Answer:
To examine the importance of light to a plant's growth.
Step-by-step explanation:
To give a background to this question, I will define a key term "Photosynthesis". Photosynthesis is a compound word made up of photo (which means "light") and synthesis (which means "to put together").
It is the process by green plants use sunlight to obtain nutrients from carbon dioxide and water.
The experiment us meant to examine the effect of deprivation of sunlight on a plant's growth.
Therefore, the essence of keeping one set of seedlings in the light is to compare if there is any difference in growth between the seeding kept in the dark cupboard and that kept in the light.
This enables us to conclude whether or not light is important for the growth of a plant.