Photosynthesis

It's the process when plants use energy from sunlight to produce sugar, which cellular respiration converts into ATP, giving the plant fuel. The photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive. Plants need only light energy, CO₂, and H₂O to make sugar. The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis. Plants take in carbon dioxide and release oxygen.
http://www.phschool.com/science/biology_place/biocoach/photosynth/intro.html

 (Flowers are needed for the uptake of water as photosynthesis is the combining of sunlight, CO2 and water to make sugar and some extra water and oxygen.)