When you say 'control tube' it's probably the type of control in your specific experiment, but normally a control is used to compare your experiment with the effects of having something in it and something out. So say you run an experiment in which your testing the effects of oxygen on photosynthesis, your control would be a trail with no oxygen present and testing rate of photosynthesis. You would then compare this control with the results of having oxygen present on the rate of photosynthesis.
I hope it helps!!!!!!!!!!
Good luck :)