Ask Mr. Smarty Plants

This is a complicated question and deserves a complicated answer. Leaves appear green because the green pigment chlorophyll absorbs red and blue light from sunlight. Consequently, the light reflected by leaves is diminished in red and blue and appears green. chlorophyll is not a very stable compound and to maintain the amount of chlorophyll in their leaves, plants must continuously synthesize it. The synthesis of chlorophyll in plants requires sunlight and warm temperatures. Therefore, during summer chlorophyll is continuously broken down and regenerated in the leaves of trees. In some trees, as the concentration of sugar in the leaf increases, the sugar reacts to form anthocyanins. These pigments cause leaves to turn red. The range and intensity of the color change is greatly influenced by the weather. Low temperatures destroy chlorophyll, and if they stay above freezing, promote the formation of anthocyanins. Bright sunshine also destroys chlorophyll and enhances anthocyanin production. Dry weather, by increasing sugar concentration in sap, also increases the amount of anthocyanin. So the brightest colors are produced when dry, sunny days are followed by cool, dry nights.