How about a spherical plant large enough to have its own gravitational pull, and attract oxygen and other nutrients to it by means of an atmosphere? Orbiting a star as a planet in close orbit, it could absorb light and heat from the star, and absorb nutrients from its own atmosphere.
Would this produce a bizarre symbiotic relationship with a civilisation that may dwell there, one that couldn't occur naturally? With the amount of Carbon Dioxide a planet-sized plant would require, even if the inside of it has died and it is now feeding off it's own decaying remains, is enormous. Could a civilisation run enough combustion engines to produce that much Carbon Dioxide, and breath the Oxygen that the plant creates, thus forming a symbiotic relationship? The plant couldn't survive without the civlisation in this case, so how could they come to live on it?
If the inside of the giant plant is decaying, wouldn't that create gases like methane, and eventually rip the planet-plant apart in a massive explosion?
What is the plant was sufficiently massive to have stars orbiting it, rather than the other way round?
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I've taken this topic far away from its intended course...