Autumn is a time for the beauty of the turning leaves. Ask students what causes that transformation? As the length of days and nights change (photoperiodism) and the amount of available daylight decreases, a plant’s ability to produce chlorophyll (photosynthetic pigment) is reduced. Senescence, the process of aging and death of plant part (leaf) begins to occur. Nutrients return to the stems and roots of the plant, abandoning the leaves. When the green chlorophyll is gone, yellow and orange carotinoids that have been always present are left.