Plants kept inside a room always grow in the direction of the window. In woodlands where there is a thick canopy of trees and sunlight rarely falls on earth, very few plants survive. Those that do, do not require sunlight to make their food.
People have long wondered about this phenomenon until the answer was discovered and explained by the English naturalist Charles Darwin. He demonstrated that the growing shoot of a grass seedling always bends towards light. This is due to a phenomenon called phototropism.
We know that green plants make their food by the process of photosynthesis. The leaves of the green plants contain a green pigment called chlorophyll. Chlorophyll converts water from the soil and carbon dioxide from the air into oxygen and sugar.
Oxygen is then released into the air and sugar is taken as food by the plant. But this entire process of making food can only be done in the presence of sunlight. The leaves, therefore, lean towards the light for photosynthesis. Without sunlight green plants cannot survive.
How does a rigid plant bend or grow to one side? Plant cells contain a substance called aurins. This substance has a tendency to move away from the light. Aurins make the cells on the darker side grow faster than the cells on the lighted side.
This causes the stems and the leaves to bend or lean towards the lighted side. The concentration of aurins on the dark side may be due to the fact that sunlight slows or kills these aurins when light falls on them. However, as long as one part of the plant gets sunlight, it can make food and the whole plant survives.
Try this experiment at home or in school. Take a potted plant and put it near a window where there is sunlight. Take another potted plant and keep it in the darkest corner of the room. After a week or 10 days, you will notice that the plant near the window is healthy while that in the dark corner is not.