Take a potted plant and keep it in the dark for two days for destarching the leaves. For the experiment as shown in the figure,

Take a wide-mouth bottle and fill 1/3 rd of it with caustic potash solution. Now with the help of a split cork insert half-part of the leaf of that potted plant in the bottle. The leaf should not touch the solution. The apparatus is kept in sunlight for a few hours. The leaf should not touch the solution. The leaf is plucked and tested for starch. (If you want to know how refer to this link)

http://www.nuffieldfoundation.org/practical-biology/testing-leaves-starch-technique

Only the half portion which is outside the bottle turns blue and the part of the leaf which was inside the bottle remains colourless. This is because that part of leaf inside the bottle did not get carbon dioxide for photosynthesis as all carbon dioxide present in the bottle was absorbed by the caustic solution.