Leaves are actually little “factories” that manufacture food to help the plant grow. In the spring and summer, these factories run at top speed, taking in carbon dioxide from the air and water from their roots.
Then sunlight enters the leaf and sets the factory in motion. But a leaf could not begin its manufacturing process without a chemical already present in its cells. That chemical is chlorophyll, which also gives the leaf its green color.
There are other colors present, too, in most leaves (red, orange, yellow, purple), but they have been hidden by the green of the chlorophyll.
When the fall comes, bringing cold weather, the veins in the leaf, which once brought water to it, become blocked. This causes the chlorophyll to break down and stop its job of keeping the leaf green. Then the reds, oranges, yellows, and purples appear.
The color that each tree turns in the fall depends on the hidden pigment, or coloring matter, present in the leaf: xanthophyll, for the yellows; carotene, for the oranges; and anthocyanin, for the reds and purples.
When an autumn leaf falls from a tree, the scar, or mark, it leaves on the branch actually has a tiny “face,” with two eyes, a nose, and even an expression!