You're right, a Tree in a Cave IS odd, but then again grass is odd as well. THEN again this whole mess takes place in a universe I created back at the end of the 90s, which is called "Strange Universe". After all, "Strange Universe" is a universe, where the technology to built airtight Spaceships was lost during some really long war some aeons ago, so the ancient (since every universe got some ancients :3) had to fill the universe with oxygen.
I think I'll simply add some more plants as light sources... maybe Sunflowers :3. With smiley faces. My first thought was to make the tree glow as well, but then the tree would be hard to differ if its fore or background.