Changing colour

A leaf is green because of the presence of chlorophyll, which is found inside an organelle called a chloroplast. When chloroplasts are abundant in the leaf's cells, as they are during the growing season, the chlorophyll's green color dominates and masks out the colors of any other pigments, such as carotenoids, that may be present in the leaf.

Chlorophyll has a vital function: that of capturing the sun's energy to use in photosynthesis. The sugars produced are the only source of the carbohydrates needed for growth and development. In their food-manufacturing process, the chloroplasts break down and thus are being continually "used up". During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.

In late summer, as daylight hours shorten and temperatures cool, the veins that carry fluids and minerals into and sugars out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease.

In this plume poppy leaf, the veins are still green while the other tissue is turning yellow, and has produced a fractal-like pattern. If there is a period of warm sunny weather in autumn, some photosynthesis will continue, and in certain species, especially maples and dogwoods, the trapped sugars will be converted into anthocyanins, which colour leaves red. Therefore the best autumn colours tend to occur during a sunny autumn, with cool but not frosty nights.