This is a cylinder world (hand-waved), but with gravity definitely "down." Imagine a cylinder whose curved surface is the size of, the United States. However, its gravity is as much like if it were placed on the surface of the Earth.
That is to say, there's definitely a 'floor' of mountains, deserts, tropics, lakes; and there's sides and a ceiling.
On the ceiling, I'd like to have a savannah-type environment that only a few intrepid explorers have used cables and hooks to investigate. Lots of bugs up there.
I'm not concerned with the artificial parts (the glowing orb in the center, the construction of it) yet; but strictly, how can I have upside down trees and grass?
(The structure is artificial, but I'd like plants to grow upside-down towards the light in the center of the massive structure, without artificial assistance.)