Gums and resins are forms of tree sap, the most familiar of which to those in temperate climates might be the sticky substances found on some pine cones. In deserts as well as humid tropical regions, there are many famous exudates that have significant medicinal properties. Many of these gums are wonderfully aromatic and shockingly bitter. Myrrh is a good example of this. Most resins, such as frankincense, have a history of use as incense, but they can also be distilled, extracted, and sometimes cold pressed. They usually gum up diffusers but they can be used in the types of diffusers that rely on heat rather than nebulization.