Plants have leaves so they can get nourishment. The process of photosynthesis, in which the plant absorbs sunlight in order to turn carbon dioxide and water into food, occurs in the leaves.
Photosynthesis occurs in the chloroplasts of cells, which are located in plant leaves. Therefore, the majority of photosynthesis takes place in the leaves, making them extremely important to the survival of the plant.
During the process of photosynthesis, water reaches the leaves through the roots of the plants. At the same time, carbon dioxide enters the leaves through little openings, or pores, on the undersides of the leaves. Chlorophyll, a green pigment found inside plants, enables the leaves to absorb sunlight. The energy from the sunlight becomes the energy source that drives the chemical reaction between the water and carbon dioxide. Once the water and carbon dioxide combine, sugar and oxygen are created. The oxygen is secreted back into the air, while the sugars are stored in the plant.
During the winter, when the ground freezes, the plant's roots can no longer pull moisture from the soil and send it to the leaves for photosynthesis. As a result, the leaves stop making food and the cells no longer produce chlorophyll. The leaves then change colors and fall off the plant.