Question

Why was the plant kept in the dark before beginning the experiment?

Answer 1

The diagram shows the test for the importance of light in photosynthesis. Before starting the experiment, the plant was kept in dark so that all the starch present in the leaf could be removed. Since no photosynthesis occurs in dark, the plant won't be able to synthesise starch and the starch which was synthesised earlier and stored in the leaves will get used up.

So, to remove all the starch from the leaves, the plant was kept in dark.