If you were testing the affects of lights on plant photosynthesis what would be the control group?

If you bought 6 plants to perform light tests on (1 plant gets 4 hours of light a day, another plant gets 5 hours a day and so on) what would be the control group?|||I would say a plant left in "normal" light meaning your control would be as though it were a normal plant growing wild with no restriction on light except for loss of sunlight due to setting of the sun. Then you restrict the amount of light to the other plants and then see how photosynthesis varied. It's just an idea.|||If you have 6 plants and you change the amount of light for each plant by one hour, start out with no light for a plant. No light would show you the affect the light plays on the plant.