In archaeology, what is the primary and secondary center of plant domestication? pls define?

Question

In archaeology, what is the primary and secondary center of plant domestication? pls define?

Answer 1

wikipedia.org

Answer 2

All the way back to Paleolithic settlements, people settled near some water and food sources, and that those food sources had a strange way of renewing themselves.  (example: man picks apples, man throws away/passes seeds, a new apple tree springs up later)

Man's discarding pile actually becomes a source of stability, and what was originally unintentional, became the beginnings of primary agriculture.    

Primary source would be the original apple tree, and secondary source would the the unintentionally harvested seeds that fourished.  The Yellow River cultures established this earlier on than the west did, but most historians recognize that most civilizations had discovered this through chance or nature's changing course by 3500bc.