How do trees grow tall with hollow cores?

Another interesting article from The Conversation:
How the first trees grew so tall with hollow cores ??? new research

Imagine a world without trees, and then try to think about the changes that would need to happen for these trees to evolve from the small primitive plants that came before them. I spend as much time as I can trying to find evidence for this transition, which is currently estimated to have happened between 390-380m years ago, in the Mid-Devonian Epoch.
 
aa trees