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Have you ever wondered…why leaves change colour in autumn?

Autumn is a season of change; the weather gets colder, there’s less daylight and leaves change colour and fall from plants. But why does this happen?

Why Do Plants Have Leaves?

Leaves contain a chemical called chlorophyll (pronounced KLO-ro-fil), which as well as giving them their lovely green colour also helps create food for the plant. The leaves act like tiny solar panels, and use the sun’s energy to convert water (from the ground) and a gas called carbon dioxide (from the air) into sugar and oxygen. This process is called photosynthesis (pronounced foto-SIN-theh-sis), and the sugar is what the plant lives on.

What Causes The Colour Change?

As the weather becomes colder and there’s less sunlight, the plant is unable to make so much food for itself. It begins to break down the chlorophyll in its leaves and reabsorb the nutrients, storing them in its roots for the winter. This means that the green colour fades and we’re able to see other colours in the leaf, which were always there but invisible under the green. These are usually yellow and orange pigments, but sometimes there are red shades too. The colour of the leaf depends on how much chlorophyll is left in the leaf, and what other colours are present; this means that some leaves are yellow, some orange, some red, some purple and some are a mixture!

Why Do Leaves Fall Off the Trees?

During the warmer seasons the plant loses water through tiny holes in its leaves. It can’t afford to do this during autumn and winter because it wouldn’t be able to replace it all that it lost.  To solve this problem, once the plant has absorbed all the nutrients in the leaves and changed their colour, it grows special cells that gradually sever each leaf from its stem. Another type of cell grows in a thin, protective layer over where the leaf used to be attached. When spring brings warmer weather and more water, the plant will begin to grow new leaves and start the process all over again.

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Sam has worked as a forensic scientist as well as for the British government, and has degrees in both archaeology and osteoarchaeology. She has 2 children, is passionate about science, reading, history and music, and loves dyeing her hair bright colours!

Sam blogs about all kinds of science at www.samanthagouldson.com.
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