I want to know the answer to the following question:
When was the theory of plant hormones introduced?
All the background I'm provided with is that "The finding in 1939 that plant callus could be maintained in culture containing IAA, followed by the observation in 1947 that it could be induced to form roots and shoots by controlling the concentration of growth hormones were key steps in the development of plant biotechnology and genetic modification.". Is that enough to answer the question?