SO I was just thinking aimlessly, and an idea occured to me. Aside from the fuel, a big restriction on space exploration is the problem of running out of oxygen. Why not just put a bunch of plants on the ship/station?

Trees convert CO2 into O2, right? And we convert it back, so it'd be a never ending cycle. This sounds a bit too easy, and there's got to be a reason why we haven't done this yet. So, who wants to poke holes in my theory?