Here We Go Round...

"Mistress Mary" talks about Mary I of England.
"Rock-a-bye Baby" talks about James II of England.
"Ring Around a Rosy" is about the bubonic plague.
"Humpty Dumpty" is apparently about a canon that fell off a castle rampart during the British Civil War.
And "Baa Baa Black Sheep" is about taxes imposed on wool during the Middle Ages.

But I've got to wonder about "Here We Go Round the Mulberry Bush." Where does it come from?

It's not the poem itself. That may come from the fact that the plant happens to grow in the courtyard of a particular British prison, and that the inmates used to exercise around it everyday.

But I am curious about the wording. Because mulberries don't grow on bushes.

The Bible says that mulberries grow on trees (Luke 17, if you're interested).
The above mentioned prison says that the foliage within its grounds is a tree.
The forestry department at Virginia Tech says that mulberries grow on trees.
And, (most importantly in my opinion) the mulberries that grow wild right outside my apartment building (and are free game to any of the tenants) grow on trees.

So why does the rhyme say bush? It's not the only one that references mulberry bushes -- alternate words to "Pop Goes the Weasel" talks about mulberry bushes, too.

Any thoughts?