There really is a "burning bush"!

Dictamnus albus is a flowering plant native to parts of Europe and Asia (but not North Africa or the Middle East, and it doesn't come with the voice of a higher being). For most of the year it acts as a normal plant, but over the summer months, it develops a sticky, flammable oily substance which sometimes spontaneously sparks in the heat. The excretion, when lit, burns rapidly -- so rapidly, in fact, that the plant itself is typically unscathed. Watch it in action:



Found at Dan Lewis' "Now I Know"