Given the following passage  "Heartwood (or duramen) is wood that as a result of a naturally occurring chemical transformation has become more resistant to decay. Heartwood formation occurs spontaneously (it is a genetically programmed process). Once heartwood formation is complete, the heartwood is dead. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once.",  answer the following question. Note that the answer is present within the text.  Question: What is the characteristic of heartwood formation?
Answer:
spontaneously