I bought a book 'Turning Green Wood' by Michael O'Donnell. In it he explains
reaction wood in branches, he says:

"Hardwood in the northern hemisphere build the reation wood on the tension
side of the pith, which gives rise to growth rings in branches being wider
on the tension side (the upper side) than on the compression side. In the
southern hemisphere this is reversed."

This poses some interesting questions:

1. How does a tree know whether it is in the northern or southern
hemisphere?
2. Do tree branches on the equator have equal size growth rings above and
below the pith?
3. If I send an acorn from England to Australia will the resulting oak have
reaction wood as its parents or as its environment?

Something to muse on?

BillR