Leaves Change Colour in the fall! A Phenomenon

Sham S. Misri

Most ever green trees do not shed all their leaves at the approach of the winter, but lose them gradually through the year; thus they are always green. But there are many green plants which have a different story to tell.

A look at trees in summer time shows only one colour:  “green,” with various shades in green. In the fall, the same leaves take on a variety of colours. One is amazed to know where from all these colours come from? Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into oxygen and glucose.

Plants use glucose as food for energy and for growing. The way plants turn water and carbon dioxide into oxygen and sugar is called photosynthesis. A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green colour.

As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter. The trees will rest, and live on the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green colour fades away, yellow, orange and some other colours appear.

What are some of these other colours? A substance called ‘Xanthophylls,’ which consists of Carbon, Hydrogen and Oxygen, is yellow. It makes nearly 23% of the pigmentation of the leaf. Carotin, the substance which makes carrots the colour they are, is also present in the leaf and makes about ten % of the pigment. Another pigment is anthocyanin, which gives the sugar maple and the scarlet oak their bright red colours. During summer we see none of these other pigments. We only see the green chlorophyll.  Though small amounts of these colours are in the leaves all summer, but the green chlorophyll covers them up.

When it becomes cold, the food that has been stored away in the leaf by the trees begins to flow out to the branches and the trunks. Since no food is produced in the winter, the chlorophyll food factory closes down and the chlorophyll disintegrates. And as the chlorophyll disappears, the other pigments that have been present all the time become visible. Then we can see orange and yellow colours. The leaves take on all these beautiful colours which we enjoy to see!

Before the leaves fall, a compact layer of cells is formed at the base of each leaf; then when the wind blows, the leaves are dislodged and the leaves fall. As plants grow, they shed older leaves and grow new ones. This is important because the leaves become damaged over time by insects, disease and weather. The shedding and replacement continues all the time.