Plants "poo" by exchanging gases through their leaves or the green part of their anatomy. At night, they take in carbon dioxide, and in the daytime they exhale oxygen. This is done through a proces called photsynthesis. The active catalyst that permits this to happen is chloropyll. The plant's inner metabolism can exchange carbon dioxide for oxygen. Of course, they need water as a carrier to carry the oxygen and carbon dioxide to the individual cells. Neat huh?