Have you ever wondered why leaves change colour in autumn, from green to orange and red? Photosynthesis is the process trees use to make their food. Using energy from the sun, water from the ground, and carbon dioxide from the air, they make glucose. The tree uses the glucose for energy and growth.
Photosynthesis happens in the leaves of plants, because they contain chlorophyll. Chlorophyll is also what makes leaves green. As long as there are plenty of daylight hours, the leaves can absorb enough energy from the sun for photosynthesis to take place. This means the leaves on the tree stay green.
As the days grow shorter, there are not as many hours of sunlight, and the days get colder. Leaves will start to change colour. This is because it is harder for the chlorophyll in the leaves to make glucose. Instead the leaves start using food they have stored away. As the leaves use glucose they have stored instead, a layer of spongy cells forms at the bottom of each leaf called the abscission layer. These cells act like a door between the leaf and the rest of the tree, which is closing really slowly. It stops water and nutrients reaching the leaf, and once all the leaf’s food is gone, it shuts.
When all the food is used up, the leaves turn brown, and fall off the tree.
To see the guide, click on the picture below.
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