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.