Saturday, July 30, 2011

Why Leaves Change Colour

Changes to the colour of leaves are most obvious in countries which have clear seasons – summer, winter, spring and autumn.

Inside a leaf, there are millions of tiny packages of colour – yellow, orange and green. These colours have special names. The yellow is called xanthophylls, the orange is carotene and the green is chlorophyll. The green colour is stronger than the others, so leaves stay green while chlorophyll works under the summer sunlight.

Water enters the leaf through tiny tubes in the leaf’s stem and near the end of summer, a thin layer of cork grows over these tubes and seals them up. Now, no more water can get into the leaf. Without water, the green chlorophyll fades and disappears. Then, the yellow xanthophylls and orange carotene can be seen. This is why many leaves turn yellow or orange in autumn.

Some autumn leaves look purple or red. All autumn long, the leaves make sugar for the plant’s food. The sugar is carried from the leaf to other parts of the plant in a liquid called sap. But sometimes sugar gets trapped inside the leaves when the tubes are sealed up. Then, the sugar turns the sap into purple or red.

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