Explain the conclusion drawn from this experiment after the iodine test.

In plant X, the potassium hydroxide kept in the watch glass absorbed all the carbon dioxide present inside the bell jar. In the absence of carbon dioxide, photosynthesis could not take place, so no starch was formed and the leaves did not turn blue black with iodine.

In the control plant Y, carbon dioxide was available, so photosynthesis took place and starch was formed. Hence its leaves turned blue black with iodine.

This proves that carbon dioxide is essential for photosynthesis.