To answer that, it is important to understand what leaves are and what they do. Leaves are nature s food factories. Plants take water from the ground through their roots and take a gas called carbon dioxide from the air. They use sunlight to turn the water and carbon dioxide into glucose. Glucose is a kind of sugar.
Plants use glucose as food for energy and as a building block for growing.
The way plants turn water and carbon dioxide into sugar is called photosynthesis. A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color. As summer ends and autumn arrives, the period of daylight gets shorter and shorter. During winter, there is not enough light or water for photosynthesis.