Put the cups on a windowsill and cover each cup with a small cardboard box. Make a hole in the side of one box and leave for several days. Check the cotton daily in both boxes to ensure it remains moist. Observe that the seeds under the box with no hole have grown straight up in search of light. The watercress seeds under the cardboard box with a hole in the side has grown toward the light.
Green plants contain a chemical called chlorophyll .
Chlorophyll is a chemical which traps light, and when combined with air and water helps the plant produce food and grow. This process is known as photosynthesis . Plants do not see light, but bend and grow toward where it comes from.