Why do leaves change colours?
I see this tree everyday and I am fascinated at how only one side of it is turning yellow.
I know that some trees turn colours earlier than others
but usually the changes are scattered throughout the tree.
Or sometimes it’s the top part that usually turns yellow or red first.
But the tree in the above photo, it’s just that side. I thought that maybe it’s because that’s the side hit by sunlight the most. But then again, I remember learning in Science that it is sunlight that gives leaves their green colour, the chlorophyll, through the process of photosynthesis. So it didn’t make sense to me that the leaves on that side were the ones turning yellow first.
I found myself googling the reason why leaves turn colours. This is what I found out on this website.
First, let’s have a refresher course on photosynthesis.
“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 glucose. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into sugar is called photosynthesis. That means ‘putting together with light.’ A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color.”
How do leaves change colours?
“As summer ends and autumn comes, the days get shorter and shorter. This is how the trees ‘know’ to begin getting ready for winter. During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off 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 fades away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along. We just can’t see them in the summer, because they are covered up by the green chlorophyll.
The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red color.
The brown color of trees like oaks is made from wastes left in the leaves.
It is the combination of all these things that make the beautiful colors we enjoy in the fall.”
Since we have shorter days now, it is actually the lack of sunlight that causes the leaves to lose their green colour. And going back to the tree in the above photo, I guess that side is the one exposed to the cold the most so the leaves on that side are the ones changing colours first, wither and and then fall off the branches. The other side is protected from the cold temperatures by the other trees and the house behind it.
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