Ed said:

It's my understanding that if you graft a branch up side down, it will assume a weeping structure. The physics of it escapes me.

Nope.

In a normal plant the shoot tips will grow up, and the roots will grow
down. It's a process called geotropism. There are subtle variations:
secondary branches and secondary roots don't grow straight up or
straight down; they grow at angles. And rhizomes grow horizontally.
Amazingly, scientists don't fully understand how plants do this. It
has to do with hormone concentrations within individual cells, and with
starch granules that sink with gravity and allow plants to tell "up"
from "down". At any rate, weeping cultivars are mutants. The shoots
don't respond in the normal way to gravity. If you graft the branch
upside down, it doesn't matter; up is still up, and down is still down,
and the branch will respond however its genes tell it to.

Nina