs and Canada balsam; the gum-resins, such as asafetida, myrrh, ammoniacum and galbanum; and the true balsams, such as benzoin, storax, balsam of Tolu and balsam of Peru.
The resins when taken internally have much the same action as essential oils, which are closely allied chemically, while the benzoic and cinnamic acids in the balsams modify their actions very slightly.
Some of these, such as resins, gums, essential oils and fats, are readily obtained as natural exudations or by very simple manipulations, while others, such as the alkaloids, glucosides and vegetable acids, often require to be extracted by very complex processes.
The oils are usually contained in special cells, glands, cavities, or canals within the plants either as such or intermixed with resinous substances; in the latter case the mixtures form oleo-resins, balsams or resins according as the product is viscid, or solid and hard.