Oh, I just had another thought. The older, bigger stems of Phytolacca americana have this very noticeable chambered pith. If you split the older stems longitudinally then you can see the odd pith.
I've not dealt with Mulberry before so I don't know about the pith style on that plant.
Old stem with deteriorating pith.
http://joshfecteau.com/wp-cont...
Link to a photo of a fresh stem on a google search. I wasn't sure about the safety of the website.
https://www.google.com/search?...
