Just a little tidbit on plants, they don't only breate in carbon dioxide in fact they are constantly taking oxygen in for celluar respiration. When sunny out photosynthesis (c6 h12 o6 + co2 + sunlight = o2) takes place, because sun breaks the hydrogen bond in water which releases ATP (i'm sure terra can do a run down on this). Respiration takes place throughout the entire day as it is necessary for cell mitosis; this function use's oxygen and glusoce from the cell and produces co2 and 36 ATP. So basically a plant can run by itself if a dynamic equlibrum exists between the photosyntesis and respiration. Also note the o2 we breathe is not necessarily from plants, but pre-existed when the earth's atmosphere formed due to gravity. So that's just a little refresher course in photosynthesis and respiration.