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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.)

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    $\begingroup$ Can you think of a reason it wouldn't work? Air Bonsai comes to mind, as do countless indoor decorations $\endgroup$
    – Cort Ammon
    Commented Oct 4, 2016 at 23:38
  • $\begingroup$ With upside-down water obviously. What else? $\endgroup$
    – Durakken
    Commented Oct 5, 2016 at 0:34
  • $\begingroup$ @CortAmmon That is very interesting. Thanks for links. $\endgroup$
    – NuWin
    Commented Oct 5, 2016 at 16:24
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    $\begingroup$ How does the soil stay in place? $\endgroup$
    – JDługosz
    Commented Oct 6, 2016 at 6:49
  • $\begingroup$ @JDługosz - that is a question I have. Tough roots maybe. The sides are going to be an interesting 'side' question later as well. $\endgroup$
    – Mikey
    Commented Oct 7, 2016 at 20:53

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Anchorage

There are plenty of real-world examples of plants growing without being anchored to the ground: they're called epiphytes. Assuming your cylindrical world has a reasonably rough surface, plants will have no trouble finding something to anchor themselves to.

Water

This is easy: you have a cloud forest. Instead of getting water from rain, the plants on the ceiling get their moisture by condensing it from the air. This could occur naturally, with clouds forming whenever the humidity exceeds the dew point, or the plants could have evolved to encourage condensation by actively cooling themselves.

Trace nutrients

This is the hard part. Although the macronutrients for plant growth (water, carbon dioxide, and oxygen) have no trouble reaching a high-altitude inverted surface, the micronutrients are a different matter. Plants might be able to evolve to pull nitrogen out of the air, but other elements such as phosphorus are harder to get. You'll either need epic storms to bring dust high enough in the air, or your plants will need to develop a carnivorous lifestyle to get their nutrients from bugs (or birds).

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